I've "inherited" a very poorly 6128 board. By poorly I mean absolutely dead.
When I got it. the 40015 and 40025 rom's had been removed and there were a few damaged tracks on the board. There was no Z80 cpu. Other than that it looked ok.
I've repaired the tracks and continuity back to the z80 seems good. I've put sockets where needed and tried to power it on with an eprom in the firmware socket.
Unfortunately nothing happens other than the LED lights up. There is no video signal... my telly says no signal and my monitor just glows slightly. no border.
The voltage before powering on is 5.2 and this drops to 5.01 when powered. The z80 is getting power. no noise is made when pressing delete etc, nor does the relay click when the right commands are typed. It's deffo not the switch as I have a jumper across the power connector.
Where should I check next? I'm quickly getting out of my depth here... :o
Paging Dr Bryce - he is the guy to ask - i'm as much use as a chocolate fireguard.
Hi Chinnery,
first check would be that all ICs are getting 5V. It doesn't sound like they are, because a drop from 5.2V to 5.01V isn't very much (unless you are using a huge 10A PSU or something. Next would be to check that the CPU has fired up. Check for the clock signal to the CPU and then check if there is any activity on the address or data buses. If all those are present, you'll need to start tracking particular signals to see how far into the boot process it's getting.
Bryce.
Hi Bryce,
Thanks for helping out once again!
I dug out the old multimeter, downloaded many data sheets, and have probed every IC on the board for a 5v signal. They all have one. I don't have the tools to measure clock frequencies, but assume an el-cheapo dvm will average out the clock pulses? So ~ 2.18 on the clock would also seem good.
One thing I did notice on the Z80 is that INT (pin 16) was low, and NMI (pin 17) is high. Is it fair to say that NMI should be set high only to reboot? Neither of the pins are shorted to ground or +5.
Other than that, there wasn't anything exciting about the address pins. They all seemed stable at either 2.43 or 1.6v, although D6 was at 0.5v (I'm not sure if this is of any use, but I may as well put it in anyway).
Also, the PSU is a 2A one from an old Belkin USB Hub.
I'll have a quick look and try find what flips the NMI, but if you have any more pointers I'm all ears :)
Oh, and did I mention that the Z80 was from a working CPC - whether this board has frazzled it or not is another matter!
Quote from: Chinnery on 21:07, 05 February 18
Hi Bryce,
Thanks for helping out once again!
I dug out the old multimeter, downloaded many data sheets, and have probed every IC on the board for a 5v signal. They all have one. I don't have the tools to measure clock frequencies, but assume an el-cheapo dvm will average out the clock pulses? So ~ 2.18 on the clock would also seem good.
One thing I did notice on the Z80 is that INT (pin 16) was low, and NMI (pin 17) is high. Is it fair to say that NMI should be set high only to reboot? Neither of the pins are shorted to ground or +5.
Other than that, there wasn't anything exciting about the address pins. They all seemed stable at either 2.43 or 1.6v, although D6 was at 0.5v (I'm not sure if this is of any use, but I may as well put it in anyway).
Also, the PSU is a 2A one from an old Belkin USB Hub.
I'll have a quick look and try find what flips the NMI, but if you have any more pointers I'm all ears :)
Nothing will flip the NMI on a bare CPC, it's just connected to a pull-up ;)
Int always low mean that the GA has issued an interrupt, but the CPC did not acknowled it.
Since the clock seems to work, I would double check the system ROM connection to the Z80.
The 1st thing the system is doing is to configure the CRTC and from then you should at least have a visible border and screen.
Do you also have a way to check the gate array is fine ?
Hi Gerald,
Thanks for helping out!
I've just double checked on the ROM/Z80 connections, and they all seem to be there. I will give it a third check when my eyes are less tired - the layout diagram I am using from the service manual is a bit tricky to follow.
I've tried reseating the 40010 but that made no difference. I have another working 6128 which I could swap the ic out and see if that makes a difference.
I haven't put a DOS rom in - though I would have thought that'd make no difference? :-\
Try swapping out the Z80 from the working CPC. It's the most likely culprit at the moment.
Bryce.
:(
Again, thanks for your help.
I tried swapping both the z80 and gate array with my working 6128. The working 6128 still works, the broken one still broken. I haven't done any more probing yet to see if anything has changed, nor have I had chance to triple check my track rebuild... That's probably a weekend job now with good board and broken board next to each other.
Ok, I've had a bit of free time to do a bit of probing, and a bit of spare cash to get a slightly better multimeter.
I can confirm that there is indeed a 4mhz clock on the Z80 and the CRTC has a 1mhz clock on pin 21. However Pin39/40 for the vsync and hsync measure as 0hz.
Does this mean that the CRTC is a-gonner, or it's not being initialised? What further checks can I do?
The signals on pins 39 and 40 of the CRTC would be very difficult to assess with a multimeter. The frequency measurement of a multimeter definitely would be able to read them, so it's difficult to say. Without a scope you won't be able to check these pins.
Bryce.
Ok, I managed to snipe a cheapo pocket scope from ebay... this is costing more than a replacement board!!! :doh:
Anyway, on the crtc clock pin, I get this:
(https://i.imgur.com/JokGUA3.png)
Looks ok I suppose - I thought it would be more square than that, but hey ho.
However, the hsync and vsync are essentially flatline...
A check of address 0 (pin30) on the Z80 goes all wavey, suggesting the Z80 is running.
Does this suggest that the CRTC has passed away, or that it's not initialised (or either or neither)?
It probably is square (or at least squarer than that shows) because the pocket scope you bought has a very low bandwidth. Grab a coffee and read this post: http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/understanding-retro-electronics/msg144677/#msg144677
What do you see on pins 39 and 40 of the crtc?
Bryce.
Quote from: Bryce on 09:36, 26 February 18
It probably is square (or at least squarer than that shows) because the pocket scope you bought has a very low bandwidth. Grab a coffee and read this post: http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/understanding-retro-electronics/msg144677/#msg144677 (http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/understanding-retro-electronics/msg144677/#msg144677)
I had read that a while back, but it must have been pushed to the back of my memory. Yes, my scope has a very low bandwidth but I couldn't afford much more - it was a luxury mainly aimed at my Arduino messing about... I mean development...
Anyway, a quick RTFM later, and I have the following:
HSYNC:
(https://i.imgur.com/X2ZsD0q.png)
I had to set the range to 10mV to get this, and it was reasonably stable.
VSYNC:
(https://i.imgur.com/c3mtvev.png)
The "blips" are just blips. No mater what time period I set to there was no pattern, the scope couldn't lock on to anything.
I must again thank you for your time and patience on this. It is a bit over my head at the moment but I am slowly learning...
I highly suspect that your CRTC is an ex-CRTC unfortunately.
Bryce.
@Chinnery (http://www.cpcwiki.eu/forum/index.php?action=profile;u=2434)
Do you have a link to the " Pocket Scope " That you purchased Please
or a Name and model number.
Thanks Ray
Quote from: Bryce on 22:10, 26 February 18
I highly suspect that your CRTC is an ex-CRTC unfortunately.
Fortunately ex CRTCs are not that expensive to replace...
Quote from: Audronic on 00:03, 27 February 18
@Chinnery (http://www.cpcwiki.eu/forum/index.php?action=profile;u=2434)
Do you have a link to the " Pocket Scope " That you purchased Please
or a Name and model number.
Thanks Ray
Yes, it is a Seeed DSO Nano V3. [Yes, Seeed]. As it is based on an open-source project, There are plenty of other similar specced scopes that may be a bit cheaper (DSO211, DSO203 amongst others).
My reasons for choosing this were obviously price (although not the cheapest), the fact it has real buttons, not touch panel style, its in a metal case and finally, the probes connect to a 3.5mm jack, so making my own probes up should be easy.
Quote from: Chinnery on 10:20, 27 February 18
Fortunately ex CRTCs are not that expensive to replace...
Yes, it is a Seeed DSO Nano V3. [Yes, Seeed]. As it is based on an open-source project, There are plenty of other similar specced scopes that may be a bit cheaper (DSO211, DSO203 amongst others).
My reasons for choosing this were obviously price (although not the cheapest), the fact it has real buttons, not touch panel style, its in a metal case and finally, the probes connect to a 3.5mm jack, so making my own probes up should be easy.
You'll only be able to make 1x probes though, not 10x, so the bandwidth issues will be even worse with self made probes.
Bryce.
Quote from: Bryce on 08:22, 05 March 18
You'll only be able to make 1x probes though, not 10x, so the bandwidth issues will be even worse with self made probes.
Bryce.
Well, I'm sorry I am unable to afford a £500 scope, just being a hobbiest. But, as I said the scope was bought mainly for my other hobby - arduino which will not need anything much more than the one I bought.
Quote from: Bryce on 22:10, 26 February 18
I highly suspect that your CRTC is an ex-CRTC unfortunately.
Bryce.
Unfortunately, swapping the crtc results in the same. No signal, no noise on hsync/vsync, no TV picture
At this point, I've decided to scrap the board. Thanks every one for their assistance.
Funny that, I'm dealing with a CPC 464 that has almost the same symptoms! (notice that this is very confusing because my avatar is very similar to Chinnery, but I walk to the right and he walks to the left :) ).
Turning on my 464 results in no video. Voltages are fine all over. Replacing the Z80 and the GA (40010) made no difference.
Looking around with a logic analyzer, some things are fine: clock, RAM Dout, address and data buses, and lots of pulses. But it seems like the system is kind of stuck: no HSYNC or VSYNC, no /IORQ from the Z80, and about half the address bits on the RAMs are stuck at 0.
Are there any tests we can do on the CRTC to see if it's functional? At first I thought it was a goner, but then I noticed all the MA bits are low, so it's like it's not being driven at all. Maybe it's a bad ROM? (I thought that never failed pretty much).
Anyway, if someone has any thoughts on tests to run on this board, I would love to hear them.
All right, I finally got X-MEM (Thanks Toto!!). First thing I did was to try it on this CPC 464 and the result is...
... all memory passed. So at first it made me think it was the CRTC like I was suspecting due to the lack of HSYNC or VSYNC signals.
But then I realized... wait... how come I *am* able to see the border and the colors in the RamTest loaded from X-MEM? I shouldn't be able to if the CRTC was busted. So that left pretty much one thing: the ROM! I reconfigured X-MEM, told it to boot its own ROM and everything looks good!! That seems pretty unusual for a ROM to go bad, right? I might even try and read it with the EPROM programmer to see if it matches what I expect.
What's the best way to replace it? Just a regular EPROM with the original ROM burned, or is there something better/more permanent.
So, Chinnery, if you're still reading this, try the ROM if you haven't thrown the computer in the garbage yet :)
Hello Avatar Sharer :)
Quote from: llopis on 09:09, 08 August 18
That seems pretty unusual for a ROM to go bad, right? I might even try and read it with the EPROM programmer to see if it matches what I expect.
I have had an instance where the firmware/basic rom from a 464 wouldn't read in my reader, but worked in the CPC. The rom was socketed so not much chance of it being damaged by removing it. The rom did eventually die though.
Quote from: llopis on 09:09, 08 August 18What's the best way to replace it? Just a regular EPROM with the original ROM burned, or is there something better/more permanent.
For a 6128, I would stick with 6128 Firmware and Basic 1.1 - essentially a copy of the existing one.
Quote from: llopis on 09:09, 08 August 18
So, Chinnery, if you're still reading this, try the ROM if you haven't thrown the computer in the garbage yet
I had tried replacing all socketed roms, also the firmware and disk rom, and got nothing. Strangely enough, I did get some flickering when I had partially desoldered the AY... but I gave up. It had cost too much by then and I'd got another one up and running.
The board is now in a gurus pile of boards to scavenge for spares.
I see that the ROM that comes with the Amstrad is a 32K chip and it includes the OS and and Basic. How do I go about burning an EPROM with both of them? Does the OS go in the lower 16K and Basic in the upper 16K?
Also, looking at the ROMs in the wiki (http://www.cpcwiki.eu/index.php/ROM_List) I see there's a Spanish version for the 464 OS, but there's only one version of the 464 Basic. Does anyone know if they need to match up, or is putting those two together fine?
Quote from: llopis on 10:21, 08 August 18
I see that the ROM that comes with the Amstrad is a 32K chip and it includes the OS and and Basic. How do I go about burning an EPROM with both of them? Does the OS go in the lower 16K and Basic in the upper 16K?
Also, looking at the ROMs in the wiki (http://www.cpcwiki.eu/index.php/ROM_List (http://www.cpcwiki.eu/index.php/ROM_List)) I see there's a Spanish version for the 464 OS, but there's only one version of the 464 Basic. Does anyone know if they need to match up, or is putting those two together fine?
Yes, it is a 32 pin chip, lower half is the OS and upper is basic.
I *believe* you need the correct firmware for the machine, but it is possible to run any basic on top - for example I have a 464 running 664 basic (v1.1) which works rather well.
OK, I pulled out the ROM chip and put a socket there, but I don't have any 32K EPROMS yet.
Just for fun I tried reading the ROM I extracted on my EPROM programmer, and even though the chip looks really good, it complains about error in pin 5. So I'm going to guess it's detecting the internal failure of the chip.
Here's hoping that it's all fixed as soon as I receive those EPROMS.
The problem with mine was indeed the ROM. I was able to use the ROM from another 464 and it works like a charm. As a bonus, I was able to copy the ROM on the EPROM programmer, so I'll be able to make an exact copy when I get the chips.
I do a binary diff against the 464 Basic ROM that's on the Wiki, and if it's different for being the Spanish ROM, I'll upload it.
To answer my own question, I did a binary diff of the Spanish 464 BASIC ROM with the generic (English?) 464 BASIC ROM, and they're just off by 2 bytes, so it's probably just some IDs and they're the same otherwise.