Hi All,
I have a non-working 464 Plus, I found that both the RAM chips were dead, so I've replaced them but the machine still doesn't work.
When I power it up, I get a grey screen with diagonal lines for a second-or-two, then the TV says no signal, which is exactly what happens on a good working 464 Plus without a ROM cartridge fitted.
The fact that the two RAM chips had gone, makes me think that the computer might have been connected to a too higher voltage PSU, and therefore, maybe the ASIC has been destroyed. BUT would I get the grey diagonal lines if the ASIC was completely dead?
I have an oscilloscope, and would be grateful for any diagnostic advice on how and what to check next.
You checked the cartridge port is clean? I had a scare with my 464 Plus recently and although the cart edge connectors were clean it turned out something was fouling the socket end. Some contact cleaner (the strong stuff) fixed it.
Just worth checking. I'd thought the RAM or ASIC had blown but its now just fine.
Good thinking :) I sprayed it with contact cleaner but maybe I should check it more carefully.
Quote from: Overkill on 20:40, 10 December 17
Good thinking :) I sprayed it with contact cleaner but maybe I should check it more carefully.
Hi Overkill
Did that fix the Problem ? ?
Thanks Ray
I've looked at the contacts and they look fine, I will test them with a meter but I don't think that is the problem.
Quote from: Overkill on 17:11, 10 December 17
The fact that the two RAM chips had gone, makes me think that the computer might have been connected to a too higher voltage PSU, and therefore, maybe the ASIC has been destroyed. BUT would I get the grey diagonal lines if the ASIC was completely dead?
Dead asic / no cartridge will usually give the same result.
Quote from: Overkill on 17:11, 10 December 17
I have an oscilloscope, and would be grateful for any diagnostic advice on how and what to check next.
1st check that the Z80 have a clock (pin 6), then that the WAITn (24) is working as well.
If you do not get a clock, check that the main oscillator is working as well.
I will do some testing later today and let you know the results, thanks.
Quote from: gerald on 12:08, 11 December 17
Dead asic / no cartridge will usually give the same result.
1st check that the Z80 have a clock (pin 6), then that the WAITn (24) is working as well.
If you do not get a clock, check that the main oscillator is working as well.
I get the clock at pin 6 and 5v at pin 24.
I've tested all the connections to the ROM cartridge and they seem fine.
Quote from: Overkill on 19:29, 11 December 17
I get the clock at pin 6 and 5v at pin 24.
I've tested all the connections to the ROM cartridge and they seem fine.
Are you sure of you replacement RAM ?
Any chance you can run this : http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/quick-and-dirty-ram-test-for-cpc
Quote from: gerald on 19:43, 11 December 17
Are you sure of you replacement RAM ?
Any chance you can run this : http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/quick-and-dirty-ram-test-for-cpc (http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/quick-and-dirty-ram-test-for-cpc)
That sounds like a handy tool to have, I can program an EPROM.
I've since found that the lumi signal is missing (I think), comparing with a working 6128+ at the RGB out socket, I find no voltage at pin 3 (that's if the attached diagram is correct).
hmm.... hang-on, I might have got the pin-outs the wrong way around. Going for my dinner, I'll check again later :)
I've tested the output of the RGB socket and pins 1 and 3 seem to be the same as the working 6128+.
I've checked the working temperature of the ASIC and it seems about the same as the working 6128+ 43-45C.
I'll try the RAM diagnostic ROM tomorrow and keep you posted :)
Sorry I need more help, see error while trying to program the RAMTEST.BIN
Ignore above, it was a bad EPROM I think :)
Hi Gerald, the computer is the same with your RamTest inserted (grey lines), I tried the EPROM on my working 6128+ and it worked on that one.
What would happen if the SN74LS27N IC16 was dead? I'm not sure what that IC does.
Quote from: Overkill on 17:02, 12 December 17
What would happen if the SN74LS27N IC16 was dead? I'm not sure what that IC does.
All could happen, as it's masking IORQn on some address range and could prevent CRTC init.
More on that in http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/amstrad-plus-and-im2-bug/msg147349/#msg147349 (http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/amstrad-plus-and-im2-bug/msg147349/#msg147349)
Did you check the Z80 as well ?
I swapped the Z80 with my 6128+
So there is not much remaining to check.
All access to the ROM/RAM need the ASIC to be working.
Only a logic analyser would help confirming the death by checking what really happen on boot.
It's a shame that IC1 is so hard to swap :(
Could also check the video out path from ASIC to DAC.
IC1 to IC15?
IC15, check that digital RGB input match analogue output.
I've found something (maybe) comparing the signal on IC15, BLU3 I get a 300mv wave (looks like curved waves that you see at the seaside :) ), BLU2 I get a good 5v square wave, but BLU1 and BLU0 is at a constant 5v (flat lined,no wave). On my working 6128+ I get a good 5v square wave on all of those.
The OUTB at IC15 is a 600mv wave that is similar to the working 6128+.
I know I'm a novice, but the fact I get a good wave pattern at BLU2 makes me think that IC1 must be working, at least a little ????? :)
Is the NSYNC similar on the din output when you compare with the working one ?
They are identical, a 5v square wave.
The biggest difference at the RGB socket is the LUMI, 1.25v on the 464+ and 1.57 on the 6128+.
I was wondering if it was just a video fault, so I left the BASIC ROM cartridge in to see if I got the Burning Rubber sound, but nothing.
Just to add, I do seem to get a very high pinched low volume noise when plugged into the TV.
Quote from: Overkill on 21:03, 12 December 17
They are identical, a 5v square wave.
NSYNC is not supposed to be a square wave or we do not have the same definition of a square wave ;)
For me a square wave is a a signal that alternate 0 and 5V with a duty cycle of 50% : half time low, half time high, repeat.
NSYNC , on a CPC, is a digital signal that should look like this
http://kudelsko.free.fr/analy_logic/utilisation/LM1881.gif (http://kudelsko.free.fr/analy_logic/utilisation/LM1881.gif)
Yes, it wasn't square, it was 5v high but dropped to 0v a bit like the composite sync in that gif.
I still wonder if I'm on to something with some of the pins on IC15 being at a steady 5v without any wave form. I was thinking of removing IC15 and then testing those points again.
Testing the extra diodes still in circuit (seen in the picture below), I get a very low resistance. Testing the same on my working 6128+, I get expected readings.
I'm getting 72 ohms across 3 of the 4 diodes in that circuit.
Quote from: Overkill on 11:29, 13 December 17
Testing the extra diodes still in circuit (seen in the picture below), I get a very low resistance. Testing the same on my working 6128+, I get expected readings.
Measuring resistance on a diode is pointless. It tells you almost nothing and is dependent on the direction you connected the meters leads. Voltage drop is all that's important, so measure the voltage drop and you'll know if they are ok. Also, if you measured them in-circuit, you have no way of knowing whether the 72R is from the diode or some other part that's in parallel to the diodes.
Bryce.
OK, work this out ?????????????????????????? :o
I put a meter across a lot of the components in that circuit, then I went back and tested across those same diodes, the readings were NOW OK and what I'd expect for diodes in circuit. So I thought I might have accidently fixed a short, so I plugged it in and still got the same grey screen. I now test the diodes for a third time, and they are back to the low ohms >:(
Quote from: Bryce on 11:41, 13 December 17
Measuring resistance on a diode is pointless. It tells you almost nothing and is dependent on the direction you connected the meters leads. Voltage drop is all that's important, so measure the voltage drop and you'll know if they are ok. Also, if you measured them in-circuit, you have no way of knowing whether the 72R is from the diode or some other part that's in parallel to the diodes.
Bryce.
Thanks Bryce, I do understand the problems you mention but I am able to compere readings with a working machine.
Ignore all that, it was the difference between have the RGB cable plugged in or not DOH! :doh:
As I said, too many other things are being measured at the same time. The 72R was most likely the terminating 75R of the video lines.
Bryce.
I'm running out of ideas, its looking more like IC1 is at fault. I think I'll shelve this repair for a while. I've just ordered a cheap IC tester from China, so I can test IC15 and IC16.
Can you use the names/part No.s of the IC's rather than IC15 / IC16. Even I have to check the schematics to know what you're talking about! :o
Bryce.
Sorry, they were named in posts above :)
[font=]SN74LS27N IC16[/font]
[font=]40464 DAC IC15[/font]
What sort of tester did you buy to test them? A 74LS27 cost 50c, you don't test them, you swap them. As for the 40464, I doubt any Chinese tester will be able to test this.
Bryce.
Yes, I was wondering if it would test the 40464, I didn't know how custom that chip is.
My logic is, I'd rather know an IC is faulty than just replace them. The IC tester was £25 and I'm sure it will be a handy tool for me :)
https://www.youtube.com/watch?v=66hPg_r4IOw
I prefer to manually test chips. Those testers may say the chip is good and it still doesn't work in the circuit.
Bryce.
I think this tester, although cheap is very good. It will only identify an IC if all the functions pass the test.
If it's a custom part and there's interest, I can send one for decapping.
Quote from: robcfg on 21:11, 13 December 17
If it's a custom part and there's interest, I can send one for decapping.
I think it's only IC1 Gate Array custom chip that we could do with, but I can't see anyone manufacturing them again :(
You can get the decapped ASIC pictures by following the links on this post (http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/gate-array-decapped!/msg133264/#msg133264).
Regarding 40464, if there's no info about it, it may be worth to decap it just out of curiosity. Besides the ASIC, ACID and 40464, there are no custom chips there.
Gerald, have you ever tried lying a ASIC chip on the contacts? I was thinking of removing IC1 from a working GX4000 and seeing if I could test the IC1 chip from this 464+ by just placing it on the board and adding some weight, instead of soldering the chip in. Is this possible, do you think?
Quote from: Overkill on 19:48, 14 December 17
Gerald, have you ever tried lying a ASIC chip on the contacts? I was thinking of removing IC1 from a working GX4000 and seeing if I could test the IC1 chip from this 464+ by just placing it on the board and adding some weight, instead of soldering the chip in. Is this possible, do you think?
You have about a 1% chance of that working. Because the pads won't ever be 100% smooth, the chances of getting every pin to make sufficient contact is extremely low.
Bryce.
Quote from: Bryce on 08:25, 15 December 17
You have about a 1% chance of that working. Because the pads won't ever be 100% smooth, the chances of getting every pin to make sufficient contact is extremely low.
Bryce.
I'd make a jig to press all the contacts down, I don't know if it would work, but I might give it a go.
By doing that you will bend the pins to different levels to force it to fit the uneven pads. It will make the IC very difficult to solder in afterwards or may even damage the part.
Bryce.
Quote from: Bryce on 10:26, 15 December 17
By doing that you will bend the pins to different levels to force it to fit the uneven pads. It will make the IC very difficult to solder in afterwards or may even damage the part.
Bryce.
The ideal solution would be if I could get some of that rubber conductive layered contact strip that they used on LCD displays, although I would guess that they might have a too higher resistance.
You'd have to find some with the correct pitch and even then it would be an unreliable balancing act.
Bryce.
Quote from: Bryce on 10:34, 15 December 17
You'd have to find some with the correct pitch and even then it would be an unreliable balancing act.
Bryce.
No, the reason that would be ideal is that the pitch is micro small, so it conducts across the width and the height, but not across it's length.
I found some on eBay, although I've not checked the size suitability yet. https://www.ebay.co.uk/itm/MULTIPLE-CONTACTS-ELASTOMERIC-CONNECTORS-FOR-LCD-66mm-4-PCS/322280471642?epid=881782255&hash=item4b0969bc5a:g:oMUAAOSw8w1X8PCf
Seems like an aweful lot of (unreliable) hassle, when soldering it in would only take 2 minutes. The part you have linked to has nonconductive walls, only the dark section in the middle conducts, so you would need to line up the pins exactly on the dark part on four sides of a floating chip while trying to keep the pressure even on an uneven pad surface... Good luck with that!
Bryce.
Yes, that linked isn't ideal, but you can get others. The idea isn't really to just fix this computer, but to have a rig that would make it easy to test those IC's in future.
If that's the goal, then I'd build something more robust which is quickly repeatable and reliable. Maybe you should consider wiring a QFP ZIF socket to the PCB?
Like this but with the correct pin count:
http://www.ebay.de/itm/IC-Sockel-Programmier-Adapter-SMD-TQFP48-QFP48-0-5mm-auf-DIP48-2-54mm-Pins/282665211851
Bryce.
That would be good, if there is such a thing.
Maybe this one for $460 :o www.aprilog.com/images/160QFS26A-QF-ZL-SD.jpg
You'll find them a lot cheaper, but unfortunately the QFP160 isn't as common as some of the other sizes like the 44, 64 or 128 so they won't be going for <€20.
Bryce.