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General Category => Amstrad CPC hardware => Topic started by: RobertM on 10:36, 11 February 14

Title: Need some help fixing CPC6128 not booting
Post by: RobertM on 10:36, 11 February 14
Hi all,
        My CPC6128 has been in storage for some time as I don't have an original monitor. I wanted to convert it S-Video and perhaps make a uC SD floppy emulator. I don't know if it ever worked.

No good lol. It doesn't boot. It did once or twice out of about 20 power cycles.

I get a mostly blank screen with a square where the active video normally is. ie white screen and black border.

I am using the composite out to a TV and a small 5 Volt switch mode PSU rated at 3 Amps.

I noticed the board was a bit dirty and there was corrosion in some of the IC sockets (CPU) so I replaced three IC sockets and cleaned the board with metho. I used snap off SIL machined pins with gold inserts.

The three sockets I replaced were for the CPU, a small PAL (Programmable Logic Array) and another chip marked 40010.

Thinking the problem may be noise from the SMPSU, I put a 100nF cap across the 5 Volt input and upgraded the 47uF 5 Volt filter cap to 220uF.

I am guessing that the CPU is running as the registers in the 6845 need to set by the CPU to get video sync and I have sync.

The 5 Volt rail is at 5.05 Volts.

My next guess would be the DRAM chips but I don't want to socket 16 chips if I don't need to.

Does anyone have some experience or advice / suggestions that mat save me some time fixing this??

Any help would be greatly appreciated.
Title: Re: Need some help fixing CPC6128 not booting
Post by: TFM on 23:34, 11 February 14
Posting once is enough here. A screenshot as told would be helpful.  ;)
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 04:25, 12 February 14
Hi, Sorry to double post. I didn't register before my attempt and it gave an error "Topic / Subject does not exist" so I registered thinking that may be the problem and then posted.

I have some pics below. In order they are -

A screen shot of the TV
The CPU socket before it was replaced
The CPU socket after it was replaced
The front of the main board after cleaning and socket replacement
The back of the main board after cleaning and socket replacement

I am happy that the PSU is ok but I will test with another PSU today.
I was hoping to use more modern DRAM chips as replacements something like 64K x 4 but the newer chips wont work and the older ones are getting hard to find.

As you can see from the screen shot, I have sync and the active field is white which to me indicates that the CPU is ok and something else is failing in the boot sequence possibly DRAM.

Also, I noticed you don't have a pic of the back of this main board on this wiki. I will take out the extra cap and remove the composite video connection (wires from video socket) and take a pic and post it here.

Thanks again, Rob.
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 10:16, 12 February 14
64Kx4 are just as old and difficult to find as 64Kx1. You could use 64Kx4 in a 6128, but you'd have to do quite a bit of manual wiring to connect up the signals as they should be.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: gerald on 10:39, 12 February 14
If the main bank of ram is failing, you can switch to the second one:

   - remove the PAL from its socket
   - on the socket :
      - link pin 9 to pin 19 (A14 to A14out) (Eq to LK5)
      - link pin 8 to pin 12 (A15 to A15out) (Eq to LK6)
      - link pin 7 to pin 16 (NCAS to NCAS1)
      - link pin 17 to pin 20 (NCAS0 to VCC)

If your CPC boot, then either the PLD or ram bank 0 is failing.
You can now test bank 0 by changing connecting NCAS(pin7) to NCAS0(pin17) and NCAS1(pin16) to VCC(pin20) on the socket.

You can also double check the schematic for pin connection : http://www.cpcwiki.eu/manuals/service.manual.cpc6128.ctm644.gt65.zip (http://www.cpcwiki.eu/manuals/service.manual.cpc6128.ctm644.gt65.zip)


Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 10:57, 12 February 14
If the main bank of ram is failing, you can switch to the second one:

   - remove the PAL from its socket
   - on the socket :
      - link pin 9 to pin 19 (A14 to A14out) (Eq to LK5)
      - link pin 8 to pin 12 (A15 to A15out) (Eq to LK6)
      - link pin 7 to pin 16 (NCAS to NCAS1)
      - link pin 17 to pin 20 (NCAS0 to VCC)

If your CPC boot, then either the PLD or ram bank 0 is failing.
You can now test bank 0 by changing connecting NCAS(pin7) to NCAS0(pin17) and NCAS1(pin16) to VCC(pin20) on the socket.

You can also double check the schematic for pin connection : http://www.cpcwiki.eu/manuals/service.manual.cpc6128.ctm644.gt65.zip (http://www.cpcwiki.eu/manuals/service.manual.cpc6128.ctm644.gt65.zip)

What a cool trick! I would never have thought of doing that, despite it being obvious :)

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 11:43, 12 February 14
Thanks Gerald,
                       I tried that but unfortunately I get exactly the same result. But it does help in that I am less suspicious of DRAM now.

I noticed an number of electro's near the sound chip that I can't see on the schematic. Some of them are 1uF/50V which are likely to have dried out. If they're in an audio amp then I can't see how they would be a problem but I will replace them next.

I just tested bank 2 so I will go and try bank 1 and see how that goes.

Here is what I believe so far.

I have measured the voltage on board at 5.05 Volts so I think the PSU is OK. I did try another PSU anyway. I am not connecting 12 Volts as I think it is only used for the floppy.

I am getting some voltage noise from the on board power connector but apart from that I consistently get exactly the same result at the same time from power on. I am using a jumper strap instead of the power switch.

I am using the composite video out so I assume I am just getting white on the screen and not random colour pixels that all look the same in black and white.

As far as I know the CPU needs to set up the 6845 registers before I can get sync so I believe the CPU is running and at least executing some of the boot code even though it consistently fails at a specific point.

The white area of the screen looks to me to be the active area of the screen where DRAM is converted to pixels.

This CPC did boot 2 times out of about 20 before I changed the sockets. I used metho to clean the board. Metho isn't the best for electro's so I will change them. However it has not booted since and appears to get stuck at the same place it did before I cleaned the board.

I don't have a CRO / Scope so this is really hard to debug. I am tempted to make a quick and dirty logic probe as anything would be better than what I have now.

I even turned the sound up and listened to the speaker. I get a faint high pitched sound in the background and a very brief pulse of white noise every second or so. All of this is very faint, I am putting the speaker to my ear with the volume at max.

I tried typing in sound 1, 240 just to see if the CPC is actually working but with no video output, no sound came out.

I will change out the crystal to a lower frequency say about 10 - 12 MHz. If my suspicions are correct about the CPU setting the 6845 registers the I should get a torn image on the screen due to loss of sync.

Once any suggestions / advice would be most appreciated.

And , Gerald thank you once again, you experience has saved me much time and is greatly appreciated.

Thanks all, Rob.
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 12:07, 12 February 14
Have you connected the keyboard and tested whether the CPC beeps when you hold down "Del" ?

One other thing: Pin 22 of the Gate array looks like it hasn't been soldered? Is this just the photo?

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 12:38, 12 February 14
OK, I tried DRAM bank 0 and it was the same as bank 1.

I changed the 16MHz XTAL for a 12.288MHz crystal and it did exactly what I expected. It gave a torn image due to the sync being to slow for the PAL TV.

I used a multi meter on all of the pins of one chip in each bank looking for any shorts. The only pin the was not as I expected was A5 but when I looked at the diagram I see a resistor on this bus pin for some reason. I have assumed one bank is on the left and the other bank is on the right, I don't know if that's true. I tested 16 pins of one chip on the left and 16 pins of one chip on the right which means I have NOT tested the whole 16 pins of the data bus.

So it seem the CPC is booting to a point. Perhaps it sets up the 6845 registers from ROM and then jumps to (Faulty)RAM??? or perhaps faulty ROM ???

I can't remember if the CPC6128 changes the address space of the ROM through the PAL during boot or if it copies low ROM to high RAM and runs from RAM (disabling ROM). I can't remember ... it's been 20 years since I had one of these.

I haven't changed the electro caps yet. Night here, too dark can't see. Unfortunately the warranty on my eyeballs expire long ago lol. A job for tomorrow.

I looked at this board and though well I could just strip it and start over with new parts but that may NOT be a 'fix'.

Then I though what is the worst and in order they are -

Open circuit via, I will never find it with my sight.
The bigger PAL faulty - I doubt I can get a replacement.
The smaller PAL is more conventional, can I get code for it (Fuse map). Can they still be bought.
I am not worried about the ROMS the code will be out there somewhere.
I doubt I would get the sound chip but I have no reason to believe that it's faulty.
All the other chips I reckon I could still get except perhaps for the MFM MOD/DEMOD that works with the FDC.

I did find another Z80 CPU lying around but it didn't work. It's probably faulty or the wrong type ie NOT CMOS.

Perhaps I should order another CPU ? Do they have to be a particular substrate ie CMOS and NOT TTL or NMOS?

Thanks, Rob.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 13:02, 12 February 14
Hi again,
    Bryce, thanks for your suggestion.

I put the 16MHz crystal back in and re-touched pin 22 of the PLA. I used my ultra zoom camera to look at again and it sort of looked soldered but I wasn't sure so I re-did it.

I tried tapping the DEL button but no sound. Thanks for your suggestion, I knew there was a way to make it beep but I couldn't remember how.

With the crystal back to 16MHz, I am now back to exactly where I was before, white box on screen.

I will see if I can replace the electro caps tomorrow.

In the mean time any suggestions are appreciated.

Thanks, Rob.


Title: Re: Need some help fixing CPC6128 not booting
Post by: 00WReX on 13:27, 12 February 14
Hey Rob, where abouts in Australia are you located.
I'm in the South Eastern suburbs of Melbourne.
I have a few bits and pieces you can try if you you are close by.
CPU, Gate Array, AY sound chip, RAM, etc + Amstrad monitors.

Cheers,
Shane
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 13:38, 12 February 14
It's very unlikely that any of those caps are damaged, and even if they were, they wouldn't cause that problem. Concentrate on the Gate Array, CRTC, ROM (the one marked 40015) and maybe the Z80. Just because the CPU was able to initialise the CRTC doesn't mean that it's fully functional.

Bryce. 
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 13:42, 12 February 14
Hi Shane,
              I am in Far North Queensland so not at all close lol.

If I get to the point of knowing what is wrong with CPC and it's one of the chip I can't and you have then I may be interested in buying it from you though.

I think I can get the RAM / CPU chips but I doubt I could get a gate array.

Would the modern Z84C0020PEC (http://www.ebay.com.au/itm/140991935896?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649) (20Mhz) CPU work?
Title: Re: Need some help fixing CPC6128 not booting
Post by: 00WReX on 14:15, 12 February 14
Yeh Rob, that's a little more than just down the road  :D

For our European friends,  that's over three thousand kilometers away.

The Gate Array is a chip I don't have a spare of, I would have just pulled one from a working CPC to test / prove yours.

I do have a couple of Z80's though, but they are relatively easy to get.

Cheers,
Shane
Title: Re: Need some help fixing CPC6128 not booting
Post by: gerald on 14:31, 12 February 14
Some points :

- The PAL removal and socket wiring cannot exclude a failing RAM device if this device is shorting any data bit low even when not selected.
- Since the CPC booted few times before the socket change, double check the socket connection either visually or using a continuity tester from socket to any other connected chip or exp connector (use the schematic for matching pin).
- Also, check that the socketed IC pin are not broken/bended or oxidised . Again, you can check continuity from socketed IC pin to any connected chip.
Title: Re: Need some help fixing CPC6128 not booting
Post by: redbox on 14:41, 12 February 14
I think I can get the RAM / CPU chips but I doubt I could get a gate array.

I have a spare Gate Array (40010) and ROM (40015) if you get stuck.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 14:57, 12 February 14
Hi Bryce,
              I had a couple of 40 pin chips here on my desk that are just bits of memorabilia.

The CPU in the CPC is marked Z8400APS which I assume is a 4MHz NMOS CPU.

I shuffled around the desk and found a Z8400BPS under my keyboard lol. I assume it is a 6MHz NMOS CPU. What luck lol, well if it works.

I have already socketed the CPU so I will try CPU chip tomorrow.

I think I have a 6845 floating around somewhere so I will look for that and see if it is compatible. If it is then I will socket the CRTC and try it to.

If these things fail to fix the problem then I will have to order stuff.

As for the gate array I think I would have no luck finding a replacement unless there is some VHDL out there somewhere.

I downloaded the ROM (16KB) and tried to disassemble it but I got rubbish. It looks like there is data in amongst the code which is throwing the disassembler out of sync with the instructions.

There are two ROMS in the CPC6128, is that 2 of 8KB Roms? what (E)EPROMS can I use as replacements. Or could I use Flash? I see I can still get 27C64A-200 EPROMS (8KB x 8bit - 200nS) would they do? I just looked and the smallest flash I can get is 32KB (too many pins).

I don't have a (E)EPROM reader programmer but I could probably throw something together with a micro-controller and run it via USB serial.

Is there any VHDL for the two programmable logic arrays available? I have only just started VHDL by the way.

If I have to order stuff I will also get a Z84C0020PEC/PEG and try that (modern 20MHz CMOS CPU). If I do then I will let other know if it works.

Fingers crossed I will get back to it tomorrow (my time GMT+10).

One thing that still gets me is that this fault started as somewhat intermittent and is now permanent.

Mask ROMS and PAL's aren't normally flaky like that but I suppose they're 30 years old. It makes me worry that I have a corroded track/trace or fractured via.

From memory RAM, CPU's failed often, more often the (D)RAM. I have never had to replace a 6845 though.

I see most of the smaller chips are 74LS and a couple of 74HC's. From memory the HC's were more prone to static (ESD) so I might change them to as I can get them locally.

Thanks again everyone for your help. I will let you know how things go tomorrow.
Title: Re: Need some help fixing CPC6128 not booting
Post by: arnoldemu on 15:08, 12 February 14
The CRTC signals are used to drive the interrupts.

Gate-Array counts HSYNC. If CRTC is not initialised, there will not be any HSYNCs, no interrupts will happen, the firmware will not see interrupts, keyboard will not work so you can't type anything or test with delete.

So the Z80 may not be getting as far as setting up the CRTC, it may not be getting as far as initialising the palette.

So it's much harder work.

So, concentrate on the Z80, ROM and the Gate-Array.

See if you can see if interrupts are being activated and check the NMI is not being triggered incorrectly.
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 17:12, 12 February 14
First off: I made a mistake above, I meant the 40025, not the 40015. Here's the general info on the ROMs:

40025 is a 32K ROM. The lower 16K contains the Firmware and the upper 16K contains BASIC 1.1
40015 is a 16K ROM and contains AMSDOS

Both are pin compatible with standard 27C... Parts, so you can easily burn new ones to replace them if needed.
The 40010 Gate Array hasn't been reverse engineered yet, but there are some available still. I don't have any at the moment, but I sent two of them to someone in Australia a while back, so you should be able to get a replacement if needed.

Either way, my money is still on the CPU. If the gate array isn't working you don't usually get even a border.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: Munchausen on 21:15, 12 February 14
You can buy the 40010 here: 40010 Amstrad IC in UK - 121spares (http://www.121spares.co.uk/40010/product-detail)


Not sure if they deliver outside the UK, and the price is wrong, when you order he asks you for £6.95 instead. A couple of people here have bought them. If it comes to it, someone here could order one and send it on to you (I'd be happy to do so).


More info: http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/40010-ulaasics-available!/
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 01:16, 15 February 14
Just a quick update.

This morning I am going to buy a logic tester if it is in stock locally. From there I will decide which way I am going with this.

I looked over the board to see what tech it uses.

The CPU is NMOS.

All the smaller chips (except the PAL) and two others are 74LS TTL chips that can be replaced with 74HCT chips if need which are all available for that matter the 74LS versions are still around.

74LSxx is (L)ow power (S)chottky TTL which was the next chip after 74xx TTL and used less power and had better noise rejection.

74HCT is a (H)igh speed (C)MOS that had (T)TL input compatibility and was an even lower direct replacement for TTL like 74LS.

74HC is a (H)igh speed (C)MOS that has CMOS input compatibility and was not intended to be a direct replacement for 74LS (TTL) but in many cases can be used.

The small logic array (40031 - HAL16L8) may still be available as a blank (PAL16L8) and I can get a programmer for it. It only has four registers so it wouldn't be hard to reverse engineer but there is no point for me as I would need a working one to this.

The two other exceptions are a 74HCU (IC117) chip that is used for the reset. The 'U' in 'HCU' is for unbuffered so I doubt anything but a HCU would be suitable and they should be available. The other exception is a 74HC (IC203) in the FDC section and they're available.

The blank EPROM's (40015 40025) and a programmer are still available but the 27C128 is harder to find. The 27C64 is common still.

The larger logic array (40010) may be available as a blank (different chip number). It would be easier to read the code and schematic to write VHDL for this than trying to reverse engineer it. However, as mentioned, some mask programmed ones are still available.

Apart from that I didn't look into availability too much. I have seen the FDC (Z765) online also the 8255 PIO. The 6845 CRTC would be harder to find. The sound chip is probably not available. I didn't look into the MFM decoder that works with the FDC at all. The DRAM is still available as 4164 not 4264 (150nS or faster) but most of what you find is second have, some new chips are still out there.

If the members here are interested I can do a write up with pictures on how to replace chips. The one that is here is OK but a bit risky by using a drill through PTH (Plated Through Holes). The PTH of this era was generally poor and using a drill is dicing luck. Often one side of the PTH had small coverage and the rest had none. If the drill happens to hit the side that has coverage then the board is toast as you never going to find the faulty PTH VIA.




Title: Re: Need some help fixing CPC6128 not booting
Post by: CanonMan on 13:57, 15 February 14
If the members here are interested I can do a write up with pictures on how to replace chips. The one that is here is OK but a bit risky by using a drill through PTH (Plated Through Holes). The PTH of this era was generally poor and using a drill is dicing luck. Often one side of the PTH had small coverage and the rest had none. If the drill happens to hit the side that has coverage then the board is toast as you never going to find the faulty PTH VIA.


Using a drill to clear plated through holes? I've never heard such a stupid idea before!


What's wrong with using a solder sucker or desoldering wick? Far safer unless you're completely cack-handed  :laugh:
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 16:45, 15 February 14
@CanonMan: I think he's referring to my guide here: IC Repair - CPCWiki (http://www.cpcwiki.eu/index.php/IC_Repair) where it suggests using a small drill if all other methods to remove the solder have failed.

@RobertM: You're not seriously going to replace every IC on the board instead of finding out what's broken are you???
The PAL has already been reverse engineered and the jed file is available.

Bryce.

Title: Re: Need some help fixing CPC6128 not booting
Post by: Munchausen on 22:37, 15 February 14
I've bought the 6845 on ebay, still pretty easy to find.


By the time you've bought all these parts you could have bought another CPC. Not that I'm trying to discourage you from resurrecting it, it's sad to see these machines go, as it isn't as though there will be more showing up any time!
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 11:08, 16 February 14
Hi all again,
I had a quick look at it with a logic tester. I didn't go right through everything but I did go over the CPU and RAM.

All I/O pins of the RAM have activity (pulsing).

On the CPU I see activity on the data and address buses.

The exception is A14 which represents a 16Kbyte difference in address space and the interrupt pin.

A14 is mostly low but has a pulse of activity (high and low) about 2 times a second.

The INT pin is ALWAYS low except for what looks like a single high pulse that happens once a second or exactly half as often as the A14 pin. 

Does that mean anything to anyone here.

Thanks.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 06:42, 18 February 14
Since my last post I tried a couple of things.

I pulled out the smaller logic array and I get exactly the same blank screen with a white box BUT I don't see the slow pulse on the INT pin.

I then pulled a DRAM chip out of bank 2 and piggybacked it to chips 1 to 8 of bank 1 and applied power, that made no difference to the screen.

I am now in the process of ordering the larger gate array (40010) from the UK.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 13:01, 20 February 14
It's very unlikely that any of those caps are damaged, and even if they were, they wouldn't cause that problem. Concentrate on the Gate Array, CRTC, ROM (the one marked 40015) and maybe the Z80. Just because the CPU was able to initialise the CRTC doesn't mean that it's fully functional.

Bryce.

Thanks for that Bryce. I've tried another CPU so my money is on the gate array (on order). I recon the 6845 is ok and I get synce an raster on the screen but I did notice that one of the two sync signals coming out of it looked odd on the logic tester. It seemed to spend just as much time low as high but that may just be the quality of the tester.

If the gate array doesn't fix it then I would probably try the 6845 next due the the slow pulsing INT signal I mentioned later on here.

Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 13:06, 20 February 14
Some points :

- The PAL removal and socket wiring cannot exclude a failing RAM device if this device is shorting any data bit low even when not selected.
- Since the CPC booted few times before the socket change, double check the socket connection either visually or using a continuity tester from socket to any other connected chip or exp connector (use the schematic for matching pin).
- Also, check that the socketed IC pin are not broken/bended or oxidised . Again, you can check continuity from socketed IC pin to any connected chip.

Hello Gerald,
I did check that I get transitions on all data and address pins of the DRAM but that doesn't exclude an internal failure of a DRAM chip.

I didn't do a continuity test on the sockets as you suggested and I have a bad feeling that might come back to bite me lol. It's not at all easy to do with my ailing sight. I did clean the chips up with a rubber and they seem ok.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 13:07, 20 February 14
I have a spare Gate Array (40010) and ROM (40015) if you get stuck.

Thanks for you offer redbox, I have a gate array on order.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 13:12, 20 February 14
The CRTC signals are used to drive the interrupts.

Gate-Array counts HSYNC. If CRTC is not initialised, there will not be any HSYNCs, no interrupts will happen, the firmware will not see interrupts, keyboard will not work so you can't type anything or test with delete.

So the Z80 may not be getting as far as setting up the CRTC, it may not be getting as far as initialising the palette.

So it's much harder work.

So, concentrate on the Z80, ROM and the Gate-Array.

See if you can see if interrupts are being activated and check the NMI is not being triggered incorrectly.

I am hoping the CRTC is working as I have sync and I would have that if it wasn't configured but the CPU / ROM. I am not sure that it is setting up the pallet as I an using the LUMA out and have black and white.

I mentioned elsewhere that the INT is nothing like I expected. I see nothing on NMI though.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 13:16, 20 February 14
@CanonMan: I think he's referring to my guide here: IC Repair - CPCWiki (http://www.cpcwiki.eu/index.php/IC_Repair) where it suggests using a small drill if all other methods to remove the solder have failed.

@RobertM: You're not seriously going to replace every IC on the board instead of finding out what's broken are you???
The PAL has already been reverse engineered and the jed file is available.

Bryce.

Hi Bryce,
I was just considering what may end up being the fail point of fixing this. I have long delays here when ordering parts so I have to see what I can do that might be helpful whale I am waiting for parts.

In you comments where you say "PAL" do you mean the smaller one of the bigger one? Is a JED file stand for JEDIC? Wasn't that the standard for the early smaller CPLD's?


Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 13:20, 20 February 14
I've bought the 6845 on ebay, still pretty easy to find.


By the time you've bought all these parts you could have bought another CPC. Not that I'm trying to discourage you from resurrecting it, it's sad to see these machines go, as it isn't as though there will be more showing up any time!

I could only find one other cpc6128 up for offer in my country and it was over 5000 Km away so the freight is a killer here. Hopefully I wont have to replace to many parts before it's fixed.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 13:21, 20 February 14
You can buy the 40010 here: 40010 Amstrad IC in UK - 121spares (http://www.121spares.co.uk/40010/product-detail)


Not sure if they deliver outside the UK, and the price is wrong, when you order he asks you for £6.95 instead. A couple of people here have bought them. If it comes to it, someone here could order one and send it on to you (I'd be happy to do so).


More info: http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/40010-ulaasics-available!/

Your a gem Munchausen, I have one on order.
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 14:12, 20 February 14
Hi Bryce,
I was just considering what may end up being the fail point of fixing this. I have long delays here when ordering parts so I have to see what I can do that might be helpful whale I am waiting for parts.

In you comments where you say "PAL" do you mean the smaller one of the bigger one? Is a JED file stand for JEDIC? Wasn't that the standard for the early smaller CPLD's?

The PAL is a small 20pin IC with HAL16L8 written on it. The Gate Array is a 40pin IC with 40010 (in a 6128) written on it. these are the only two custom ICs the CPC has. The PAL has been reverse engineered and can be easily reproduced if required. Yes, .jed is a JEDEC file containing the code for the PAL. Unfortunately, no-one has even begun looking at a Gate Array replacement (as far as I know).

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: arnoldemu on 19:42, 20 February 14
I am hoping the CRTC is working as I have sync and I would have that if it wasn't configured but the CPU / ROM. I am not sure that it is setting up the pallet as I an using the LUMA out and have black and white.

I mentioned elsewhere that the INT is nothing like I expected. I see nothing on NMI though.
The LUM is derived from R,G,B so it's definitely setting up the palette.

NMI isn't used on the CPC, some devices use it (Multiface II).

INT can sometimes be held low for a while. The Gate-Array will assert it until the Z80 acknowledges it. Under normal operation in BASIC it will be low for a short time.


Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 05:40, 22 February 14
Just a thought... I was intending to build a RGB to s-Video converter for this CPC6128.

I really did want to use it with a monitor and I have a uncommon monitor that is dual purpose - VGA and CCTV monitor.

Because it accepts CCTV (PAL) it will sync down to the PAL 50Hz frame rate. Most computer type monitors won't sync below 60Hz.

After reading some disassembly of the CPC6128 ROM, I see it 'detects' PAL / NTSC configuration and makes some changes as a result.

So my question is, Can I simply change a jumper or something like that and have my CPC6128 output NTSC?

If that is possible then it would be much easier to interface it to a standard computer monitor and give a better picture. 
Title: Re: Need some help fixing CPC6128 not booting
Post by: ralferoo on 11:51, 22 February 14
After reading some disassembly of the CPC6128 ROM, I see it 'detects' PAL / NTSC configuration and makes some changes as a result.

So my question is, Can I simply change a jumper or something like that and have my CPC6128 output NTSC?
Yes, if you can find LK1-LK3 which select the manufacturer name on boot, I think it's LK4 which should be next to them that selects NTSC mode.

Note that this will only really be useful for BASIC programs. All it does is configure the CRTC to have a shorter display length (260 lines instead of 312), which it turn increases the display frequency from 50Hz to 60Hz. Any games or demos that reconfigure the CRTC themselves will almost certainly reset these values to things suitable for PAL.

If you're going to use Bryce's s/video adapter, you'll also want to use an NTSC crystal instead of PAL and change one of the pins to select how the colour signal is encoded (pin 1 on the AD724 is HIGH for NTSC, LOW of PAL). Of course, this is only needed if you want colour output. If you're looking to drive a black-and-white CCTV display, you might as well just use the SYNC and LUM pins from the display connector.
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 16:06, 22 February 14
Just a thought... I was intending to build a RGB to s-Video converter for this CPC6128.

I still have some of these left over if you are interested: Limited batch of new S-Video Adapters. (http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/limited-batch-of-new-s-video-adapters/)

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 04:15, 23 February 14
I still have some of these left over if you are interested: Limited batch of new S-Video Adapters. (http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/limited-batch-of-new-s-video-adapters/)

Bryce.

Thanks for the offer Bryce but I have already bought all the parts. I have both the AD724 and AD722 so I was going to see which has a better picture. The AD722 has directly coupled inputs.

I cut a SOIC adaptor in half so that I can use standard 0.1" proto board.

Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 13:37, 23 February 14
Nice little hack there. I haven't tried the 722, but I doubt there's any difference in the picture quality. I'd add a jumper  and both crystals though, so that you can choose between PAL and NTSC. There might be a quality difference there.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 06:28, 10 March 14
Hi again,
             I received the new gate array and put that in and there is no difference. Still a dark screen with a white box on the screen. I have bought some socket strips to socket the ram which I might do soon. I tried the piggyback ram trick and it didn't help.

So it's time to order more parts by the looks of things.

I did try another Z80 but I don't know that it works so I would like to order another Z80 as well.

The original is marked Z8400APS and the one I tried was marked Z8400BPS and I just found a Sharp LH0080A which I might try as well.

I assume these are all NMOS Z80's (A = 4MHz, B=6MHz) and I am wondering if the newer Z80's will work.

The newer ones are marked Z84C00xxPEx, like Z84C0020PEC which is sold as a NMOS/CMOS chip, would one of these work?

Apart from that I don't really know which way to go. I was thinking to replace the Z80 and 6845. From memory there were different speed versions of the 6845 and some even had other differences. I don't relay think there is a problem with the 6845 as I have a stable raster.

I don't have a EPROM programmer any more otherwise I would burn another ROM. I do have some programmers but they're mostly for micro controllers, I will check if one of them will do old EPROM's.

How do I get the exact ROM image I need. Even if I can program a blank I have no UV eraser so I want to get it right first time. Would I need to do both ROM's?

Thanks all.

Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 16:00, 05 April 14
I have a similar problem with an amstrad cpc 6128, black screen, no beep sound pressing 'del'. According to service manual this means that problem is z80, gate array, crtc or ram chips. Fortunately i have a second amstrad cpc in perfect working order, so i managed to test cpu and gate array as these are on dip bases and cen be easily removed/plugged from their places. Result? Nothing, still no picture. So i will probably need to concentrate on crtc and ram chips.
I already ordered a 6845 crtc (i confirm that hey are still rather cheap and easily to find on ebay), and i was thinking of ordering these (http://www.ebay.co.uk/itm/10x-MCM4164BP15-4164-DRAM-IC-64k-x-1-/231185725337?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item35d3be3799) ram chips. I believe they are ok for cpc right?
I also ordered a cheap special "desoldering" gun (http://www.ebay.co.uk/itm/281157440634?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649) which i think it will render chip removal "piece of cake", without needing to destroy either the chip or the board!  ;)
Imo this would prove especially useful as usually you are not 100% that the chip you are going to remove is indeed faulty! For example, if the problem is a faulty ram chip, how you will find out which of the 16 ram chips has the problem?
Finally i would like to ask bryce, what exactly you will need to program the PAL16L8 (http://cpcwiki.eu/index.php/PAL16L8)?

Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 16:24, 05 April 14
Don't believe everything you read, those cheap desoldering guns aren't great. To properly desolder a chip without destroying it only requires a standard soldering iron and some wick. The most important part is that you limit and control the heat getting to the chip.

As for the RAMs, broken RAMs will usually get hot. Either way I doubt it's RAM, even with all RAMs broken you'd get more than a black screen, so the CRTC is most likely your problem.

GAL / PAL programming can be expensive. The device I use cost around €500, I'm not sure if there are many decent cheaper alternatives.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 16:40, 05 April 14
Thanks for your quick response bryce!  :)
So what you suggest is replace the 6845 (which i already ordered anyway) and if i still have problem, then go for ram right?
To tell you the truth,i already made the "classic" temperature "finger-sensing"  :) and i found out that all ram chips seem to heat the same (and that's why i ordered first the 6845...).
I had a small experience in the past, by discovering a faulty ram chip from an amiga 500, using this method, one of the 4 ram chips was heated more than the others 3,i changed it, and amiga was back to life again!  ;)

As for the desoldering gun, i dont know if you noticed, but this is not the usual cheap desoldering pump. It is actually an electric iron with a hollow tip in the end (i didn't even know that these kind of special iron exist!), which  gives you the unique ability to "wrap" the soldered pin,heated very fast and quick, and as soon as solder becomes liquid, you press the button and all solder is sucked away!This way you minimize the time of heating and also you will get a very "clean" hole! It's like having both soldering iron and a pump in one tool! 
At least this is how it should work, "in theory"...  ::)   
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 06:49, 06 April 14
Hi, ikonsgr

Are you getting a blank box on the screen where the text window is normally? What monitor are you using, there's more that can go wrong with the monitor? Do you see a clean 5Volts going to the CPC? What test equipment do you have?

I started removing the DRAM using a soldering iron and a solder sucker tool and I managed to lift a track so I put the whole thing aside and ordered DRAM and I will cut the pins on the installed DRAM and take them out one by one. I will be using machine pin sockets.

I got second hand DRAM from jameco.com

On this tread someone pointed me to a place to get the ASIC and I replaced that but it didn't help. At least I have a spare ASIC now as one day the will be impossible to get.

There are programmers for the HAL chip on ebay called 'universal programmer' but there is no code available to program into it. I take this to mean that the HAL doesn't fail. I will probably be buying a programmer anyway as it can do the ROM's. On this thread someone told me how to remove the HAL and test by using jumpers in the socket. You can jumper to select Bank1 or 2 of the DRAM. Because I replaced the HAL socket with machine pins, my cheep breadboard jumpers fitted perfectly in the socket. The HAL has only four registers and a 16 x 8 logic array, it can't be hard to write the VHDL for that.

I have yet to socket and replace the DRAM, after that I expect I will be ordering a 6845 as well.

I haven't done the research on the 6845 yet. There were different versions of this chip that had different speeds. I suspect the one in the CPC is the slower version and if that is true using a faster one would be fine. However the reverse is not true.

I have ordered and received 4164 DRAMs. The originals are 4264. The difference between the two is a page mode for burst reading but I don't think the CPC uses the page mode anyway. the 4164 / 4264 are 64K x 1. Don't buy 4464 as they're 64K x 4. The ones you linked to look fine. The originals are 150nS so they can be replaced with 120nS or 150nS but not 200nS.

There is another 'trick' for testing DRAM that works most of the time. Be warned though when it doesn't work you can damage the 'test' DRAM. If you bend the pins of a spare (test) DRAM in a little then it can piggy back onto the DRAM in the board. Place it on each DRAM in turn with power off and then try to boot. If it springs to life then the DRAM it is piggy backed onto is faulty. Of course this wont work if you have two or more faulty DRAMs in the lower bank.

Another thing I found on this site is that the power switch gets noisy. I have been using a jumper block on where the power switch normally connects.

When I get my CPC working (haven't had time lately) I will be making a SD memory interface to replace the FDD. I am also thinking of a way to get it to work with a VGA monitor.

Please keep in touch.
Title: Re: Need some help fixing CPC6128 not booting
Post by: gerald on 12:13, 06 April 14
There are programmers for the HAL chip on ebay called 'universal programmer' but there is no code available to program into it. I take this to mean that the HAL doesn't fail.
The PAL code is available on the wiki : Amstrad 6128 HAL/PAL16L8 chip (http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/amstrad-6128-halpal16l8-chip/)  or PAL16L8 - CPCWiki (http://cpcwiki.eu/index.php/PAL16L8)

I have yet to socket and replace the DRAM, after that I expect I will be ordering a 6845 as well.

I haven't done the research on the 6845 yet. There were different versions of this chip that had different speeds. I suspect the one in the CPC is the slower version and if that is true using a faster one would be fine. However the reverse is not true.
CPC use the lower speed : 1MHz.

I have ordered and received 4164 DRAMs. The originals are 4264. The difference between the two is a page mode for burst reading but I don't think the CPC uses the page mode anyway. the 4164 / 4264 are 64K x 1. Don't buy 4464 as they're 64K x 4. The ones you linked to look fine. The originals are 150nS so they can be replaced with 120nS or 150nS but not 200nS.
Amstrad used amongs others :
   micron MT4264-15, which support page mode
   mitsubishi NM4164-20, so I will say that 200ns are OK,

And indeed, the CPC use the page mode for fetching the video data !

Other RAM found on CPC range (non plus) are : OKI M3764-20RS, NEC D4164C-2, TI TMS4164-15NL, TMM4164P-3, KM4164B-15
Title: Re: Need some help fixing CPC6128 not booting
Post by: gerald on 14:09, 06 April 14
Mitsubishi reference is MN4164-20, not NM4164  ???
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 14:11, 06 April 14
There are programmers for the HAL chip on ebay called 'universal programmer' but there is no code available to program into it.

The code is available. It's here: Amstrad 6128 HAL/PAL16L8 chip (http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/amstrad-6128-halpal16l8-chip/msg71580/#msg71580) and I'Ve tested it and it works fine.

Bryce.

Edit: I see Gerald was faster :)
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 01:16, 07 April 14
The code is available. It's here: Amstrad 6128 HAL/PAL16L8 chip (http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/amstrad-6128-halpal16l8-chip/msg71580/#msg71580) and I'Ve tested it and it works fine.

Bryce.

I read the code but I don't recognise the language. I assume -
! = NOT
& = AND
# = OR

But what makes X1, X2, X3 registers?
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 10:30, 07 April 14
Explaining it all in a post is difficult. The language is called CUPL, you'll find loads of tutorials online.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 20:57, 25 April 14
Finally today the desoldering gun arrived along with the 6845 replacement!
I solder the new 6845 but unfortunately still, no picture at all  :(
Now, according to the service manual the "no picture at all" symptom can caused by faulty:

Z80
6845
40010
RAM
CRYSTAL

 CPU,6845 and 40010 i use are 100% ok (i know for sure because i have a second amstrad cpc where all these work ok) so next,is crystal. I put a new 16Mhz crystal but still same symptom. So the only thing left is ram!
The problem is that none of my ram chips seems to get hot, in fact one row doesn't get hot at all, and the other is heated very little!
Because ram change is a lot of work,and it would be very disappointed to have no success again, i was wondering if there is anything else on amstrad motherboard that could cause the "no picture at all" symptom. For example, could a  faulty 74lsxx chip cause this symptom?
And if i end up on replacing the ram chips, since i have no heat difference among chips meaning there is no way to know which are the faulty ones,i was wondering, if i replace the 1st bank only (i think it's the right row of chips closer to the 40010, as these seem to get hotter on both amstrad cpc's i have) would i get any picture even if there is a faulty ram chip on the second bank of 64k?

Title: Re: Need some help fixing CPC6128 not booting
Post by: TotO on 21:04, 25 April 14
I hope that you have put sockets for doing better tests without the need to unsolder each time, else you may damage the main board.
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 21:12, 25 April 14
Of course i will use dip sockets! I used a 40pin dip socket for replacing the 6845 too!
Btw, is there a chance that a faulty rom chip could cause the "no picture at all" symptom or not?
Title: Re: Need some help fixing CPC6128 not booting
Post by: TotO on 21:13, 25 April 14
As I know, a faulty ROM let the screen grey with a black border.
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 23:44, 25 April 14
I desolder the 1st ram bank, and now i'm waiting for the new ram chips...  ::)

Btw, the desoldering iron makes very good job, apart from rather fast and easy desoldering, i managed to remove the old 6845 and all of the 8 ram chips without breaking or cutting any legs,so i will be able to test some of the old ram chips to see if there are any good ones!   ;)
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 08:17, 26 April 14
Finally today the desoldering gun arrived along with the 6845 replacement!
I solder the new 6845 but unfortunately still, no picture at all  :(
Now, according to the service manual the "no picture at all" symptom can caused by faulty:

Z80
6845
40010
RAM
CRYSTAL

 CPU,6845 and 40010 i use are 100% ok (i know for sure because i have a second amstrad cpc where all these work ok) so next,is crystal. I put a new 16Mhz crystal but still same symptom. So the only thing left is ram!
The problem is that none of my ram chips seems to get hot, in fact one row doesn't get hot at all, and the other is heated very little!
Because ram change is a lot of work,and it would be very disappointed to have no success again, i was wondering if there is anything else on amstrad motherboard that could cause the "no picture at all" symptom. For example, could a  faulty 74lsxx chip cause this symptom?
And if i end up on replacing the ram chips, since i have no heat difference among chips meaning there is no way to know which are the faulty ones,i was wondering, if i replace the 1st bank only (i think it's the right row of chips closer to the 40010, as these seem to get hotter on both amstrad cpc's i have) would i get any picture even if there is a faulty ram chip on the second bank of 64k?

Hi, The diagnostic path (chip replacement) in the manual is selected to cover only the most probable faults. You say you have "no picture at all" ... if you have a box or window where the picture normally goes then the problem is most likely very different to problems when you have "no raster". Most of the chips "have to work" for there to be any sign of life so it is possible that a smaller chip is at fault (but this is not most likely).

If you have a stable box or windows on the screen then the crystal and oscillator chip are fine.

A faulty ROM can cause various faults depending on what is wrong with it. If you have a stuck address or data bit then you will get nothing! If it is just corrupted later on in the address space you will get the blank screen with the box as that is one of the first things the CPU sets up.

The RAM BANKS shares all signals except the data bus and /CAS so a stuck address bit on bank 2 can cause a fault. The CPC doesn't need bank 2 to run. On this thread someone gave instructions on how to remove the PAL(HAL) and use jumpers to disable bank 2 but remember a stuck address bit in bank two will still cause a fault.

It may be helpful for you to offer a clear and concise description of your fault. The very helpful people here cannot give advice that is any better in quality and accuracy than the quality and accuracy of the fault description. It may also be helpful to know what test equipment you have access to.

If you position the main board in front of you as it would be when using the CPC then bank 1 is on the right and bank 2 is on the left.

If you go as far as buying a programmer for the ROM then choose one that can also set the RAM if you want to test them.

Also, to others, can someone please confirm the BANK positions. I would hate to be the cause of someone pulling the wrong bank out.
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 13:39, 26 April 14
Thanks for your answer RobertM.
Regarding ram banks, according to service manual, ic's 127-134 are the 1st bank, and ic's 119-126 is the 2nd one (on both cpc's i have 1st bank is the one closer to the 40010, the right one as you see the mobo from the front).
But if ,as you say, any of the 16 ram chips is faulty i get no picture, changing only the 1st bank will not help much if i have faulty chips on both banks...

Now about my symptom, when i say "no picture at all" i mean LITERALLY NOTHING!  :)
There is no "window", no colour change on the screen, it's like having video cable disconnected!
As i have a second amstrad cpc ,which functions ok,i can compare the "thermal" behavior of various chips.
The only difference i found out is, that on good amstrad, ram chips are getting hotter compared to the ones on the faulty cpc which barely get warm (especially the 1st bank). What i also note is a difference in the buzzer noise when you turn up volume at max.Good cpc, gives a steady "buzz" while faulty one, has also a repeated "ticking" sound. And this noise remains exactly the same  when i removed the 1st ram bank.
I also make some tests removing 40010 or cpu and what i found out is that noise pattern is changing depending on what chip was removed!
It might be like a "sound diagnostic" where you can findout the faulty ic by hearing the noise pattern! Something like the error beep codes on a pc!  ;)


P.s. Since video ram is placed at address #C000 and it's 16kb long, it means thar all video ram is between byte 49152-65535. That means that all video ram is at the 1st bank.So, if i change all chips of 1st bank, then shouldn't i get something on screen (other than nothing at all....) , even if a ram chip of the 2nd bank is faulty?
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 20:22, 26 April 14
OK, The fail point is very early or immediately at power on. While RAM could be the problem I would forget about it for now.

My first suspect would be that there is a short or low resistance somewhere and the 'tick' is the result of the power supply in the monitor shutting down and restarting (what monitor are you using). My second suspect would be a resistance in the power supply line some where.

The first point I would test in this situation is the on board 5 Volt rail. If you have a multi meter then check that the 5 Volt rail is actually 5 Volts. First disconnect the 12 Volt line and Floppy Drive. The 12 Volt line is only used for the floppy. The specification is 5 Volts plus or minus 10 percent (4.5 - 5.5 Volts) however in practice it should be between 4.8 and 5.2 Volts.

Try to test the 5 Volts after the switch. There have been reported problems with the switch after aging. I take the switch connector off the main board and use a jumper block in the socket instead. The ground is easy to find on the board as it is the larger thicker areas that are most common. Most of the smaller chips have 5 Volts on one pin at one end of the chip and ground on one pin at the other end and other side. The circuit diagram will help and look up the pinout of any of the smaller chips. Google the number with 'IC' after the number and select images.

https://www.google.com.au/search?q=7400+ic&source=lnms&tbm=isch&sa=X&ei=TtRbU6C6OM2kkQWX6IDYCg&sqi=2&ved=0CAYQ_AUoAQ&biw=1024&bih=616 (https://www.google.com.au/search?q=7400+ic&source=lnms&tbm=isch&sa=X&ei=TtRbU6C6OM2kkQWX6IDYCg&sqi=2&ved=0CAYQ_AUoAQ&biw=1024&bih=616)

All the smaller IC's starting with 74xx have Vcc and ground like this. Vcc is 5 Volts.

If you don't have a multi meter then plug in the little power light making sure none of the circuit on the back of the little board it is on touches anything conductive or the main board. While listening to the 'tick' sound see if the LED light dims briefly or flashes briefly when you hear the 'tick'. If it dims when you hear the 'tick' then disconnect the main board from the monitor as it may case damage to the PSU power supply in the monitor.

How often (fast) is the tick sound and is it consistent or random?

Test equipment that would help from cheapest upwards -

Digital MultiMeter (ebay $5) (http://www.ebay.com.au/itm/New-LCD-Digital-Multimeter-DT830B-not-include-battery-/271289420839?pt=AU_Gadgets&hash=item3f2a1c0c27)

Logic Tester (ebay $11) (http://www.ebay.com.au/itm/New-LP-1-DTL-TTL-Digital-Logic-Probe-Circuit-Tester-Pen-DC-18V-Max-/150872984621?pt=AU_Electronics_Vintage_Electronics&hash=item2320bb0c2d)

If you have the money then -
DSO (ebay $66) (http://www.ebay.com.au/itm/Mini-SainSmart-ARM-DSO201-Nano-Pocket-Sized-Digital-Storage-Oscilloscope-Kit-/191092059611?pt=AU_B_I_Electrical_Test_Equipment&hash=item2c7df96ddb)

I am NOT recommending the sellers above. The links are just for example pictures.
If you buy a DSO then you don't need the Multimeter of Logic Tester. The DSO I linked to is the bottom of the range and it's usefulness is very limited as a DSO when working on computer board clocked at 4 Mhz. You ideally would want a DSO of 10 Mhz or 20 Ms/s or better. Let me know if you want a DSO and I will look for something suitable. I don't have one unfortunately.

If you get a logic tester then this is the diagnostic path after verifying that you have a stable 5 volt supply.

Test that /RESET on the CPU pin 26 is close to 5 Volts (high and not pulsing). If not replace R101, R111, R145, IC117, IC110 in that order testing as you go so that you don't replace more than needed. or ask for mor detailed info.

Test that you have a constant pulse signal on the CLOCK pin 6 of the CPU. If not test IC117 pin 4. If not pulsing then change IC 117. IC 117 is 74HCU04 and needs to be that so don't use the more common 74HC04.
By 'constant pulse signal' I mean that the 'PULSE' light stays on.

Test that you have a constant pulse signal on the /INT pin of the CPU. If so then CPU and gate array and all associated addressing (glue logic IC's) should be good and most probably the 6845 is ok as well.

Use the working board to compare voltages on CPU /RFESH /M1 /INT /MREQ /NMI /IORQ /BUSRQ, /RESET /READ /WRITE

With a Logic tester they should be -
/RFSH (28) constant pulse.
/M1 (27) constant pulse.
/INT (16) constant pulse.
/MREQ (19) constant pulse.
/NMI (17) permanently high.
/IORQ (20) constant pulse. I should verify this.
/BUSRQ (25) permanently high.
/RESET (26) permanently high.
CLOCK (6) constant pulse. Shown as the slashed 0.
/RD (21) constant pulse.
/WR (22) constant pulse.
/WAIT (24) permanently high.
/BUSRQ (25) permanently high.
/BUSAK (23) permanently high.
/HALT (18) permanently high.

Other places -
Any (only test one per bank) /RAS (pin 4)BANK 1 and BANK 2 constant pulse.
Any (only test one) /CAS (pin 15) BANK 1 constant pulse (CAS0).
Any (only test one) /CAS (pin 15) BANK 2 permanently high (CAS1). I should verify this. Not sure if the refresh uses CAS or RAS.
All DO pins (one per chip pin 2) on BANK 1 constant pulse.
All DI pins (one per chip pin 14) on BANK 1 constant pulse.
All Address pins in BANK 1 (pins 5, 6, 7, 9, 10, 11, 12, 13) constant pulse.
If you have a DI data pin that is not pulsing then that RAM chip or the CPU is faulty.
If you have a DO that is not pulsing then that RAM chip or IC115 or the gate array are faulty.
If you have an address pin that is not pulsing then test two pins on the CPU if it is A0 then test A0 and A8 if it is A6 then test A6 and A14 etc on the CPU. If pulsing on the CPU then several chips could cause this.

Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 20:44, 26 April 14
If the clicking is getting slower when you remove the larger gate array or CPU then that greatly increases the chance that it is a power problem. It could even be the monitor at fault especially if the two CPC are slightly different models.

The location and BANK of the video RAM is determined by the CPU programming the smaller logic array IC118.

With the RAM BANK 1 removed, I suspect,  you should still get the box on the screen unless there is a stuck data or address line in BANK 2. With all RAM (both banks) removed then I expect you would get the box on the screen.


Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 23:44, 26 April 14
First, thanks for you answers and concern mate!  :)
Second,i'm not using an amstrad monitor i use a 5volt power supply and a scart cable to a tv.
Third, i have many multimeters and i've already checked supply pins on many ic's  on the board (z80,6845,40010,ram chips, and some 74xx) and all seem to have the right voltage.
Fourth, the ticking sound is continuously  with a period of , i say, about 4-5ticks/second. When the 40010 is removed no ticking sound is heared only a low noise "whistle".

A digital oscilloscope would be a marvelous tool indeed, but at the moment i can't afford for something like that. The logic tester on the other side, seems more interesting, and if i could make all those checks you mention, then i might get one!
Now, since i already removed the 1st bank of ram chips and ordered new ones,i will wait until i get them, and if i still have problem then i might get a logic tester to check all the signals you mention.

But i was wondering, since i don't have a power supply problem and all the basic ic's are 100% ok  (z80, 6845,40010 are all tested an working on my other cpc) is there any chance to have problems to any of the signals you mention?

p.s. i just ordered this (http://www.ebay.com/itm/EISTAR-Digital-Logic-Probe-DTL-TTL-Circuit-Tester-Pen-DC-18V-Max-ge-/261225928344?pt=LH_DefaultDomain_0&hash=item3cd2475698) logic tester for only 6euros! So now i will be ready to make a full "check up" of my cpc! ;)
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 23:59, 26 April 14
Amstrad monitors use a Sanyo L78MG to regulate the 5V supply. This chip has internal thermal shutdown and over-current protection. If the current is too high or it gets too hot it shuts down completely. They are very robust and survive even a complete short circuit. If there was a short or very low resistance on the CPCs 5V rail the regulator would shut down almost immediately and won't go back on as long as the short remains. So there wouldn't be any ticking and the LED would never light.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 02:02, 27 April 14
Hi,
    That's the same Logic Tester I bought for this. It's cheap but not great. I also just last night ordered the desolder tool that you bought.

With my last post I was focussed on what happens before you get a box on the screen.

The first thing that needs to happen is that the Z80 has to read and run code from the ROM. For this it needs a clean data and address bus and the right control signal inputs so there is the place to start testing. It will run code from the ROM even with bad RAM and get to the point of showing a box on the screen if enough of the other parts are working.

If you have the correct control signals to the CPU and the buses are OK and you can see activity on all the address and data pins then then you can move on to other testing. If however one CPU pin is not as expected then which pin that may be is a big clue as to where the problem is. If it IS running code from the ROM and the other major chips are working then the best bet is to change the ROM but first check the control signals to the ROM. IF the ROM control is wrong then it's the GLUE logic (smaller chips and small HAL) at fault. If the ROM control signals are right then it's most probably the ROM.

The fault you have is more difficult than the one I have. I haven't had time for it lately but my case is very likely to be RAM or ROM. In my case I am getting a box on the screen but the /INT signal at the CPU is totally wrong.
I have replaced CPU and the larger gate array. Getting the box on the screen means that the CPU is reading and running code from the ROM. The fact that the box is blank and not a garbled random mess of pixels indicates the CPU is also writing zero's to the Video RAM. The box also means that the larger gate array is working and that the Z80 is also able to set the registers in the 6845 so the 6845 is probably OK to.

After the CPU gets the blank box on the screen it then has to set up jump tables in RAM. Before it does this it is NOT dependant on RAM providing the RAM is not interfering with the buses to the CPU. Once the tables are set up it needs the RAM and address glue logic to work (some smaller chips and the HAL). This is why I don't think the fault in your case is RAM unless it is interfering with the buses to the CPU.

I will be removing all the RAM on my CPC. I expect I will still get a blank box on the screen with no RAM installed. I will test that and let you know. After that I will install BANK 1 only and see if it then works. If it doesn't then my next step is the ROM. I am also replacing the 6845 and CPU even though I don't think that is the problem. I have only changed the larger gate array with a new part. I used an old Z80 CPU. I have a new 6845 and Z80 on order. The Z80 I ordered is one of the modern 10Mhz versions. The old Z80 in there seems to be working so I wanted to verify that the newer Z80's will work in this board.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 02:12, 27 April 14
Hi Bryce,
            Thanks once again for your help. The ticking sound has got me confused lol. I thought it may be transitions on the power rail getting through the audio amp. It may however be the CPU initializing the Audio section in which case the CPU would be looping at around 4 Hz. 4 Hz is an unusual frequency for a CPU clocked at 4 MHz and having a 16 bit address space but still possible.
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 21:14, 27 April 14
I just finished testing  signals with a voltmeter of ram chips of bank 2 on faulty amstrad cpc.
The ras/cas signals of ram chips seem to be at the same level as with the working cpc (most probable this means that 6845+40010 are ok which i already know anyway  ::) )
All Dout are constantly at ~5volts, and Din are at 2-2.5 volts except one that gives 0.6volt and another that gives 3.7volts!
On the good amstrad cpc, all Din (of all 16 ram chips) are giving ~1.7volt and Dout ~0.2-0.3volt
I also check all the cpu signals and seem to be ok.
Of course i have removed the ram chips of bank 1,so i don't know how this can affect the signals (although Din and Dout are separated).

Next, i'm going to solder dip cases and put back the old ram chips to see what measurements i will take.


Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 03:57, 28 April 14
All DO's of bank 1 are directly connected to the co-responding DO of bank 2, they are NOT separate between banks but also are NOT directly connected to the CPU data bus. Instead they are buffered by IC114.

The same is NOT true of the DI's. Not only are all the DI's in bank 1 connected to the co-responding DI in bank 2, they ARE also DIRECTLY connected to the Data bus D0 - D7 of the CPU.

So if you have a DI that is stuck as 1(5V) and a DI that is stuck as 0(0V) then the co-responding Dx's of the CPU will also show the same voltage or logic level.

This means that the CPU is NOT able to run code from the ROM. On the CPU the D0-D7 are both output and input.

Test the co-responding Dx's directly on the CPU you should see the same voltages. If you do then remove the socketed chips (including the CPU) one by one testing to see if the problem goes away.

Do the following tests without power. Test the resistance from 5V and ground to some of the 'working' Dx's on the CPU socket as a reference. Then test the suspect Dx's on the CPU socket with all the chips in place. Then repeat removing the socketed chips to see if it goes away.

If it doesn't go away then you are on your way to finding the problem as you have identified the problem and now just need to locate it. That is assuming the resistance of one pin is greatly different to the reference.

If the resistance is very low then you have a short. The short may me in a chip. Unfortunately the data bus goes practically everywhere.

If you have a short it is hard to find as you have to move around the board disconnecting pins as you go. The resistance of the traces on the board is very low so testing from close to the short will give the same result as testing far from the short with a standard multimeter.

If you are looking for a short then the best solution is to use an ESR meter (Equivalent Series Resistance) or low ohms meter. ESR meters are also excellent for testing capacitors in circuit. You will find ESR meters on ebay.

I am probably jumping the gun a bit with the voltage testing as you WILL see different results with the RAM bank removed. However the results you mentioned definitely do NOT look healthy. A logic tester is better for this as it shows transitions. If there are no transitions then you definitely have a fault. The problem is that although the start code of the ROM will still give predictable and similar results to the working CPC for the start period, the start period is likely to be far less than a second after which the results are not predictable for me.

However the resistance tests should still be valid. I am probably writing ahead of where you are as there is a delay between our posts. I am at GMT +10:00.

I hope this helps, and it looks like you are on you way to fixing this definitively now.

I would suggest doing the resistance test before anything else as a low or very low resistance will give you a definitive "go, no go" test.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 04:08, 28 April 14
I just had another look at the ESR Meters on ebay. Some of them have a resolution of 0.5 OHM.

The one I have used for finding shorts in circuit boards has a resolution of 0.01 OHM or 50 times the resolution.

I don't think a resolution of 0.5 OHM will cut it for you. You will need better than that.

I will wait to hear what resistances you see.

Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 10:21, 28 April 14
I just finished testing  signals with a voltmeter of ram chips of bank 2 on faulty amstrad cpc.
The ras/cas signals of ram chips seem to be at the same level as with the working cpc (most probable this means that 6845+40010 are ok which i already know anyway  ::) )

Measuring RAS/ and CAS or any other signals in a CPC with a multimeter is pointless, because they are (or should be) switching on and off quicker than your Multimeter can display.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: Munchausen on 10:57, 28 April 14
Test equipment that would help from cheapest upwards -

Digital MultiMeter (ebay $5) (http://www.ebay.com.au/itm/New-LCD-Digital-Multimeter-DT830B-not-include-battery-/271289420839?pt=AU_Gadgets&hash=item3f2a1c0c27)

Logic Tester (ebay $11) (http://www.ebay.com.au/itm/New-LP-1-DTL-TTL-Digital-Logic-Probe-Circuit-Tester-Pen-DC-18V-Max-/150872984621?pt=AU_Electronics_Vintage_Electronics&hash=item2320bb0c2d)

If you have the money then -
DSO (ebay $66) (http://www.ebay.com.au/itm/Mini-SainSmart-ARM-DSO201-Nano-Pocket-Sized-Digital-Storage-Oscilloscope-Kit-/191092059611?pt=AU_B_I_Electrical_Test_Equipment&hash=item2c7df96ddb)


A bit more expensive (~£100) but for a scope I got a hantek USB one recently. I've had a really good experience with it, 40MHz dual channel so will do you for most things, there's software for linux available and it comes with a pretty good pair of probes. It isa bit more expensive but worth it if you want to use it for more things and especially if you need something better than a few MHz for a low price (a stand-alone DSO with the same spec would cost at least 3 times the price): Hantek DSO2090 PC Based USB Digital Oscilloscope 100M/s 40MHz Bandwidth 2CH (http://www.ebay.co.uk/itm/310475526648)


And you can get a 24MHz logic analyser very cheap too (£10). Uses the seleae logic software: Hobby Components USB 24M 8CH 24MHz Logic Analyser, 1.1.16 | eBay (http://www.ebay.co.uk/itm/130993520591)
Test clips (£5): Test Hook Clips Ideal for Logic Analyser 10-way in Yellow | eBay (http://www.ebay.co.uk/itm/141039116605)


For a multimeter I have a low end fluke, but I guess a cheap meter is probably good enough for most things (mostly continuity testing you'll need I think).


Good luck with your repairs

Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 13:09, 28 April 14
@robertm : thanks for your analysis and suggestions. I will make the resistance measurements to all ram chips to see if i can locate the "culprit"!  :)
@bryce: yes multimeter can only sample about 3-4 values/sec, BUT what you can actually see is the rms (or something like "average") value of the pulsing signal! For instance,i measure pin 6 of cpu to be ~2.5volt and that's because pulses are at 4Mhz beteween 0-5volt and with ~50% duty cycle, which gives an "average" value of ~2.5volt. So, in most cases if i get a value more than 0volts and less than 5volts this means that the particular pin is pulsing and you will get an rms value according to duty cycle of the pulse!  ;) .Ofcourse by using a multimeter you can't distinguish if you have a truly pulsed signal or just a constant voltage, and that's where a logic tester is useful to!  :)
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 13:43, 28 April 14
That's fine for a clock signal where the duty factor is 50%, but a CAS signal has pulse of about 25ns per clock cycle, that gives a duty factor of about 97%, so assuming the positive level is exactly 5V (which it rarely is) a multimeter would read about 4.88V - 4.9V, which could be a pulse, could be a stable positive level or maybe just the accuracy of the multimeter. So you've no idea what's really happening. Accurate measurement is crucial when trying to repair a computer, otherwise you are just stumbling in the dark and guessing.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 18:33, 28 April 14
I think i made a great discovery! ALL Dout on the good amstrad cpc has only ~25kΩ resistance, while ALL Dout on the faulty one has ~0.4MΩ(resistance i measure between 0v and Dout  on all ram chips i unsoldered is ~2MΩ)!
Mind also that  i removed the 40010 as Dout's has direct connection there, so each Dout is shared between the 2 ram chips in the same row (from bank 1 and bank2),and  the 2ic's (74ls244 and 74ls373) (i check this with the multimeter)
I believe that ram chips are ok (at least none of the 8 chips i unsolder, had a short circuit between Vcc and 0V pins and none was getting hot either) and as i happen to have another 74ls244, i "piggy bag" it above the one on the motherboard, but resistance of all Dout remain the same! So would it be safe to assume that the 74LS373 (the octal latch) is the TRUE culprit?  :)
 
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 00:08, 29 April 14
Is there a short or very low resistance between Dout and GND on the RAM ICs? That's where the short would be, not between Vcc and GND.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 00:15, 29 April 14
Is there a short or very low resistance between Dout and GND on the RAM ICs? That's where the short would be, not between Vcc and GND.

Bryce.

On the contrary, there is a rather big resistance (~400KΩ) between Dout and ground, while on the working cpc resistance of Dout-grd is ~25KΩ!
You think that this indicates a problem with all ram chips(as all ram chips have the same measurement), or maybe the problem is with the octal latch?
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 00:21, 29 April 14
No, it doesn't really point directly to anything. Comparing the ICs resistance between a loose part and one in a circuit doesn't really tell you anything unless it really is a pure short circuit. But now that they are out, replace them with known good ones and you've removed another factor from the equation.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 00:32, 29 April 14
The Dout resistance i mention in both cases (fault and working cpc) are with the ram chips ON BOARD not "in the air". The Dout of each pair of ram chips (bank 1 and bank 2) are directly  connected to specific pins of the 74LS244, 74LS373 and also the 40010 (which in both cases was removed from board).
So,when i measure 25KΩ on all Douts of the working cpc and ~400KΩ on the faulty one, doesn't this mean that the problem would be either with ram chips and/or the 2 74LS ic's?
But, since none of the ram chips of the faulty cpc has short circuit (neither got very hot which i think it's a typical symptom of a bad ram chip) will it be more probable the problem lies in one of the 2 74LXXX ic's?

Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 00:49, 29 April 14
Ah ok, I thought you'd removed the RAM ICs. In that case I'd try swapping the logic first.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 01:04, 29 April 14
Well,i remove the 1st bank indeed, but i have installed dip sockets and put them back again!  :)
Tomorrow i will go buy a nice 74HC373 (better cmos for lower consumption, after all even in the schematics of the service manual they propose 74LS or 74HC anyway  ;) ),i already have some 20pin dip sockets and a 74HC244 and.... let the desoldering begin (once again...)!  :D
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 12:35, 29 April 14
I would be careful substituting different spec chips at this time as you may introduce more variables into the equation while you still in diagnosis stage.

While chip manufacture has come a long way since the CPC's were made, the manufacturers still target the same specification as the old originals.

No only does CMOS have different Vil and Vih (Without looking it up) the fanout (how many chips it can drive) is different from my memory.

If your socketing then you can always change to CMOS later, once the fault is found.

CMOS is most probably fine but if it isn't then your in for a long journey to find out.

Also I did notice a jumper on the main board that had CMOS written beside it but I havent looked at the circuit diagram to see what it is for.

Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 14:01, 06 May 14
If you have the money then -
DSO (ebay $66) (http://www.ebay.com.au/itm/Mini-SainSmart-ARM-DSO201-Nano-Pocket-Sized-Digital-Storage-Oscilloscope-Kit-/191092059611?pt=AU_B_I_Electrical_Test_Equipment&hash=item2c7df96ddb)

By pure coincidence I had the chance to try one of these out at the weekend. I would definitely NOT recommend getting one of these, not even for just checking if a signal is present. It's a toy, not a meter. It's meant to be a 1Mhz device, but the results on signals above 100Khz were completely wrong and pretty random too, so it would be useless for even retro electronics. Connected to a 1Mhz square wave displayed a straight line suggesting that no signal was present. In reality the clock signal was there and running fine. It only displayed a halfway realistic result when I turned the clock down to about 70Khz!!! And even then the compensation was completely off.
There is a more expensive 12Mhz (some sites claim this to be 36 or even 72Mhz) version called the DSO203 which might be better suited, but at €135 it's too expensive. For €110 you can already get a "real" 70Mhz USB oscilloscope which will serve you much better.

The only thing worse than having no test equipment, is having test equipment that you can't trust.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 20:58, 06 May 14
[UPDATE]
Finally the ram chips arrived and so i installed all the first bank with brand new chips (the packet was 10 items), remove the second bank completely and also change the 74ls244 and 74ls373 with new ones. So EVERYTHING in the path of the Dout signal of all ram chips is replaced!
Unfortunately the result is the same: completely nothing on the screen. Although now the ram chips seem to get warm like in the working cpc (the old ones barely got warm)
So now my question is: If you remove the 2nd bank of a working cpc 6128, can it still works, or at least give some video signal (a window or something) other than nothing at all? Because it would be really frustrate to order another pack of ram chips to fill in the 2nd bank, and still get the same problem. And in that case 99% chances mean that the problem is a faulty rom chip....  :(
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 21:00, 06 May 14
Are you sure the Gate Array works? Sounds like it's gone.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 22:04, 06 May 14
Yeap, because i have a second cpc which works perfect, and i'm using the 40010 from there!  ;)
Title: Re: Need some help fixing CPC6128 not booting
Post by: gerald on 22:07, 06 May 14
So now my question is: If you remove the 2nd bank of a working cpc 6128, can it still works, or at least give some video signal (a window or something) other than nothing at all? Because it would be really frustrate to order another pack of ram chips to fill in the 2nd bank, and still get the same problem. And in that case 99% chances mean that the problem is a faulty rom chip....  :(
A 6128 will work without the second bank. SW trying to use it will obviously fail, but basic and FW does not use it at all.

The screen initialisation is done in the very beginning of the ROM code, even before using any RAM. This should give you a proper screen with border.
If you are sure the CRTC/Z80/GA are OK, the ROM may be your problem. However, it can also be an address/data/ctrl bus problem or ROMDIS forced high (no access to ROM), or WAITn forced low (Z80 waiting forever) .....
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 22:18, 06 May 14
And what faulty chips on motherboard might cause any of these you mention (address/data/ctrl bus problem or ROMDIS forced high (no access to ROM), or WAITn forced low (Z80 waiting forever)?
Up until now i have replaced almost any major chip (Z80,6845,40010,HAL16L8,74LS244,74LS373,1st bank ram) on motherboard, so is there a chance that some of the remaining chips (for example a faulty 74lsxxx) which i haven't replaced, could cause a so serious problem?
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 22:35, 06 May 14
Have you checked the resistance of each Address and Data line to Vpp? Maybe a pull-up resistor has failed?

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: gerald on 22:40, 06 May 14
Just trying to list what can conflict with rom fetch :

ROMDIS is pulled down by a resistor and connected to pin 22 (CE\) of the ROM. It is driven high by external ROM management or internally (on 6128) by the AMSDOS rom management. Check that the voltage on pin 22 is 0v. If not, the amsdos ROM management may be the problem.

WAITn It is driven by the GA through a resistor with a 3/4 ratio (3 cycle low, 1 cycle high). You should measure about 1V with a multimeter on this pin. If you find 0V, the resistor may be broken.

Regarding the data bus, faultly PPI (8255) and the printer buffer can be (unlikely) candidates. AMSDOS ROM also.

However, checking that the ROM is OK is a priority.
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 22:51, 06 May 14
Just trying to list what can conflict with rom fetch :

ROMDIS is pulled down by a resistor and connected to pin 22 (CE\) of the ROM. It is driven high by external ROM management or internally (on 6128) by the AMSDOS rom management. Check that the voltage on pin 22 is 0v. If not, the amsdos ROM management may be the problem.

WAITn It is driven by the GA through a resistor with a 3/4 ratio (3 cycle low, 1 cycle high). You should measure about 1V with a multimeter on this pin. If you find 0V, the resistor may be broken.

Regarding the data bus, faultly PPI (8255) and the printer buffer can be (unlikely) candidates. AMSDOS ROM also.

However, checking that the ROM is OK is a priority.

That's exactly the direction that I was thinking towards. Some signal is being held high / low somewhere, but that means it could be almost anything on the PCB :(

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: gerald on 23:16, 06 May 14
That's exactly the direction that I was thinking towards. Some signal is being held high / low somewhere, but that means it could be almost anything on the PCB :(

Bryce.
Yes, and that's why the ROM need to be checked first after the romdis/waitn.
If the ROM is OK, then oscilloscope/LA becomes a mandatory hunting tools ! :(
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 01:28, 07 May 14
Thanks for the info guys. I'll check it tomorrow and let you know!
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 19:51, 07 May 14
Ok, 'ive checked the 2 signals you said, romdis on pin 22 of the amsdos/basic rom and "wait" on pin 24 of the cpu.
On faulty amstrad i get ~1,5mV and 1.2Volts respectively, while on the working one i get ~0.5mV and 1,15Volts.
So you think ROM is ok?
I also try the working amstrad without the pal16l8 and it gives a grey box. The faulty one doesn't give anything on the screen, but it has only the 1st bank installed (2nd bank is completely removed)
Finally i checked almost all resistors on the board and they seem to match the values with resistors on the working cpc.
You think it's worth buying another pack of ram chips for the 2nd bank, or most probable the problem is elsewhere?


P.s i just noticed another strange thing: when i get my hand close to the ic 203 (a 74hc240 on the top right corner of the board above the 765 controller i think it's part of the floppy disk circuit) i get sound distortion from loudspeaker! Of course this is not happening on the working cpc. Does this mean this chip is faulty, and if it is, can it cause such a serious problem?

Title: Re: Need some help fixing CPC6128 not booting
Post by: gerald on 20:54, 07 May 14
Ok, 'ive checked the 2 signals you said, romdis on pin 22 of the amsdos/basic rom and "wait" on pin 24 of the cpu.
On faulty amstrad i get ~1,5mV and 1.2Volts respectively, while on the working one i get ~0.5mV and 1,15Volts.
So you think ROM is ok?
These two test result are expected. This mean that
1. The ROM is not disabled
2. The Z80 is not halted

For the ROM itself, you cannot say that it is working or not. But, if there are no problem on the memory bus (address/data/control), the ROM is likely failing.

I also try the working amstrad without the pal16l8 and it gives a grey box. The faulty one doesn't give anything on the screen, but it has only the 1st bank installed (2nd bank is completely removed)
Removing the PAL will have the same effect as a faultly RAM. Since you do not have anything on screen, you should forget about the RAM for now  ;)

P.s i just noticed another strange thing: when i get my hand close to the ic 203 (a 74hc240 on the top right corner of the board above the 765 controller i think it's part of the floppy disk circuit) i get sound distortion from loudspeaker! Of course this is not happening on the working cpc. Does this mean this chip is faulty, and if it is, can it cause such a serious problem?
This chip is buffering signals from the floppy drive and is also part of the FDC address decoding. If the decoding goes wrong, the FDC may drive the bus.
Can you check that the CS pin stays high (IC201 pin 4), as well as the AMSDOS ROM is not selected (IC204 pin 22). Both pins voltage should be close to 5V.

Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 23:23, 07 May 14
Yeap, both pins got ~4.5volts just like on the working amstrad too.
Btw, these "parados" roms you can find everywhere on ebay,is a substitute for the ic 204 (the fdd rom)?
And if i do have a rom problem, which of the 2 roms can cause this "nothing on screen" symptom?
Title: Re: Need some help fixing CPC6128 not booting
Post by: TFM on 23:56, 07 May 14
If you got no sign on message then either BASIC or the Firmware has a problem... Or the GA.
Title: Re: Need some help fixing CPC6128 not booting
Post by: gerald on 12:33, 08 May 14
Yeap, both pins got ~4.5volts just like on the working amstrad too.
Btw, these "parados" roms you can find everywhere on ebay,is a substitute for the ic 204 (the fdd rom)?
Parados is a AMSDOS replacement ROM, it can replace IC204.

And if i do have a rom problem, which of the 2 roms can cause this "nothing on screen" symptom?
The FW/BASIC ROM is IC103, and is the one responsible for setting up the hardware at power up.
Ideally, you should remove it from the board and verify it with an eprom programmer (comparing it to a valid rom lower rom and basic rom)
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 21:32, 08 May 14
That would be indeed ideally, IF i had an eprom programmer!  :)
For now,i'm waiting for the logic tester and i was thinking, if i will make a "logic" map of every pin (either low,high or pulsed) of each rom on the working cpc, and then make the same thing with the faulty one and compare them, will i be able to know for "sure" from the result,if rom chips are ok (in case of complete concurrence between the roms of the working and the faulty cpc), or NOT ok (in opposite case)?

Finally,if the basic/fw rom chip is indeed faulty, is there any way to find a replacement?
And if not, are  this (http://www.ebay.ie/itm/ATMEL-ATMEGA168-20PU-Ic-8Bit-16K-Flash-Mcu-Dip28-168-/360923056048?pt=UK_Computing_Other_Computing_Networking&hash=item5408b0c7b0) the needed eeprom to be programmed (i think there is a list of all roms somewhere in cpc wiki...  ::) )?
Title: Re: Need some help fixing CPC6128 not booting
Post by: gerald on 22:30, 08 May 14
For now,i'm waiting for the logic tester and i was thinking, if i will make a "logic" map of every pin (either low,high or pulsed) of each rom on the working cpc, and then make the same thing with the faulty one and compare them, will i be able to know for "sure" from the result,if rom chips are ok (in case of complete concurrence between the roms of the working and the faulty cpc), or NOT ok (in opposite case)?
Unless you get some static data bit, you cannot conclude as you don't know what the Z80 is doing.
You could check the rom by testing it with a working 6128. But you risk to damage that working CPC as you need to unsolder its ROM (and add a socket).

Finally,if the basic/fw rom chip is indeed faulty, is there any way to find a replacement?
You need a 27C256 (32kx8) eeprom. The lower 16k need to be programmed with the 6128 FW, the upper with the basic 1.1

And if not, are  this (http://www.ebay.ie/itm/ATMEL-ATMEGA168-20PU-Ic-8Bit-16K-Flash-Mcu-Dip28-168-/360923056048?pt=UK_Computing_Other_Computing_Networking&hash=item5408b0c7b0) the needed eeprom to be programmed (i think there is a list of all roms somewhere in cpc wiki...  ::) )?
No, this is a microcontroller. :o
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 22:43, 08 May 14
Unless you get some static data bit, you cannot conclude as you don't know what the Z80 is doing.
You could check the rom by testing it with a working 6128. But you risk to damage that working CPC as you need to unsolder its ROM (and add a socket).
And that's exactly why i'm NOT going to do this, because the other cpc is the only working cpc i have!  :)

You need a 27C256 (32kx8) eeprom. The lower 16k need to be programmed with the 6128 FW, the upper with the basic 1.1
Then, you mean this (http://www.ebay.co.uk/itm/5pcs-M27C256B-10F1-EPROM-IC-M27C256B-ST-DIP-32-AR-/390745247340?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item5afa3b4e6c)?
Title: Re: Need some help fixing CPC6128 not booting
Post by: gerald on 22:46, 08 May 14
And that's exactly why i'm NOT going to do this, because the other cpc is the only working cpc i have!  :)
Then, you mean this (http://www.ebay.co.uk/itm/5pcs-M27C256B-10F1-EPROM-IC-M27C256B-ST-DIP-32-AR-/390745247340?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item5afa3b4e6c)?
Yes !
But you need to get them programmed.
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 22:52, 08 May 14
I found another (http://www.ebay.com/itm/1PCS-IC-M27C256B-12F1-27C256-CDIP-28-ST-NEW-GOOD-QUALITY-DATE-CODE-11-/300743678579?pt=LH_DefaultDomain_0&hash=item4605b85a73) for only 2$/piece, i suppose i can use these instead?
And i also found these (http://www.ebay.com/itm/2-pieces-27C256T-15-256K-32K-x-8-CMOS-EPROM-27C256-/120908064733?pt=LH_DefaultDomain_0&hash=item1c26aea7dd) which they don't seem to have the window for UV erase.
I have a friend of mine which has a couple of programmers but i don't know if it can program the specific one.
 Is this a rather "rare" type of eeprom which needs expensive programmer or not?

Title: Re: Need some help fixing CPC6128 not booting
Post by: gerald on 23:11, 08 May 14
The 2$ one are OK, the second link point to SMD device that you cannot use on the CPC without an adaptor.
27C256 are really standard eprom. Any eprom programmer should be able to handle them.

Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 23:12, 08 May 14
Ignoring the fact that the second ones are SMD parts (so they wouldn't match the PCB anyway), they seem to be OTP (One Time Programmable) parts. I think the guy is wrongly describing them as EPROMs, they're PROMs as far as I can see. Not really what you want.

Even the cheapest and crapest of programmers can program a 27C256, I'm sure that your mates programmer will be able to, no matter what he has.

Bryce.

Edit: Oh, gerald got there before me :)
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 23:21, 08 May 14
Thanks a lot for answering my "stupid" questions my friends!  :)
So basically, a faulty rom it's not such a "big deal" after all....
And if i may ask, what is the cost of  a "crappy" programmer for these eeproms?
Because if they are really cheap, then i might actually get one and do the programming my self!  ;)
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 23:37, 08 May 14
For €27 you can get a Chinese Willem Clone, which will do quite a bit, but it's a bare PCB: EPROM BIOS Programmer/Programmierer PCB5.0E Neu | eBay (http://www.ebay.de/itm/EPROM-BIOS-Programmer-Programmierer-PCB5-0E-Neu-/260997670871?pt=Bauteile&hash=item3cc4ac67d7)

But I'd recommend spending a few euros more (€39) for one of the better Chinese devices such as this: TOP853 USB Universal Programmer Programmierer für EPROM FLASH MCU GAL PIC (http://www.ebay.de/itm/TOP853-USB-Universal-Programmer-Programmierer-fur-EPROM-FLASH-MCU-GAL-PIC-TOP853-/221377051850?pt=Wissenschaftliche_Ger%C3%A4te&hash=item338b19bcca)

The more expensive one has a case, but much more importantly, it auto-detects the inserted IC, so there's no endless dip switch setting involved. It can also program a few other things such as the GAL that's used in the CPC :) and PIC and AVR micro controllers.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 23:48, 08 May 14
You mean the GAL16L8 or the 40010 GAL? Although i don't know if the program needed for the 40010 is available  :)
Well for just programming 2 roms,i think it's a little "too much"....  ::)
Btw, why i need a 27c256 rom and not 27c128, since both roms of a cpc are 16kb so a 16Kx8=128kbit rom could do the job just fine, right?
Although, as i see on ebay, there is no difference in actual cost between the two, but i just wondering .

P.s. Hey Bryce, i was wondering does any of the 2 programmers you propose can be used for programming an amiga kickstart? If yes, then it might be a good solution after all!  ;)
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 00:08, 09 May 14
The 16L8 is the GAL, the 40010 isn't a GAL it's a more of a custom IC. There were never programmable versions of this, it's function fixed when it's manufactured.

Not sure whether they can do Kickstarts, possibly the second one with an adapter? You'd have to ask the guys over on Amibay.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 01:37, 09 May 14
Well for just programming 2 roms,i think it's a little "too much"....  ::)

Btw, why i need a 27c256 rom and not 27c128, since both roms of a cpc are 16kb so a 16Kx8=128kbit rom could do the job just fine, right?

Although, as i see on ebay, there is no difference in actual cost between the two, but i just wondering .
Hi ikonsgr,
This question didn't seem to be answered for you.

You don't need to replace both ROMS. The Firmware and BASIC are both in the same ROM and that's why you need a 27C256 and NOT a 27C128. From memory the second ROM on the board is just for the floppy and is a 27C128 but you don't need to worry about it at this time.

Also from memory the ROMs are labeled on the board as (40025) PC-3 and (40015) FDD. I expect it is the 40025 you need to change.

The 27Cxx's were the most common ROMS/EPROMS of this era so as Bryce said just about any programmer will do them. If you have a friend that has a programmer then that is the best solution. If you are going to buy a programmer then it would be better to have one that does the Gate Array as well. This may not be the end of your fault finding so I suggest you save your money at this stage and have your friend program them and use the saved money down the track so you can buy a Scope or Logic Analyzer, or better programmer.

The 27C256 and 27C128 are not interchangeable.

How did you go with the resistance testing of the two data bus bits that were different? That looked to be the best lead you had.

I have removed all RAM chips from my board and I will be socketing them today. Fingers crossed.

I have new RAM. The Z80, 6856 and de-solder tool haven't arrived yet. If the new RAM doesn't fix my CPC then I will socket the 6845 and wait for the replacement.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 05:30, 09 May 14
Hi all,
           I tried the CPC6128 with NO RAM on the board and I can confirm that you should still see the white square on the screen even without any RAM.

I then repaired the track that I lifted and socketed RAM BANK 1.

I put new RAM in BANK 1 and tested. Now I get the BASIC screen so it looks like all is good!!!!

When the other chips arrive I will have a spare Gate Array (the large one), spare Z80, spare 6845 and four spare RAM chips (I ordered 20).

I will try the modern Z80 (10 MHz) when it arrives and let you know how that goes.

I don't know if the problem was a RAM chip or if the Via where the track lifted was fractured and caused the lifting. I didn't have any trouble with any other chips that I had removed at that stage but after the track lifted I cut the others out as a precaution and removed the pins one by one. So I can't test the RAM that was removed.

Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 05:35, 09 May 14
Now I will socket BANK 2, put the RAM in it and test again. If all goes well that I can move on to my original intention.

I was going to use a micro controller to emulate the floppy with a SD card. I am not sure now that this is what I want.

The reason I was going to do it this way is that I wanted you be able to easily edit CPC files on my windows PC. I was going to have the RAW files on SD cards and get the micro controller to emulate the file system that the CPC expects. This is different to the emulators on the market as they use a file on the SD card that is an 'image' of a CPC disk.

Now I am wondering of I could just use 2.5 inch disks on the CPC. I did this once and found that I could use DOS on the PC to format CPC (3.5 inch) disks using the DOS format switches that allow you to specify cylinders head sectors. From memory the CPC uses 40 tracks, 1 head and 9 sectors per track. Where as DOS (360K) used 40 tracks, 2 heads and 8 sectors per track.


Perhaps I could just use 3.5 inch disks. CAn windows read / edit the CPC format disks???
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 11:12, 09 May 14
If you are going to buy a programmer then it would be better to have one that does the Gate Array as well. 

Please don't confuse the terms. There is no programmer in the world that will program a Gate Array. Gate arrays are not programmable devices. The GAL (Generic Array Logic) is a programmable device, there's a small one in the 6128 to do the bank switching and it is programmable. The Gate Array is a non-programmable 40pin custom IC (40007, 40010) found in all classic CPCs and replaced by the ASIC in Plus CPCs.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 11:54, 09 May 14
Hi Bryce,
            When poking around the web looking for details on the gate array I found someone who had put some effort into reverse engineering the gate array. They had a site on git hub. Unfortunately all their work is in a foreign language.

According to them the ASIC is acually a mask programmed gate array. They even had a chip number for the non-mask programmed version. I found a programmer on ebay that will program the programmable version but the site I was looking at didn't go as far as having any VHDL or Verilog.

I ended up buying a gate array as at the price it wasn't worth even looking at a programmer.

So it seems that just as the HAL is a mask programmed version of a PAL, also the ASIC is just a mask programmed version of a more conventional programmable gate array.

If you want I will go dig up the details. It was a work in progress at the time I last looked. As I mentioned the site was in a foreign language but google translate does ok with more technical language.
Title: Re: Need some help fixing CPC6128 not booting
Post by: gerald on 12:35, 09 May 14
Btw, why i need a 27c256 rom and not 27c128, since both roms of a cpc are 16kb so a 16Kx8=128kbit rom could do the job just fine, right?
Sytem rom is a 32k rom and contains the FW (lower 16k) and the basic (upper 16k). Just look at the schematics  ;)
Amsdos is 16k.

Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 14:47, 09 May 14
Hi Bryce,
            When poking around the web looking for details on the gate array I found someone who had put some effort into reverse engineering the gate array. They had a site on git hub. Unfortunately all their work is in a foreign language.

According to them the ASIC is acually a mask programmed gate array. They even had a chip number for the non-mask programmed version. I found a programmer on ebay that will program the programmable version but the site I was looking at didn't go as far as having any VHDL or Verilog.

I ended up buying a gate array as at the price it wasn't worth even looking at a programmer.

So it seems that just as the HAL is a mask programmed version of a PAL, also the ASIC is just a mask programmed version of a more conventional programmable gate array.

If you want I will go dig up the details. It was a work in progress at the time I last looked. As I mentioned the site was in a foreign language but google translate does ok with more technical language.

Can you send me a link to this information, they also seem to be confusing things a bit, or maybe something was lost in the translation.

HALs and Gate Arrays are mask programmed: ie: The chip manufacturer sets the function using a mask while the silicon is being etched and before it gets put into its IC package.
Later, user programmable versions of some HALs got released called PALs, but the old Gate Arrays were never released as user programmable versions. Newer devices have been released known as CPLDs and FPGAs. These are not the same chip, but could be used to build a modern replacement.

Getting a programmer that can do CPLDs might be useful, but they usually cost quite a bit more than a simple EPROM programmer.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 22:28, 09 May 14
Now I will socket BANK 2, put the RAM in it and test again. If all goes well that I can move on to my original intention.

I was going to use a micro controller to emulate the floppy with a SD card. I am not sure now that this is what I want.

The reason I was going to do it this way is that I wanted you be able to easily edit CPC files on my windows PC. I was going to have the RAW files on SD cards and get the micro controller to emulate the file system that the CPC expects. This is different to the emulators on the market as they use a file on the SD card that is an 'image' of a CPC disk.

Now I am wondering of I could just use 2.5 inch disks on the CPC. I did this once and found that I could use DOS on the PC to format CPC (3.5 inch) disks using the DOS format switches that allow you to specify cylinders head sectors. From memory the CPC uses 40 tracks, 1 head and 9 sectors per track. Where as DOS (360K) used 40 tracks, 2 heads and 8 sectors per track.


Perhaps I could just use 3.5 inch disks. CAn windows read / edit the CPC format disks???

I think that everything you need can be done directly using cpcdiskxp (http://www.cpcmania.com/)! The latest versions also support the use of usb floppy drive where you can make disks with much more capacity (~255Kb/side instead of 178kb),which also a cpc can directly read them!  ;)
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 22:35, 09 May 14

How did you go with the resistance testing of the two data bus bits that were different? That looked to be the best lead you had.


Well since you verify that even without any ram you get a window,and also putting bank 1 you get the basic screen, while i still get nothing (note that i replace with 100% working ones, almost every "big" chip in the board: cpu,40010,PAL16L8, crt6845,74LS373, 74LS244, 8 ram chips of 1st bank) i believe 99% chance i have a faulty basic/os rom.

I suppose that the 2 different  16kb rom files can be transffered to the one 27C256 eeprom chip is not a problem with any programmer right?


P.s. about your concerns for the modern cmos z80/10Mhz,i used a modern Z80/20mhz and still worked just fine on my working cpc,so i dont thnik you will have any problems with that!
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 23:35, 09 May 14
I suppose that the 2 different  16kb rom files can be transffered to the one 27C256 eeprom chip is not a problem with any programmer right?


P.s. about your concerns for the modern cmos z80/10Mhz,i used a modern Z80/20mhz and still worked just fine on my working cpc,so i dont thnik you will have any problems with that!

Normally you just combine the two 16K files on a PC into one 32K file and burn it as normal.

Most of the modern Z80s will electrically work in a CPC, I think the only problem is that some of the undocumented commands that are/were frequently taken of advantage of, aren't present in the modern versions.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 05:05, 10 May 14
Well since you verify that even without any ram you get a window,and also putting bank 1 you get the basic screen, while i still get nothing (note that i replace with 100% working ones, almost every "big" chip in the board: cpu,40010,PAL16L8, crt6845,74LS373, 74LS244, 8 ram chips of 1st bank) i believe 99% chance i have a faulty basic/os rom.

I suppose that the 2 different  16kb rom files can be transffered to the one 27C256 eeprom chip is not a problem with any programmer right?

Hi,
    What's left on the data bus that you haven't replaced is the ROM's as you mentioned and the 8255 (IC107), the 765 (IC201), 74LS273 (IC106) and IC209 to IC212 in the Floppy section. So if it's data bus a short you are looking for then these are the 9 suspects. Of course it could just be one of the control signals to these that is causing the problem as well. That is why I was wondering what the resistance tests showed?

There are 3 parts to the code and only 2 ROM chips. Amstrad originally put the Firmware and BASIC in the first ROM (27C256 - IC103 - 40025) so that is how it has to be. The third part of the code Floppy firmware(27C128 - IC204 - 40015) has it's own ROM.

To program the ROM that the CPC boots from you need to append the BASIC code (16KB) to the end of the Firmware code (16Kb) and program it into a 27C256 (32KB).

And yes almost any EPROM / Universal programmer will program a 27Cxxx as Bryce mentioned (he is a guru here and knows the CPC exceptional well).

You mentioned EEPROMS but these are EPROMS.

EPROMS have a window in the top and are erased with ultra violet light so you have to block off the window so ambient UV doesn't eventually corrupt them.

EEPROMS are (E)lectrically (E)rasable and don't have a window.

You mentioned noise when you put your hand near IC203, this IC is involved in /CS line of the 765 FDC (IC201) and any noise on the /CS line (IC201 Pin 4) will certainly case problems on the data bus.

I still think the resistance test is worth while, you have 9 IC's left to replace hoping to get the right one when the fault could actually be a short on the circuit board on one hand, on the other hand it may be a control signal and not be any of the chips you now suspect.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 05:08, 10 May 14
PS: The /CS line of the 765 FDC (IC201 Pin 4) should read very close to 5 Volts.
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 15:22, 10 May 14
I've already test the pin 4 on ic 201 (the 765 controller) and it's indeed ~4.5Volt exactly like on the working cpc.
So if this means that 765 is probably ok (or at least can't cause the "no screen" symptom), could a faulty ic203  cause the "no screen at all" symptom?

In my case we have a very serious and fundamental problem: nothing onscreen!
So the most important thing to know is, if this can caused PRIMARY from a faulty fw/basic rom alone or if ANY (or all...) of the chips you mention above (I/O, FDC, buffers etc) are faulty, can cause the same symptom, even if rom is ok!
And if that's the case, could a logic map of high/low/pulsed on the pins of the above chips could reveal which one has the problem?


Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 02:04, 11 May 14
I've already test the pin 4 on ic 201 (the 765 controller) and it's indeed ~4.5Volt exactly like on the working cpc.
So if this means that 765 is probably ok (or at least can't cause the "no screen" symptom), could a faulty ic203  cause the "no screen at all" symptom?

In my case we have a very serious and fundamental problem: nothing onscreen!
So the most important thing to know is, if this can caused PRIMARY from a faulty fw/basic rom alone or if ANY (or all...) of the chips you mention above (I/O, FDC, buffers etc) are faulty, can cause the same symptom, even if rom is ok!
And if that's the case, could a logic map of high/low/pulsed on the pins of the above chips could reveal which one has the problem?

For the CPC to 'work' practically every chips has to function properly. Chips don't just have one mode of 'failure', they have many different ways in which they can fail especially the larger and more complex chips. That is why the manual suggests changing the larger chips first, diagnosing a complex failure mode with a complex chips is extremely hard even with very good test equipment. There is also the reasoning that the larger chips generate more heat and are therefore more prone to failure to a limited degree.

Your diagnostic process has gone well beyond the scope of faults the manual was intended for. The manual was intended for only the most basic faults and the fault you have in not a basic one.

In my case I could tell that 'some' of the chips were at least working to some degree. To get the box on the screen the Z80 has to be able to run code from the ROM and the data / address space needs to be clean. Or in other words, I had a rough point at which the failure occurred. ie the Z80 can run code, set up the 6845 Regs, write 0's to the screen buffer and seemed to fail when it first needed to run code from RAM.

In your case we don't have a point at which it fails and it appears to failing right from the very start. This is a very difficult fault to diagnose and especialy so with such rudermentery test equipment such as a multi meter.

If I had the equipment then the first thing I would test on your board would be to check the Vcc (5 Volt) rail with a scope looking for noise. Then I would test all the pins on the CPU (as it has the buses attached) and look for anything that is abnormal. In doing these things I am not looking for the 'fault' itself but I am looking for a lead that will take me to the 'fault'.

In your case we don't yet have a lead to follow.

The changing chips is almost certain to fix it. I say almost certain because when you have changed ALL of the chips and it still doesn't work then you know it's a short or fractured via in the circuit board.

You have almost exhausted the changing chips option by way of a logical process. You're more or less at the point where the fault can be narrowed down to being caused by "any of the chips you haven't changed" and there is not really any diagnostic reason to describe it better than that because you don't have a 'lead' to follow.

So in other words, yes the ROM(s) can cause this fault but there is not really any greater reason that it is a ROM over any of the other chips. To continue the chip changing the most (and minimally) logical path is to change the larger and more complex chips first. That would be the ROM's, 6845, 8255 and 765.

If however you can find a 'lead' then you will have a logical reason to change a particular chip and a very high probability that this will fix the CPC.

If you have a logic tester you can test each pin of the CPU and compare that to the other board. If there is a difference that you can see then 'perhaps' and only 'perhaps' that might point to a lead. I say perhaps because of two reasons. 1) Your eyes can see to frequencies up to about 20Hz and that is exceptional LOW compared to the 1MHz that most of these signals will be running at. and 2) Most of the signals are going to be different and that is simply a symptom of the fault and not the cause.

I suspect that what you WILL find is much more conclusive than a 'perhaps'. I suspect you will find a data or address or control pin that is stuck HIGH or LOW and even with something as simple as a logic tester this would be blatantly obvious.

The information that is most useful from a logic tester is status of the 'pulse' indicator. ie constantly OFF, constantly ON, constantly FLASHING. If you find a pin that is very different in this regard then the next clue is the status of the other leds ie constantly RED, constantly GREEN, mostly RED, mostly GREEN, both RED and GREEN. In any case it would be better to record both results to compare ie Pulse status and Logic status.

In my case I saw all pins (on the CPU) were as expected except the /INT pin. I didn't have another CPC to compare to and if I did then the results between the two would have been quite different in several ways. The key is in what you 'Expect' to see. I could list the 'expectations' here but it would take some time for me to do. It would be far easier for you to test and post the results.

If you want to save some effort then you can look for any pin on the CPU that is Pulsing on one CPC and not on the other. I you find one then report the levels (RED / GREEN).

If none of the above then compare the levels (RED / GREEN) of any pins that are NOT pulsing.

This diagnostic process cannot find ALL of the possible faults as it is (very) limited by the fact that you are using a simple Logic Tester. However for 'some' of the possible faults, it can provide a lead that is capable of leading you to the specific fault you have.

Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 02:38, 11 May 14
Here is what I would expect to see on the Z80 CPU, I haven't tested this on my CPC so it's just what I expect to see.

Your probe will have a led (red/green) that is brighter than the other. If you test pin 6 then you will see what I mean by both red and green.

The address buses should be about the same. Look for any that are different.

Also the data buses should be about the same. Look for any that are different.

If you copy and paste the text below into a text viewer that doesn't support rich text or set the font to courier then it will be aligned and easier to read.

Address bus Pins 1 to 5 and pins 30 to 40 Constant pulse both red and green

Data bus Pins 7 to 10 and pins 12 to 15 Constant pulse both red and green

Pin 6  Clock   Constant pulse both red and green
Pin 11 Vcc     No pulse red
Pin 16 /INT    Constant pulse red
Pin 17 /NMI    No pulse red
Pin 18 /HALT   No pulse red
Pin 19 /MREQ   Constant pulse mostly red
Pin 20 /IORQ   Constant pulse mostly red
Pin 21 /RD     Constant pulse mostly red
Pin 22 /WR     Constant pulse mostly red
Pin 23 /BUSACK No pulse red
Pin 24 /WAIT   No pulse red
Pin 25 /BUSREQ No pulse red
Pin 26 /RESET  No pulse red
Pin 27 /M1     Constant pulse mostly red
Pin 28 /RFSH   Constant pulse mostly red
Pin 29 GND     No pulse green
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 03:15, 11 May 14
The address buses should be about the same. Look for any that are different.

Also the data buses should be about the same. Look for any that are different.

To clarify the above -

You don't need to compare A0 of the working CPC with A0 of the faulty one ect.

Just compare A0 to A1 to A2 etc of the faulty one, and D0 to D1 to D2 etc of the faulty one, to each other pin. They should all be about the same.

If the CPU is running these buses will just be random 1's and 0's so what you will see is just the average state.

Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 03:34, 11 May 14
Can you send me a link to this information, they also seem to be confusing things a bit, or maybe something was lost in the translation.

HALs and Gate Arrays are mask programmed: ie: The chip manufacturer sets the function using a mask while the silicon is being etched and before it gets put into its IC package.
Later, user programmable versions of some HALs got released called PALs, but the old Gate Arrays were never released as user programmable versions. Newer devices have been released known as CPLDs and FPGAs. These are not the same chip, but could be used to build a modern replacement.

Getting a programmer that can do CPLDs might be useful, but they usually cost quite a bit more than a simple EPROM programmer.

Bryce.
Hi Bryce,
             I haven't forgotten you. I spent about an hour yesterday looking for the rabbit hole I found that info in previously. I can find some of what I found previously but not the info that went as far as a chip number.

Most of what's out there is just repeated in many locations. There are two documents in a foreign language, one in Spanish and one in French. I can't find the Spanish one. My search ends in 'Server not found'.

I will look again today and post back.
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 22:59, 12 May 14
RobertM, thanks a lot for all your time and effort to help me out in "reviving" my very own first amstrad cpc from 1985!

Moerover, i'm sure your detailed descriptions could help more people having "difficult" problems like myself!

For the moment, i ordered an eprom for the fw/basic rom, mainly because i have a friend of mine who can make the flashing and also because eprom costs 2$ including postage!  :D
I'm also waiting for the logic tester, as soon as i get it, i will start probing all the signals of the cpu to see if they are as you mention.
As soon as i have any news i will let you know!  ;)
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 05:28, 14 May 14
Well I am making progress with fixing this CPC. BANK 2 of the RAM is in and all is working.

Now I am trying to decide if I should use a Floppy Emulator or actual Floppy Drives.

I see cheep emulators on ebay here (http://www.ebay.com.au/itm/121087653625?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649) but I don't know if it will work. I like some of the better emulators but they're too expensive for me.

I looked through my junk box and found 2 Sony MPF920 1.44 Floppy Drives. They're new, still in plastic wrappers.

So I might just give these a go while I decide weather to make an emulator.

The connector is a hassle. I don't want to use an edge connector like the CPC has for Drive 2.

I was thinking of using the 26 pin header connector that was intended for the internal Drive (1).

I noticed there is also pads on the circuit board for a 36 pin external drive connector. The pin spacing is 0.1" from left to right but the spacing between the rows looks to be about 0.2". What connector fits this?

I did do something like this about 20 years ago and I remember shifting data back and forth from the CPC with a 3.5" Floppy Drive and an IBM clone PC. I remember using the dos format switches to select tracks/sectors but I don't remember much more than that.

Can some one point me in the direction of data about what formats the Amdos supports??? From memory the differences were PC(Amstard): Tracks: 80(40) Sectors: 8(9) Heads: 2(1). I think I was using something like the 5.25" 360K format on a 1.44MB Floppy with the hole covered to force the double density mode.

Someone mentioned a Windows utility that will format 3.5" floppies on a PC with a format that the CPC can use. The problem is that my PC main board doesn't have a floppy interface so I would have to use a USB floppy. It seems that a USB floppy can't read the CPC format, only write it. I want to edit CPC programs on the PC so that has only limited use for me.

Also is there some data on the signal format for the read data and write data to the floppy. I think it's just MFM but I don't know it's speed. I am hoping I can get enough speed out of a atMEGA or PIC micro-controller to bit bang the signals. If I can then all I need is a cheap micro-controller, a couple of buttons, a 16x2 LCD display and a SD card.

Thanks in advance to anyone willing to help.
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 10:17, 14 May 14
The cheap floppy emulator you've linked to will only support PC formats so it's useless for a CPC. The HxC works on a CPC but cost a bit more. The real PC drives you have will work, but you will need to make some modifications to change it to DS0 and so that it sends a ready signal. The connector that fits on those holes in the CPC is this one: SE 5736FR - Centronic-Buchse, 36-polig, Printmontage bei reichelt elektronik (http://www.reichelt.de/Centronics-Verbinder/SE-5736FR/3/index.html?&ACTION=3&LA=2&ARTICLE=17437&GROUPID=5209&artnr=SE+5736FR) which I'm sure you can source locally too.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: arnoldemu on 10:56, 14 May 14
http://info-coach.fr/atari/hardware/fd-hard/cd-8002-1.pdf (http://info-coach.fr/atari/hardware/fd-hard/cd-8002-1.pdf)

all in here.

mfm.
double density.
ibm system 34 floppy disc format.

the information near the end shows the gap, id and data fields.

it also describes the mfm encoding too.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 10:41, 15 May 14
Hi all, I am stuck again.

I downloaded CPCDiskXP in the hope that I could format some 3.5" disks in a format that the CPC can use.

I don't have a Floppy Port on my main board so I am using a USB floppy. Unfortunately it looks like CPCDiskXP can't format a USB floppy, it can only format a Floppy connected to the main board. Is that correct???

This is frustrating lol, If I had CP/M on a 3.5" in the correct format then I could just use the CPC to format disks.

Just to get that first disk @#$%&!

OK, It looks like I have to put together another PC and load XP and use a main board connected Floppy Drive Just to get **something** that will work on the CPC with a 3.5" drive.

OH Please ... is there any other way ????

I haven't done the hardware on the CPC yet. I am going to connect a 3.5" 1.44MB Floppy drive to the internal floppy connector. The 3.5" I have doesn't use 12 Volts so the existing 5 Volt power supply should run it. (5V - 3A). The floppy drive uses less than one AMP.

I tried the DOS Format switches with the UBS Floppy drive and the Floppy disk hole covered over (Double Density), All I get is 'Format not supported'.

Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 11:12, 15 May 14
Don't you know anyone with a PC with internal floppy that you could use once? I use ManageDSK to format CPC floppies on the PC, but I don't think it works on USB drives either.
There are also several other CPC users in Australia, maybe one of them can send you CP/M on the 3.5in floppy?

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 11:17, 18 May 14
Hi Bryce,
              I have installed a floppy into the CPC case to work as drive A.

I followed a picture you have here -
Adaptor Cable for Internal HxC (http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/adaptor-cable-for-internal-hxc/5/?wap2)
and it didn't work.

The three signals that seem to be transposed in the picture are ...
Main Board - Floppy Drive
NC         ->   /DriveB
Drive0    ->  MotorA
/Index    ->  /Index

NOT transposed is -
/MotorOn   -> /MotorB

I looked up how a 'standard' 3.5" floppy works and found that by default they are hard wired as DriveB

So I just transposed the last two wires - /Index to give -

/MotorOn     ->   /MotorB
NC               ->   /DriveA
/Drive0        ->   /DriveB
empty          ->  gap
/Index          ->  /Index

Now it seems to work .. it lights when I type |A the motor runs and I can hear it seeking. I don't yet have a formatted disk so I get an error.

I also saw this picture on another page but I can't find it now.

I also bridged the /RDY signal to ground on the Main board connector as new 3.5" use this signal for DiskChange instead.

I wanted to do it this way because when I need to replace the drive I can get *any* standard PC 3.5" and just plug it in without needing to worry about jumpers.

I don't have an Amstrad monitor so I am just using a 5 Volt 3 Amp 'Wall Wart' and NO 12 Volt supply. Newer 3.5" floppy drives don't use 12 Volts anyway.

The CPC6128 is 1.7 Amp and the Floppy Drive is 0.96 Amp so the 3 amp supply is fine.

One other thing that I might mention is that the CPC6128 Floppy Power connector uses RED for 12 Volts and Orange for 5 Volts. On my CPC they had the same colours / positions on the connector as a ATX supply meaning they were transposed when compared to a normal 3.5" floppy. I just swapped the pins in the connector at the end of the floppy power lead. Oops - except that orange is yellow (Sorry I am colour blind).

This would be an issue for people using an amstrad monitor and have 12 Volts connected to the CPC as the 12 Volts would be going into the floppies 5 Volt pin. I am hoping that this info might be helpful for someone.

The confusions started for me when I read HXC / Internal adaptor. I read this to mean HXC and Internal Floppy Drive adaptor cable. It now looks to mean HXC running on internal floppy connector adaptor cable. Or something like that.

Now I will put a PC together with a 3.5" floppy and XP and do some formatting / disk dumps.
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 16:12, 18 May 14
Hi Robert,
       the cable I posted will only work if you have changed the 3.5in floppy from DS1 (standard for PC floppies) to DS0. This is usually a small solder jumper on the floppy PCB. There's also usually a RDY jumper on the floppy PCB so that it sends the RDY signal at the right times instead of permanently connecting it to GND which causes problems with some software.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 17:33, 18 May 14
I just finished a logic mapping of all cpu pins between the working and the faulty cpc's. I found differences for the above pins:
 
PINGOODBAD
3allallFLASHING RED
4GYallFLASHING RED YEL
5allallFLASHING RED
12allYG
16YRG
20YRR
23YRR
29GYG
So, as it seems there are differences mostly on some pins of the address bus where instead of having constant light of all 3 leds,i got flashing (with a period of ~10Hz) red and red yellow leds. So, does this mean the problem lies to os/fw rom chip or any of the other larger chips (8255, AY Sound chip etc) might have problem too?
Btw, robertm, as i have a rather big expecrience using 3.5" floppy drives with a cpc, i can tell you for sure that almost ANY 3.5" 1.44mb IDE floppy disk drive for pc, can be directly connected using a ribbon cable! Only thing you need to do is to connect pins 1 and 2 for the "ready" signal! Of course you can also put switches for making the external drive "A" or "B" (pins 23,24) and choosing side of disk (pins 3,4)!I'm making such ribbon cables for some years now, and i have tried dozens of different floppy drives, up until know all seem to work just fine! ;-)
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 00:04, 19 May 14
Your tests results don't seem right.

Can you verify them.

Pin 29 (GND) should NEVER have a yellow light. A yellow light here means you either have a problem with how the probe is grounded to 0 Volts or you have an open circuit Via. On the bottom of the circuit board under the Z80 is one long vertical track. The via at the end opposite pin 29 is where the broken via would be. A dirty Z80 socket can do the same.

None of the other results can have any meaning while you have a yellow on Pin 29 (GND).
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 00:22, 19 May 14
I just finished a logic mapping of all cpu pins between the working and the faulty cpc's. I found differences for the above pins:
SIGNAL    PIN       BAD   
A13          3           all    FLASHING RED
A14          4           all    FLASHING RED YEL
A15          5           all    FLASHING RED
D2           12         YG   
/INT         16         G   
/IORQ      20         R   
/BUSAK    23         R   
GND         29         G

So, as it seems there are differences mostly on some pins of the address bus where instead of having constant light of all 3 leds,i got flashing (with a period of ~10Hz) red and red yellow leds. So, does this mean the problem lies to os/fw rom chip or any of the other larger chips (8255, AY Sound chip etc) might have problem too?
Btw, robertm, as i have a rather big expecrience using 3.5" floppy drives with a cpc, i can tell you for sure that almost ANY 3.5" 1.44mb IDE floppy disk drive for pc, can be directly connected using a ribbon cable! Only thing you need to do is to connect pins 1 and 2 for the "ready" signal! Of course you can also put switches for making the external drive "A" or "B" (pins 23,24) and choosing side of disk (pins 3,4)!I'm making such ribbon cables for some years now, and i have tried dozens of different floppy drives, up until know all seem to work just fine! ;-)

Did you compare the pins to the "expected results" I listed before?

These results show that R110 needs to be replaced but that makes no sense as you don't have a Red on Pin 25 (BUSRQ) which means the CPU is faulty and you have replaced that so it makes no sense.

This is a nonsensical result. 
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 00:26, 19 May 14
Bump

Address bus Pins 1 to 5 and pins 30 to 40 Constant pulse both red and green

Data bus Pins 7 to 10 and pins 12 to 15 Constant pulse both red and green

Pin 6  Clock   Constant pulse both red and green
Pin 11 Vcc     No pulse red
Pin 16 /INT    Constant pulse red
Pin 17 /NMI    No pulse red
Pin 18 /HALT   No pulse red
Pin 19 /MREQ   Constant pulse mostly red
Pin 20 /IORQ   Constant pulse mostly red
Pin 21 /RD     Constant pulse mostly red
Pin 22 /WR     Constant pulse mostly red
Pin 23 /BUSACK No pulse red
Pin 24 /WAIT   No pulse red
Pin 25 /BUSREQ No pulse red
Pin 26 /RESET  No pulse red
Pin 27 /M1     Constant pulse mostly red
Pin 28 /RFSH   Constant pulse mostly red
Pin 29 GND     No pulse green
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 12:14, 19 May 14
Doing this type of debugging with a logic probe is a futile exercise that's not going to solve anything. Without properly reading the signals with a scope or logic analyser is just going to go in circles.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: Munchausen on 17:48, 19 May 14

You can get a decent logic analyser for under £10 on ebay: Hobby Components USB 24M 8CH 24MHz Logic Analyser & Test Hook Clips | eBay (http://www.ebay.co.uk/itm/Hobby-Components-USB-24M-8CH-24MHz-Logic-Analyser-Test-Hook-Clips-/161309241759)


Most of them are seleae logic clones so you can use the seleae software. I bought from that actual seller.



Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 18:01, 19 May 14
Technically, that is a logic analyser, but it's far from what I would consider "decent". It might just be enough for the job here, although it would be much more complicated and slow compared to using a proper logic analyser and it definitely wouldn't be enough to fault find anything even slightly more complicated. I wouldn't recommend it to anyone who wanted to regularly fault find digital circuits. Of course a real logic analyser is going to set you back a lot more, but it's worth it if you intend making electronics your hobby.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: pelrun on 05:40, 20 May 14
Actually they're a lot better than you give them credit for! I don't have a Saleae or clone myself (although a friend does), having opted for the OpenBench Logic Sniffer (and more recently a DSLogic from the kickstarter) and those are pretty awesome. Definitely compared to the 'high-end' logic analyzers that end up having fewer features because you have to pay $$$$ for each and every protocol you want to decode.


I've debugged some pretty hairy issues using them, on all sorts of boards (running at far higher clocks than the trusty CPC, at that.) My friend has used his Saleae to good effect debugging some high-performance radio modem hardware, amongst other things.
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 10:39, 20 May 14
But how do you manage something like this:

Monitor a UART signal, trigger on a particular word and show what's on the data and Address bus for the next 5 seconds.

I know how to do this on a proper logic analyser and it's typical of the type of test I might need to do, but I've no idea how I'd do it with just 8 channels unless I repeated the test several times and even then I would be unsure of the results.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 11:18, 20 May 14
But how do you manage something like this:

Monitor a UART signal, trigger on a particular word and show what's on the data and Address bus for the next 5 seconds.

I know how to do this on a proper logic analyser and it's typical of the type of test I might need to do, but I've no idea how I'd do it with just 8 channels unless I repeated the test several times and even then I would be unsure of the results.

Bryce.

Well the simple answer is 'with FPGA'.

Back in the 80's we didn't have FPGA! and we seemed to manage with our dungy old 20MHz oscilloscopes.
Title: Re: Need some help fixing CPC6128 not booting
Post by: pelrun on 11:24, 20 May 14
On the saleae, I wouldn't know (although it's continuous streaming, so you could capture a large chunk of data without triggering, run the UART decoder on the lot, then search for the bit you're interested in.) But yeah, 8 channels isn't enough to capture a couple of wide buses simultaneously. Lucky for me most interfaces have moved from parallel to high-speed serial these days, so you need fewer channels anyway :D


My OBLS captures to BRAM in the FPGA, so I can capture up to 200MSPS but I'm limited in buffer size - so I need to set up an appropriate serial trigger on the appropriate signal line, set it to capture 99% after the trigger point, then run the capture. But it has 32 channels.


The DSLogic has a separate DRAM buffer, so it can capture a huge amount of data (only 16 channels though :P ) and do either approach :)


(and when I get my Novena board, I could roll my own off the FPGA interface...)
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 12:00, 20 May 14
Well the simple answer is 'with FPGA'.

Back in the 80's we didn't have FPGA! and we seemed to manage with our dungy old 20MHz oscilloscopes.

With FPGA??? You're suggesting building your own device for one measurement? Or how will the FPGA suddenly add channels, functions and buffer RAM to an 8 channel analyser?

Back in the 80's I was limited to a 10Mhz analogue 2 channel scope due to finances, but it was a struggle and many things just couldn't be tested. Today you can get a real 24 channel analyser with word / byte / pattern recognition and a decent sized buffer for a few hundred Euros. I am also not limited to pocket money and cutting the neighbours grass to earn money :)

Off course, as I said earlier, it all boils down to how much you intend to use it and how complicated your target device is. If you intend doing regular fixing / developing then I'd still recommend spending a little more and getting a semi-decent MSO. That way you have an oscilloscope and analyser that will serve you well. Something like this will get you a lot further than those low-cost USB solutions: Hantek MSO5102D 100MHz 2 Channel 1GSa/s Oscilloscope 16CH Analyzer FREE EXPRESS (http://www.ebay.de/itm/Hantek-MSO5102D-100MHz-2-Channel-1GSa-s-Oscilloscope-16CH-Analyzer-FREE-EXPRESS-/331152541655?pt=Mess_Pr%C3%BCftechnik&hash=item4d1a3ac7d7) 

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 12:06, 20 May 14
I am also not limited to pocket money and cutting the neighbours grass to earn money :)

Good for you Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 12:08, 20 May 14
Now I just get to cut my own grass and nobody pays me for it :(

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: Munchausen on 12:30, 20 May 14
Technically, that is a logic analyser, but it's far from what I would consider "decent". It might just be enough for the job here, although it would be much more complicated and slow compared to using a proper logic analyser and it definitely wouldn't be enough to fault find anything even slightly more complicated. I wouldn't recommend it to anyone who wanted to regularly fault find digital circuits. Of course a real logic analyser is going to set you back a lot more, but it's worth it if you intend making electronics your hobby.

Criticising this seems very harsh when we are comparing to a logic probe. It's easy when you have access to very expensive equipment, for example I know the electronics engineer at my previous job had a >£4000 scope, while at home I work with a USB one that cost ~£90, but for doing the odd job I can get away with it. The logic analyser above is better than many that cost more than ten times its price, and for a hobbyist is likely to (a) just about fit your needs and (b) be all you can afford. Electronics has been a hobby of mine for a long time, and I wanted a good scope for almost as long, but eventually I realised that I can get a £90 now or a £400 one "one day". I can't really justify the greater expense when the things I play with generally cost less than ten or fifteen pounds, and I have much greater other expenses to worry about.


More to the point, for this problem the above logic analyser is the debugging option with the minimum possible outlay, and is still almost infinitely better than a logic probe. I don't think it would be useful to suggest that someone goes and spend hundreds on a logic analyser or scope to fix a CPC, but £10 might be ok. The other option of course (which I'd probably have gone for by now) is to send it to you :D
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 12:30, 20 May 14
A digital oscilloscope would be a marvelous tool indeed, but at the moment i can't afford for something like that. The logic tester on the other side, seems more interesting, and if i could make all those checks you mention, then i might get one!
It's not so easy to fix something like a CPC on budget. Mine is working now. I have all the original disks working with a 3.5" drive installed into my CPC6128.

This was achieved with a multi meter, logic tester and some greatly appreciated advice from Bryce.

For me seeing the very low frequency of the /INT pin of the CPU gave an indication that I had a RAM issue.

I am sorry to be such a 'stick in the mud' about test equipment. I am simply trying to assist ikonsgr in a manner that is suitable to the budget.
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 12:55, 20 May 14
@Munchausen (http://www.cpcwiki.eu/forum/index.php?action=profile;u=792): I wasn't really criticising the analyser, rather just pointing out that it is only suitable for people who don't intend doing more than one repair or very occassional testing. I have seen many people "climbing the testing equipment ladder" and being frustrated that they didn't buy proper equipment in the first place. They spend €20 on a cheap analyser, €100 on a cheap scope and the same on soldering iron, DMM etc. then quickly realise that all that money was more or less wasted because they quickly "outgrow" those devices. Electronics definitely isn't a cheap hobby, but if you think you are going to be doing more of it, then it's worth dimensioning your equipment (and budget) for a higher level. A scope that you can use for years is well worth a few hundred, but spending that for one repair would be ridiculous, so a €10 analyser will hopefully be just enough to get it fixed.

Bryce.

P.s. I've no problem fixing stuff for you (or anyone else here), so send it over if you've given up. It's always good to see a CPC come back to life :)
Title: Re: Need some help fixing CPC6128 not booting
Post by: pelrun on 13:26, 20 May 14
Off course, as I said earlier, it all boils down to how much you intend to use it and how complicated your target device is. If you intend doing regular fixing / developing then I'd still recommend spending a little more and getting a semi-decent MSO. That way you have an oscilloscope and analyser that will serve you well. Something like this will get you a lot further than those low-cost USB solutions:


I'm 100% behind paying for a decent scope or not getting one at all; those dinky $100 handheld lcd ones are pretty much worthless. Either spend $100 on a good second hand analogue scope, or $400-$500+ on one of the low-end Rigols at a bare minimum.


An analyser is a bit different; it's possible to get a very capable fpga based usb logic analyser for under $100 these days - much of the power comes from the software, where it's possible to roll your own protocol decoder in code. I definitely preferred it over the 16 channel analyser built into the expensive tektronix MSO I used at work - it was clunky, and I had no licenses to unlock the protocols it could decode. But the scope functions were top-notch.
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 13:47, 20 May 14
The main difference I find is when you are fault-finding. If you are developing something you usually set up the equipment to measure a specific situation and that's easily done on both a real analyser and a USB software controlled analyser, but when you are probing around a faulty board, testing many things in different ways, then a real analyser is much faster and more comfortable to use.

Bryce.

Edit: Just looking at the offer for  the USB analyser on ebay again I noticed a "deal killer" in my opinion: The input impedance is 100K!! That means that connecting this to a circuit has quite a large influence on how the circuit reacts. This would probably be ok for the CPC, but definitely not for anything else. An analyser should have at least a 1Mohm impendance. They also don't mention the voltage range it works in, which is a bit worrying. Is there a page where all the specs are listed?
Title: Re: Need some help fixing CPC6128 not booting
Post by: gerald on 14:41, 20 May 14
Edit: Just looking at the offer for  the USB analyser on ebay again I noticed a "deal killer" in my opinion: The input impedance is 100K!! That means that connecting this to a circuit has quite a large influence on how the circuit reacts. This would probably be ok for the CPC, but definitely not for anything else. An analyser should have at least a 1Mohm impendance. They also don't mention the voltage range it works in, which is a bit worrying. Is there a page where all the specs are listed?
100k input inpedance is rather standard for a logic analyser, and you will find this on Agilent logic analyser, which are not known for being cheap. It can go down to 50k/20k on some probes, an this is just from DC point of view.
1M or even 10M is more common on scope, where you may significantly disturb the signal your looking at because the source impedance is high. This is usually not the case for digital signals, and in there you use active probes.
Title: Re: Need some help fixing CPC6128 not booting
Post by: gerald on 15:04, 20 May 14
Now, on cheap logic analyser, I would back Bryce on limited usablility of a 8 channel logic analyser to debug a CPC when you've run out of your multimer and finger sensing bullets.
A bare minimum is 24 channel, which allow you to see at the address bus and control signal of the Z80, ideally a 32/34 way is the sweet spot for a CPC/8bit.

While I understand the budget point, the (re)usability is also a strong point.
Remember that USB stuff always requires a driver, which you may miss in few years, because the manufacturer does not provide an updated one for your new OS, or because the original manufacturer found a way to detect counterfeit HW (FDTI nuked counterfeit RS232 adapter by driver update).
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 15:10, 20 May 14
Obviously impedance is much more critical on a scope, but 100K on an analyser can have a negative effect on rise times. On low frequency equipment this isn't usually an issue, but on high frequency equipment this can be critical. It all depends on what frequency range you are working in. Maybe just due to the type of stuff I do (did) at work, I tend to go for as high an impedance and as low a capacitance as possible. They don't even mention the probe capacitance on cheap devices.
The analysers at work (way outside my price limit - ie: in a different galaxy) are 10M / 0.7pf. My hobby analyser 1M / 5pf.

(FDTI nuked counterfeit RS232 adapter by driver update).
Oops, wasn't aware of that. Must check whether my cheap copy still works!!

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: pelrun on 05:09, 21 May 14
My analysers are fully open source, so no issues there :)


I spent my monies on an original HP 16550 logic pod and high quality micro test hooks. There's a mod for the OBLS that properly terminates it for the isolation network in the pod and biases it for various trigger levels. Even without that the inputs are buffered and have a pretty high input impedance (far higher than the 1Mohm of the saleae.) But yeah, it's really only for under 100MHz.
This is where it's beneficial to have a friend with a lot of expensive test equipment in the odd instance I need to work on something more demanding :D
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 10:22, 21 May 14
Then you're sorted :) Mixing the HP Pod with the low cost analyser is an interesting idea.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 13:00, 22 May 14
Bit of an update -

I installed a 3.5" floppy as drive A. It's hard to make them fit lol. I had to remove a fair amount of plastic.

I noticed the screw spacing on the side of the CPC is the same as the screw spacing on a 3.5" floppy but the 3.5" sits a little inward of the front of the CPC. It works and I copied the system disks with a floppy in a PC with CPCDiskXP. I couldn't get CPCDiskXP to do *anything* with a USB floppy so I gave up on that.

I have ordered another drive and 50 Disks.

I noticed that the edge connector for the external drive has a couple of +5 Volt pins so I am going to try an external 3.5" disk from the edge connector that uses power from the same connector.

I have been using the composite monochrome output from the video connector with a monitor that has VGA, s-Video and Composite inputs so it can scan down to the low 50Hz PAL frame rate.

I tried connecting the signals from the CPC Video connector to the VGA input by connecting the CPC composite sync to both the vertical and horizontal sync signals going into the monitor. That almost worked but the Hsync was out at the top of the screen as the Vsync was interfering.

I then tried collecting all the signals directly from the gate array and then the image was stable but the active area of the screen started right on the left edge of the screen. It looks like the gate array delays the Hsync signal in some way.

So now I will make a box with a RGBs to colour composite and s-Video converter. I will run the power plug to the box and have a second lead that then conveys power (5 Volts) to the CPC. I will also have a 6 Pin Din lead to connect the box to the CPC. On the output side I will have a composite colour output and an s-Video output for the monitor.

I am going to build the circuit with an AD722 (not AD724) as it has directly coupled inputs. I will be making it the worst possible way to test, ie vero type board in a plastic unshielded box.

I am hoping to get a better picture with the directly coupled inputs but at same time it will be far more sensitive to RF induced into any possible ground loop. The AD722 has both a digital and analog grounds and I haven't yet worked out how would be best to attempt to isolate the two. My concern is that because the circuit in the box shares the same power supply as the CPC then I have to worry about any RF that might be induced into the power cable between the box and CPC, even the earth itself.

I was going to put 100nF caps in the DC input and output of the box and run the digital rail of the AD722 from the 5 Volts. I was then going to connect the Video signal ground to the analog ground of the AD722 and use an inductor to decouple the 5 Volt rail that would otherwise be directly shared with the digital side of the AD722.

Does this sound like it could work or am I lining myself up for failure by sharing the 5 Volts power supply with the CPC and s-Video converter ??? Any comments ???
Title: Re: Need some help fixing CPC6128 not booting
Post by: gerald on 13:22, 22 May 14
I noticed that the edge connector for the external drive has a couple of +5 Volt pins so I am going to try an external 3.5" disk from the edge connector that uses power from the same connector.
Are you sure these are connected ? The 6128 schematic show them on the disk side of the connector, but not on the main board.
I think this is a left over from the DDI1 schematic where the drive indeed powered the interface.

As for powering the drive from the CPC trough that connector, there are no real issue. I have one CPC configured that way.
You will have to patch the drive and the CPC. I am using pin 6 of the connector, which is usually unconnected on drives.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 13:31, 22 May 14
Are you sure these are connected ? The 6128 schematic show them on the disk side of the connector, but not on the main board.
I think this is a left over from the DDI1 schematic where the drive indeed powered the interface.

As for powering the drive from the CPC trough that connector, there are no real issue. I have one CPC configured that way.
You will have to patch the drive and the CPC. I am using pin 6 of the connector, which is usually unconnected on drives.

Good point. I though that was just a convention in the schematic but now I look again. I see that all the other 5 Volt rails are lined in and the FDD connector 5 Volt rails are NOT. So I will check before I get too carried away.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 13:32, 22 May 14
As for the ground loop issue ... I cheated and just looked at the schematic for the monitor. They just short the grounds and hope for the best lol. I will do the same and just decouple the 5 Volt rails. If I do have trouble then I I can make changes then.
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 14:05, 22 May 14
The pins marked 5V on the 6128 floppy port aren't connected to anything. These are the pins that the FD-1 uses to send power to the DDI-1 so an incoming 5V would be expected if an FD-1 was connected.
As for the AD722 Analogue / digital supplies, there's no need to isolate them so much from each other. If you connect a 100nf capacitor directly across the power pins of each power input it will work fine. See my schematic for the AD724: RGB SVideo - CPCWiki (http://www.cpcwiki.eu/index.php/RGB_SVideo)  What's more critical is the crystal start-up. Make sure to use a capacitor less than 7pf across the crystal and that the leads between the crystal and the AD aren't too long or parallel or you will get a really shitty picture. The AD722 is also very sensitive to stray capacitance generally, using a stripboard may cause problems that you don't get on a properly laid out board.
Btw: The 724 is an improvement over the 722, the three capacitors on the RGB Inputs improve the picture over the 722 solution.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 23:30, 07 September 14
Finally i found a friend who can burn the 27C256 eeprom with fw/basic!  :)
Now i was thinking to make a new rom for amsdos too. Can i use another 27c256 eeprom chip ,and if yes,where should i put the 16kb amsdos rom,to upper or lower 16k segment?
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 10:25, 08 September 14
Ideally you should use a 27C128. If you use a 27C256 then you have to either:

A) Program AmsDOS to the lower portion.

B) Program AmsDOS to the upper portion and connect A14 (pin 27) to 5V.

Alternatively

C) You could program AmsDOS to the lower portion and ParaDOS to the upper portion and add a switch to switch pin 27 between GND and 5V so that you can select the DOS you want to use.

Pin 27 is normally connected to VCC (Pin 28) on the CPC mainboard, so you'll have to cut the connecting track on the mainboard for options B or C.

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: CraigsBar on 11:07, 08 September 14
I have several (hundred) amd 27c128 chips if you need some.
Title: Re: Need some help fixing CPC6128 not booting
Post by: Munchausen on 14:08, 08 September 14
What's more critical is the crystal start-up. Make sure to use a capacitor less than 7pf across the crystal and that the leads between the crystal and the AD aren't too long or parallel or you will get a really shitty picture. The AD722 is also very sensitive to stray capacitance generally, using a stripboard may cause problems that you don't get on a properly laid out board.


+1. I had exactly this problem with the AD724 and strip board, and it took me forever to figure out what was going wrong. Make those tracks to the crystal as short as you possibly can because they are really sensitive!
Title: Re: Need some help fixing CPC6128 not booting
Post by: ikonsgr on 16:44, 08 September 14
Ideally you should use a 27C128. If you use a 27C256 then you have to either:

A) Program AmsDOS to the lower portion.

B) Program AmsDOS to the upper portion and connect A14 (pin 27) to 5V.

Alternatively

C) You could program AmsDOS to the lower portion and ParaDOS to the upper portion and add a switch to switch pin 27 between GND and 5V so that you can select the DOS you want to use.

Pin 27 is normally connected to VCC (Pin 28) on the CPC mainboard, so you'll have to cut the connecting track on the mainboard for options B or C.

Bryce.
That's a very nice idea bryce, dual booting cpc! I will give it a try and see what happens!  ;)
Title: Re: Need some help fixing CPC6128 not booting
Post by: Bryce on 17:49, 08 September 14
I did a proper tutorial on what you need to do here: DualOS - CPCWiki (http://www.cpcwiki.eu/index.php/DualOS)

Bryce.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 07:21, 01 January 16
The pins marked 5V on the 6128 floppy port aren't connected to anything. These are the pins that the FD-1 uses to send power to the DDI-1 so an incoming 5V would be expected if an FD-1 was connected.
As for the AD722 Analogue / digital supplies, there's no need to isolate them so much from each other. If you connect a 100nf capacitor directly across the power pins of each power input it will work fine. See my schematic for the AD724: RGB SVideo - CPCWiki (http://www.cpcwiki.eu/index.php/RGB_SVideo)  What's more critical is the crystal start-up. Make sure to use a capacitor less than 7pf across the crystal and that the leads between the crystal and the AD aren't too long or parallel or you will get a really shitty picture. The AD722 is also very sensitive to stray capacitance generally, using a stripboard may cause problems that you don't get on a properly laid out board.
Btw: The 724 is an improvement over the 722, the three capacitors on the RGB Inputs improve the picture over the 722 solution.

Bryce.

Wow I fell off the planet there for a while - found this place called 2000's

The resolution is excellent but the game play sux.

Anyway - I recently got back to this PAL baseband modulator.

I had two chips that were like AD722 and AD724. I used the one with capacitive coupled inputs as the datasheet for the other had obselete across the page.

The baseband modulator works for composite but not s-video and I can't seem to tune the coulor burst freq. I see that Brice recomends a 7pF trimmer and I have been using a larger one so I will drop that and see.

I made the cable for the s-video so maybe I functioned that up.

Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 07:25, 01 January 16
I did a proper tutorial on what you need to do here: DualOS - CPCWiki (http://www.cpcwiki.eu/index.php/DualOS)

Bryce.

Thanks Bryce - I will have to get back to this.
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 07:28, 01 January 16
Ideally you should use a 27C128. If you use a 27C256 then you have to either:

A) Program AmsDOS to the lower portion.

B) Program AmsDOS to the upper portion and connect A14 (pin 27) to 5V.

Alternatively

C) You could program AmsDOS to the lower portion and ParaDOS to the upper portion and add a switch to switch pin 27 between GND and 5V so that you can select the DOS you want to use.

Pin 27 is normally connected to VCC (Pin 28) on the CPC mainboard, so you'll have to cut the connecting track on the mainboard for options B or C.

Bryce.

Hi Bryce - I will assume that the original question was about a duel operaqting system because I don't remember.

Since I was here last I have learnt VHDL so I might see what I can do with a CPLD / FLASH on the expansion port.
Title: Re: Need some help fixing CPC6128 not booting
Post by: Munchausen on 12:22, 01 January 16
The baseband modulator works for composite but not s-video and I can't seem to tune the coulor burst freq. I see that Brice recomends a 7pF trimmer and I have been using a larger one so I will drop that and see.

I built one based on an AD724 on some veroboard in the past, and had exactly the same issue. I eventually fried it and my CPC by connecting 12V instead of 5V and just bought one from Bryce later, but that's a different story! Anyway, the issue with tuning it turned out to be that the tracks to the crystal were too long - make sure they are as short as possible because parasitic capacitance can really screw this up. In fact - keep all the tracks as short as possible!
Title: Re: Need some help fixing CPC6128 not booting
Post by: RobertM on 10:10, 03 January 16
I built one based on an AD724 on some veroboard in the past, and had exactly the same issue. I eventually fried it and my CPC by connecting 12V instead of 5V and just bought one from Bryce later, but that's a different story! Anyway, the issue with tuning it turned out to be that the tracks to the crystal were too long - make sure they are as short as possible because parasitic capacitance can really screw this up. In fact - keep all the tracks as short as possible!
lol well mine was probably even worse.

I was trying (for the first time) a minium etch board and I couldn't find how to make a keep out zone for the crystal tracks in the CAD so it ended up being 20mil tracks with 20mil clearenece.

That was one of several problems lol. The tracks are long to put the trimmer cap near some color ballance pots.

Anyway - it's a proto so no biggie.