Hello. I'm hoping someone can offer some pointers.
I purchased a CPC knowing it probably wouldn't work, and I was right. ;) I'm getting screens like the following examples when powering on.
https://photos.google.com/share/AF1QipO0S3hqdoAHFCXdSPknU48Hm2iz6-LRAtWW1s3wlj3_6Wk3Zv5v7ch4GxeVYhcp4Q?key=cHIyZzlHM2lPX3ZjbjRXVlNXNmZFakxoSmRLazNB
I have some experience with fixing ZX Spectrums and I would hazard a guess that because there is an image it's either faulty RAM or associated chips/tracks. If there's an image within the centred area then I'm assuming the CPU is probably okay.
Included in the above link are some images of the board. It looks like the very first version1 PCB (Z70100). Everything looks to be in good condition and I was pleased to see most ICs are socketed—including the RAM. I couldn't detect any RAM chips that were obviously hot although IC 108 was getting hot along with whatever is under the heatsink (IC 116).
I was thinking of just getting some 4164 ram replacements as a first shot at fixing this. I have a multimeter and soldering equipment, but nothing more advanced.
Any advice, much appreciated. Thanks!
Most likely a RAM issue.
I'would first re-seat ROM / RAM on socket
Quote from: gerald on 14:02, 12 March 22Most likely a RAM issue.
I'would first re-seat ROM / RAM on socket
Thanks for this Gerald. I reseated all the ram chips and the ROM, and it didn't make any difference unfortunately. Perhaps one or more of the ram chips have stopped working, so I'll order some off ebay. They seem to be plentful.
Do you know if piggybacking tends to work for ram chips in CPCs as a way of testing?
On a side note, I read somewhere that pressing 'delete' should result in some noise/beeping. I'm not getting any sound. The volume is up and the audio out is connected to audio cable part of the SCART I have.
I also noted some try solder pads near the ram chips. Continuity was okay, I think, but just in case I resoldered those pads.
I've seen some 4164 ram chips on ebay and the speed is 100ns. Would this be fine with a CPC 464? Thanks!
Quote from: jarcher1980 on 15:08, 12 March 22Do you know if piggybacking tends to work for ram chips in CPCs as a way of testing?
Why ? the RAM are on socket. Just replace and test.
Quote from: jarcher1980 on 15:08, 12 March 22On a side note, I read somewhere that pressing 'delete' should result in some noise/beeping. I'm not getting any sound. The volume is up and the audio out is connected to audio cable part of the SCART I have.
You need working RAM for this. The delete trick only hint you of a bad video connection.
Quote from: gerald on 15:55, 12 March 22Quote from: jarcher1980 on 15:08, 12 March 22Do you know if piggybacking tends to work for ram chips in CPCs as a way of testing?
Why ? the RAM are on socket. Just replace and test.
Quote from: jarcher1980 on 15:08, 12 March 22On a side note, I read somewhere that pressing 'delete' should result in some noise/beeping. I'm not getting any sound. The volume is up and the audio out is connected to audio cable part of the SCART I have.
You need working RAM for this. The delete trick only hint you of a bad video connection.
Those pictures aren't his 464, they're just an example he found.
Bryce.
Thanks for the information both. Really helpful. It seems like ram is the first thing to replace.
I'm going to order a set of 8 4164 chips. The ones I found are 100ns, so 50ns quicker than the original memory speed. I assume this is fine and it will just run slower. If you could confirm that would be great.
Those images of the PCB are my board. All the ram is—thankfully—socketed along with most of the other chips.
Thanks again!
In that case I'd try re-seating the RAM you have first. It may just be a dodgy connection in the socket. If not get new chips and swap them out. The RAM speed doesn't matter as long as it is 200ns or less. The quoted speed is how fast they can react, not how fast they run.
Bryce.
Quote from: Bryce on 10:55, 13 March 22In that case I'd try re-seating the RAM you have first. It may just be a dodgy connection in the socket. If not get new chips and swap them out. The RAM speed doesn't matter as long as it is 200ns or less. The quoted speed is how fast they can react, not how fast they run.
Bryce.
Thanks for your advice. I did take each ram chip out, and resocketed, and it made no difference. I didn't test each socket pin though for continuity. I'll do that if new ram chips don't solve the issue. I've ordered some.
You can also try the diagnostic ROM from
@llopis .. if you have a M4 card or a Dandanator you can just load it into the card and it will tell you if the RAM modules are bad and also which one.
https://github.com/llopis/amstrad-diagnostics
Quote from: SerErris on 11:31, 16 March 22You can also try the diagnostic ROM from @llopis .. if you have a M4 card or a Dandanator you can just load it into the card and it will tell you if the RAM modules are bad and also which one.
https://github.com/llopis/amstrad-diagnostics
Thanks for the tip. 😊
Quote from: Bryce on 10:55, 13 March 22In that case I'd try re-seating the RAM you have first. It may just be a dodgy connection in the socket. If not get new chips and swap them out. The RAM speed doesn't matter as long as it is 200ns or less. The quoted speed is how fast they can react, not how fast they run.
Bryce.
Replacement ram chips arrived. I replaced them 1 at a time, trying the machine. With every memory module replaced, the machine still results in the 'garbage' images that I linked to in my first post. If anything, I did notice that the garbage seemed a bit more patterned—if that makes sense. Whereas before there were a lot of random 'dots' artefacts, now there are definitely more uniformed square shape patterns. Not sure if that's helpful; I can post some images if so.
I also took the opportunity to check continuity from each new memory chip pin to the corresponding socket pin—and continuity was fine.
If it's not the memory, any ideas what it may be. All the surrounding tracks look absolutely fine to me.
Any other suspect ICs for this sort of screen garbage?
Thanks in advance all.
The initial garbage picture hint that 256 byte (3*80 + 16) are one way, then 256 are the other way (ie A8 0, then 1) and so on.
I would check address mux IC109 and A8 from Z80.
Quote from: gerald on 21:23, 16 March 22The initial garbage picture hint that 256 byte (3*80 + 16) are one way, then 256 are the other way (ie A8 0, then 1) and so on.
I would check address mux IC109 and A8 from Z80.
Thanks gerald. Could you explain what you mean by 'mux'. I do have the pcb schematic for the 464 to hand.
Quote from: gerald on 21:23, 16 March 22The initial garbage picture hint that 256 byte (3*80 + 16) are one way, then 256 are the other way (ie A8 0, then 1) and so on.
I would check address mux IC109 and A8 from Z80.
Here are some updated garbage pictures with all the new memory fitted. Not sure if this is helpful or not. Thank you.
https://photos.google.com/share/AF1QipNolHgGXpy6h27sX7m6GXnPOSaB5g0ju5IvY2uXccjiQDwSevYQxMTj9VwCGDD8Uw?key=MWJGR2dSSmljQTNuTll2TEh2ZHc5bWZ1aWFEMG1n
I've done a bit more investigating. I checked the voltages for each ram chip and the CPU and they're drawing a consistent 2.8V. I checked the PSU (a new one, sourced online) and it's outputting a consistent 5v, so I think it's absolutely fine.
I checked the power connector socket on the CPC and it was also measuring 2.8V. I don't have the keyboard connector attached, only the power supply side.
So does this indicate there's a fault somewhere? Could it be the power socket? It does look a bit brown/rusty inside. Any advice appreciated. Thanks.
Check and clean the power socket, but the power switch may also need to be opened and cleaned.
Bryce.
Quote from: Bryce on 11:19, 18 March 22but the power switch may also need to be opened and cleaned.
Fortunately we have a WIKI article for that ;-)
https://www.cpcwiki.eu/index.php/Fixing_the_power_switch
btw: we are missing pictures. If you fix the power switch anyway, you can take a few and update the article (or send me the pictures and I'll update the article).
Quote from: Bryce on 11:19, 18 March 22Check and clean the power socket, but the power switch may also need to be opened and cleaned.
Bryce.
I will try that, thanks.
I thought I was onto a winner with this. The inside of the power socket connector definitely looks corroded although my cleaning with a trimmed cotton bud did return some of the metal sheen/colour. Unfortunately, it made no difference.
The power switch looked absolutely fine inside. The two wires going into the switch looked a bit suspect as the connection was loose; so I soldered those better. I also tried bypassing the switch entirely by connecting the two pins on the board; it made no difference. So, I think the switch is okay.
The voltage is still reading between 2.2 and 2.8 volts at the memory chips and the CPU; it sometimes reads 4 volts. It varies and I can't understand why. Nothing is changing except perhaps how the power connector is sitting in the socket.
When I take a voltage reading directly at the power socket (see photo below), it is the same reading as everywhere else on the board - at the memory chips and the CPU - between 2.2 and 4 volts. Is this normal? Or should it ALWAYS be 5v at that position?
https://photos.app.goo.gl/pn6JhWtNpN8QjRso9 (https://photos.app.goo.gl/pn6JhWtNpN8QjRso9)
Perhaps it is the power socket after all.
It should always be 5V at that position. Either the socket is still very dirty or there is a break in the circuit.
Bryce.
Quote from: Bryce on 10:42, 19 March 22It should always be 5V at that position. Either the socket is still very dirty or there is a break in the circuit.
Bryce.
I ordered a new power socket and replaced the old one. I'm now getting 5v at the connector. The RAM and CPU are getting 4.68v which is an improvement, but I don't think that's enough is it?
I checked voltage at the Gate Array. For VCC1 at pin 22 it's 4.6v and at 38 it's 4.6v. For VCC2 at pin 9 and 33 it's 2.85v. I did notice when measuring voltage at the Gate Array that it wasn't steady, it was fluctuating +/- about 0.08v; don't know if that's helpful.
I've also checked each ram chip using a tester and they're all fine, so I'm pretty confident it's not a ram issue. I'm also bypassing the switch completely atm.
Does this still suggest a break in the power rail somewhere? Many thanks for the help so far.
If the voltage is fluctuating it would suggest that the powersupply is struggling to supply the required current. What PSU are you using?
Bryce.
Quote from: Bryce on 11:22, 26 March 22If the voltage is fluctuating it would suggest that the powersupply is struggling to supply the required current. What PSU are you using?
Bryce.
I'm using a modern replacement from Coolnovelties. If I recall it's 5v 2amps. I tested it and it's providing a consistent 5v.
Should the ram/cpu etc be getting exactly 5v. What is an acceptable variation?
Anything below 4.75V could make the computer unstable.
Bryce.
Quote from: Bryce on 16:22, 26 March 22Anything below 4.75V could make the computer unstable.
Bryce.
Thanks for confirming. I'll keep checking the voltage rails and hopefully I can detect where it drops.
Quote from: jarcher1980 on 21:07, 26 March 22Quote from: Bryce on 16:22, 26 March 22Anything below 4.75V could make the computer unstable.
Bryce.
Thanks for confirming. I'll keep checking the voltage rails and hopefully I can detect where it drops.
Still investigating... when checking the power rails from CP001 I decided to check the nearby R145. I suspect this isn't connected to the issue, but the Ohms reading was 429. The schematic suggests it should be 1k. What do you think? Thanks.
If you were measuring it in-circuit, then it was in parallel with several other devices, so the value could be anything!
Bryce.
Quote from: Bryce on 15:10, 27 March 22If you were measuring it in-circuit, then it was in parallel with several other devices, so the value could be anything!
Bryce.
So, perhaps not an issue.
Thanks for helping on this Bryce. Much appreciated.
Please bear with me as I present another potential 'non-issue' as the fault may be elsewhere...
I've been checking voltages at different locations. I'm using a simple wire bridge (with two female connectors at the end so there's no contact with other pins) to connect pin 1 and 3 of CP001 to switch on the CPC. It seems to work fine—with the same garbage centre or grey centre as before showing on screen.
If I remove this bridge and measure the voltage at pin 3 (labelled 'A' in my image below) I get 5.1V—which is exactly what the power supply is giving out. If I use the bridge and measure the same pin 3 I get 4.9v. If I measure pin 1—the other side of the bridge and labelled 'B' in my photo—I get 4.81v.
4.81v is what I'm getting at the voltage pins on the CPU, RAM etc.
Could CP001 be the cause of the voltage drop? Or is the fault elsewhere in the circuit and I'm just detecting this here when I join the whole circuit?
https://photos.google.com/photo/AF1QipNkYo5-XwqcN4PLECovWXYadnTNVK4jPDkNl3M5
No, the drop from 5.1V to 4.8/4.9V is what would be expected because a load (the CPC circuit) has been added.
Can't check the photo as it brings me to a login page (and I don't have an account).
Bryce.
Quote from: Bryce on 08:43, 29 March 22No, the drop from 5.1V to 4.8/4.9V is what would be expected because a load (the CPC circuit) has been added.
Can't check the photo as it brings me to a login page (and I don't have an account).
Bryce.
Thanks for confirming. Perhaps voltage around the PCB isn't an issue then. It must be something else. If it's not the RAM, then perhaps it's one of the other ICs.
It could still be many things. Where are you located?
Bryce.
Quote from: Bryce on 15:51, 31 March 22It could still be many things. Where are you located?
Bryce.
Yorkshire, UK.
Ok, then sending it to me isn't an option, but I can recommend someone in the UK that can fix it if you don't have any success.
Bryce.
Quote from: Bryce on 08:33, 01 April 22Ok, then sending it to me isn't an option, but I can recommend someone in the UK that can fix it if you don't have any success.
Bryce.
Good to know Bryce. Many thanks.
Quote from: Bryce on 08:33, 01 April 22Ok, then sending it to me isn't an option, but I can recommend someone in the UK that can fix it if you don't have any success.
Bryce.
Hello. Still attempting to fix! :-)
I've been testing continuity around the memory modules—mainly between each module's D0 and D1 (pins 14 and 2 respectively) and the D0-D7 of the Gate Array, along with lines to IC115 and IC114. (Marked on a schematic here (https://photos.google.com/share/AF1QipOHlLNT4-0SBxpcNmJXL2--iwNCxRDd8I44f-362oi-1Wr8udvIXiczWj-k5QG5cg/photo/AF1QipOC1o0rjWaMinlujhUDe2CkaL3O4BGRF57MjREY?key=NTUzV0o0UjV3blBjR2xZOTBlNG9ITlBhR29ORzJ3).) Everything checks out fine as far as I can tell.
I noted that GadgetUK had a similar fault with a garbage screen in this Youtube video (https://www.youtube.com/watch?v=9ctY9lhIS8Q&t=512s). He identified IC112 as the issue. I decided to get a logic probe and thought it worthwhile checking the same chip. You can see my results at this link (https://photos.google.com/share/AF1QipOHlLNT4-0SBxpcNmJXL2--iwNCxRDd8I44f-362oi-1Wr8udvIXiczWj-k5QG5cg/photo/AF1QipPNp2o-PTlq5BxrI09GSSPbBuVak8hGIin6mYxs?key=NTUzV0o0UjV3blBjR2xZOTBlNG9ITlBhR29ORzJ3). Each of the 4 OR gates was showing a HI output. I think this is correct, but it would be interesting to hear your thoughts.
If IC112 isn't the issue, are there any other ICs you think it would be worth checking next?
Thanks again!
Hi, just a few thoughts from my side:
A logic probe is an interesting instrument to test a hand crafted circuit, e.g. circuits that operate at low speed. I'm not so persuaded that this will be of much help in your hands in order to hunt down the error in a circuitry that runs at 4MHz.
I would invest the money in a few cheap chips that you find on this board, and try the piggy-back trick shown here instead (one chip after the other, not all at the same time).
Another thing is to closely observe the picture the CPC is still producing, as it tells you something. It is not a good idea to refer to a picture you found on the internet saying it looks like this. If I were you, I would produce a good picture showing how it looks like in your situation. Everyting else is half hearted.
Referring to the pictures you refer to: obviously the clock is running, CRTC and Gate Array are also functional, otherwise you wouldn't see the pattern on the screen. The CPU might be okay, but apparently it crashed. There seems to be a problem with reading out the RAM, which caused the CPU to crash but which also produces the noise you can see on the screen. Try to do something which reduces this noise. As soon as it's gone, I would expect the machine to revive.
Also, people that connect an LCD monitor to the CPC obviously use a power supply not designed for it. Can be that it's a good one, but can also be the opposite, can be the power is somehow distorted and you can't see that with a simple volt meter. I would try a different power supply to double check. Alternatively, test with 4 NiMh batteries in series (at 1.2-1.3V each), this gives a very clean linear power (double check the polarity!).
Quote from: Nworc on 15:27, 12 April 22Hi, just a few thoughts from my side:
A logic probe is an interesting instrument to test a hand crafted circuit, e.g. circuits that operate at low speed. I'm not so persuaded that this will be of much help in your hands in order to hunt down the error in a circuitry that runs at 4MHz.
I would invest the money in a few cheap chips that you find on this board, and try the piggy-back trick shown here instead (one chip after the other, not all at the same time).
Another thing is to closely observe the picture the CPC is still producing, as it tells you something. It is not a good idea to refer to a picture you found on the internet saying it looks like this. If I were you, I would produce a good picture showing how it looks like in your situation. Everything else is half hearted.
Referring to the pictures you refer to: obviously the clock is running, CRTC and Gate Array are also functional, otherwise you wouldn't see the pattern on the screen. The CPU might be okay, but apparently it crashed. There seems to be a problem with reading out the RAM, which caused the CPU to crash but which also produces the noise you can see on the screen. Try to do something which reduces this noise. As soon as it's gone, I would expect the machine to revive.
Also, people that connect an LCD monitor to the CPC obviously use a power supply not designed for it. Can be that it's a good one, but can also be the opposite, can be the power is somehow distorted and you can't see that with a simple volt meter. I would try a different power supply to double check. Alternatively, test with 4 NiMh batteries in series (at 1.2-1.3V each), this gives a very clean linear power (double check the polarity!).
Hi Nworc. Thanks for taking the time to reply. :-)
I did post some images of the screens that I'm getting in earlier messages in this thread, but I've just taken some new ones. You can see them at this link (https://photos.google.com/share/AF1QipP-8FBVSbWW2xBxy04z1io36_t_7ZWNRMZVGSVNqo3AbGG8I9eeqF0z0N-xhsUlsg?key=ZVJzdjdmODM0amNlN0ZydlFEWDJ5X1hrYUYwemVn). If you can point to any likely tracks/ICs that may be causing this issue, that would be great.
I did swap the CPU with one of my spare Spectrum ones and I got the same screens so I can probably rule of the CPU. I don't have any spare gate arrays so if it's that I guess it's game over. All the memory ICs have been checked with a tester a few times and they seem fine.
For some reason, under load, the chips are getting around 4.6 - 4.8v—which I understand can cause issues below 4.75v. I guess there must be a short somewhere. Tracks all look fine so my guess is it's an IC.
I've tried two different modern replacement 464 power supplies as well as an Amstrad CTM 644 monitor.
During today's test, aside from the very hot gate array I did note that IC108 was getting quite warm. All other ICs seem cool.
IC108, the CRTC is innocent, it's apparently doing it's job.
Well, the pictures are not very sharp, but good enough to see that the vertical stripes seem to be on byte level, meaning it's always the same bit / set of bits which is returning an unexpected value.
Now following the idea, that it's always the same bits which are affected. The point I see is, if the Gate Array is accessing the RAM to read the gfx data, the only chips which are connected to Data Out of the RAM chips are the Gate Array and the Data Latch/Buffer IC114 and IC115. Apparently the Gate Array is receiving wrong data since this is what we see on the screen, so either it's having an internal problem, or one of the other mentioned chips (the data latch/buffer) puts a load on the Data Out lines interfering with the RAM.
In case it's not that, I'm also not convinced that the voltage is at the correct level at all RAM chips at all the time, for whatever reason.
I think that's all I can say.
Quote from: Nworc on 21:48, 12 April 22IC108, the CRTC is innocent, it's apparently doing it's job.
Well, the pictures are not very sharp, but good enough to see that the vertical stripes seem to be on byte level, meaning it's always the same bit / set of bits which is returning an unexpected value.
Now following the idea, that it's always the same bits which are affected. The point I see is, if the Gate Array is accessing the RAM to read the gfx data, the only chips which are connected to Data Out of the RAM chips are the Gate Array and the Data Latch/Buffer IC114 and IC115. Apparently the Gate Array is receiving wrong data since this is what we see on the screen, so either it's having an internal problem, or one of the other mentioned chips (the data latch/buffer) puts a load on the Data Out lines interfering with the RAM.
In case it's not that, I'm also not convinced that the voltage is at the correct level at all RAM chips at all the time, for whatever reason.
I think that's all I can say.
Thanks for that.
I suspected the CPU->RAM->BUFFERS->GATE ARRAY circuitry so I checked for continuity between them. You can see my checks here (https://photos.google.com/share/AF1QipOHlLNT4-0SBxpcNmJXL2--iwNCxRDd8I44f-362oi-1Wr8udvIXiczWj-k5QG5cg/photo/AF1QipOC1o0rjWaMinlujhUDe2CkaL3O4BGRF57MjREY?key=NTUzV0o0UjV3blBjR2xZOTBlNG9ITlBhR29ORzJ3). All those in red were checked for continuity and they're fine.
Do you think IC115 and IC114 may be faulty, or even IC112? Do you think it's worth purchasing some replacements? Thanks again.
Quote from: jarcher1980Do you think IC115 and IC114 may be faulty, or even IC112? Do you think it's worth purchasing some replacements? Thanks again.
I can't guarantee a return on investment if you spend a pound on each of these. It can always be something you don't expect. I would resolder all chips in question before I would order them, just to exclude that kind of an issue. BTW you can't detect weak solder joints using a multimeter. Okay, if the joint is so weak that the component is falling out, you can detect it :)
how/when do the colours change?
Quote from: eto on 12:44, 13 April 22how/when do the colours change?
It seems random whenever I turn it on. Occasionally, I get a completely grey center instead of coloured garbage. It's weird.
Quote from: Nworc on 11:54, 13 April 22Quote from: jarcher1980Do you think IC115 and IC114 may be faulty, or even IC112? Do you think it's worth purchasing some replacements? Thanks again.
I can't guarantee a return on investment if you spend a pound on each of these. It can always be something you don't expect. I would resolder all chips in question before I would order them, just to exclude that kind of an issue. BTW you can't detect weak solder joints using a multimeter. Okay, if the joint is so weak that the component is falling out, you can detect it :)
Good point. I'll solder in some new sockets and retry the same chips first before ordering new ones. Thanks!
I would not add sockets for chips which aren't socketed. I had the "bright" idea once to socket RAM chips in one of my computers, and problems started with that. In the end I threw out the sockets. By resoldering I just meant: heating up the solder joints and applying a very tiny bit of fresh solder (the one with flowing agent inside) on each joint.
Quote from: Nworc on 20:38, 13 April 22I would not add sockets for chips which aren't socketed. I had the "bright" idea once to socket RAM chips in one of my computers, and problems started with that. In the end I threw out the sockets. By resoldering I just meant: heating up the solder joints and applying a very tiny bit of fresh solder (the one with flowing agent inside) on each joint.
Are okay. Good call. Thanks for clarifying.
Quote from: Nworc on 20:38, 13 April 22I would not add sockets for chips which aren't socketed. I had the "bright" idea once to socket RAM chips in one of my computers, and problems started with that. In the end I threw out the sockets.
That's the opposite of what I heard as recommendations. It's usually said to always do sockets e.g. to avoid desoldering again if the new chip is also faulty.
Quote from: Nworc on 20:38, 13 April 22I would not add sockets for chips which aren't socketed. I had the "bright" idea once to socket RAM chips in one of my computers, and problems started with that. In the end I threw out the sockets.
Wrong !
As a techno with 40 years of electronic Maintenance the above comments are more about the installation of the sockets than the function of sockets ????
Please put in sockets it helps with further repairs and diagnostics.
Keep Safe
Ray
I saw another discussion on this forum where the CRTC was broken and threw wrong addresses at the bus. It still produced an image, but somehow garbled. Not sure if that could be an option here, as it's such a weird, logical pattern (as mentioned be gerald), but if the CRTC or something else throws complete garbage at the address bus, then the CPC could still work, we would just see the data at the resulting locations.
Maybe a bit simple, but have you tried to press DEL and check if you hear a beep? (volume level high of course).
Quote from: eto on 08:26, 14 April 22I saw another discussion on this forum where the CRTC was broken and threw wrong addresses at the bus. It still produced an image, but somehow garbled. Not sure if that could be an option here, as it's such a weird, logical pattern (as mentioned be gerald), but if the CRTC or something else throws complete garbage at the address bus, then the CPC could still work, we would just see the data at the resulting locations.
Maybe a bit simple, but have you tried to press DEL and check if you hear a beep? (volume level high of course).
Thanks for this. I'm at a bit of a loss as to what to test next to be honest—short of just buying ramdom ICs and seeing if they work. :-)
When I press the DEL key I don't hear anything.
I did notice today that when powering up two or three times, the forwards and backwards buttons on the tape deck function as they should. However, the play button sometimes works and sometimes doesn't. Can anyone think of a reason why that would be? Would fluctuating power, or insufficient power, produce this behaviour?
I'm personally thinking there is a power problem with this CPC but I've ruled out the power supplies; I've tried three. When the machine is not switched on power at the power connector is 5.1 volts but when switched on it drops to between 4.7 and 4.9; it seems to vary. Dropping would seem to be normal under load, but it seems on average to hover around the 4.75 mark and I know anything below this can cause problems.
Hoping someone can answer my above queries.
Suspecting the voltage may be an issue, I decided to test what happens to the voltage when removing certain ICs one by one. I'm getting 4.75v at the CPU, RAM, etc usually. When removing memory ICs, the ROM and the CPU the voltage stayed the same. When I removed the Gate Array the voltage jumped up to 4.92v—which I gather is healthy.
Does this suggest the Gate Array is faulty—or is this normal behaviour?
I do have a CPC 6128. Are the Gate Arrays backwards compatible? Could I swap it over to check if the Gate is the issue?
Thanks!
I've never measured the difference, but removing the Gate Array pretty much stops all activities on the entire board which would lower the current. However, that's quite a difference. Are you sure your PSU is able to supply the required current?
The 40007 and 40010 are not pin compatible, so you will need the correct part number for your 6128. If your 464 has the same GA you can swap them to check.
Bryce.
Out of curiosity I wanted to find out how low you can go on the 464, using a laboratory power supply, measured using a Voltcraft VC404 at the connector (voltage drops by 0.03V when turning the machine one, voltage measured under load):
The 464 boots stable at a voltage of 4.02V and above.
Between 3.90V and 4.02V is the range of free demos, a different one per different voltage, boot message still visible.
Below 3.90V I could not get a boot message.
These figures will be different from machine to machine, but I think it's obvious to see how stable the 464 runs even if the PSU is not ideal.
In the demo voltage area once I experienced the clicking of the cassette motor relais, another time I could observe the vertical stripes shown in this thread, so it is possible to get the component to fail at a too low voltage, I mean that component which fails regularly in the board mentioned here.
Quote from: Bryce on 10:24, 25 April 22I've never measured the difference, but removing the Gate Array pretty much stops all activities on the entire board which would lower the current. However, that's quite a difference. Are you sure your PSU is able to supply the required current?
The 40007 and 40010 are not pin compatible, so you will need the correct part number for your 6128. If your 464 has the same GA you can swap them to check.
Bryce.
Thanks for getting back to me about the Gate Arrays. I'm pretty sure the 6128 has a 40010, but I'll double check.
I decided to take out the GA, give it a good clean and remove the original metal heat sink. It was getting in the way of checking memory ICs, so I replaced it with a modern heat sink. Looks good and seems to work fine. The problem still exists though.
I'm starting to think the voltage may not be the issue. I have acquired an Amstrad Diagnostic rom and it's produced some interesting results. https://photos.google.com/share/AF1QipO5iI8JGaMO9iX7d_FN6-ZSRCcRjpGg9ddKLWcdA-2QAOBw_U1Qr9N4luPSxpFjDQ?key=eVhVb3VldG1NLTQ1bzlFeFIzVHpNZEUwb0pvTlNR (https://photos.google.com/share/AF1QipO5iI8JGaMO9iX7d_FN6-ZSRCcRjpGg9ddKLWcdA-2QAOBw_U1Qr9N4luPSxpFjDQ?key=eVhVb3VldG1NLTQ1bzlFeFIzVHpNZEUwb0pvTlNR)
The first image was the first test. It showed 3 good ICs at bit 2, 4 and 6.
I have experience of Spectrums, and I know these tests can show RAM faults when there's actually other problems. So, I deliberately moved the green (working) ICs to bit 0, 1 and 2 and they still showed green (see image 2). Does this suggest that these green memory ICs are actually good and the others aren't? I'm guessing so.
The thing is, I have a DRAM tester and I've checked all the original memory ICs and some new ones I bought, and they're all reported as fine.
I'm more confused now. :-)
Is it worth buying some more 4164 memory ICs, or could there be an another problem I'm missing?
Thanks!
Quote from: Nworc on 13:10, 25 April 22Out of curiosity I wanted to find out how low you can go on the 464, using a laboratory power supply, measured using a Voltcraft VC404 at the connector (voltage drops by 0.03V when turning the machine one, voltage measured under load):
The 464 boots stable at a voltage of 4.02V and above.
Between 3.90V and 4.02V is the range of free demos, a different one per different voltage, boot message still visible.
Below 3.90V I could not get a boot message.
These figures will be different from machine to machine, but I think it's obvious to see how stable the 464 runs even if the PSU is not ideal.
In the demo voltage area once I experienced the clicking of the cassette motor relais, another time I could observe the vertical stripes shown in this thread, so it is possible to get the component to fail at a too low voltage, I mean that component which fails regularly in the board mentioned here.
That's really interesting. Thanks. So it could still be the voltage that's causing one component to fail. Or, it's some other problem—such as the memory ICs (see my post above).
I have tested the machine with two different modern power supplies (built for the 464) and I also used an original working Amstrad colour monitor and the issue persists with all.
Thanks.
Quote from: jarcher1980 on 21:57, 25 April 22The thing is, I have a DRAM tester and I've checked all the original memory ICs and some new ones I bought, and they're all reported as fine.
Noel's retro lab just recently covered an issue where a ram chip was reported as working, but didn't work in a computer (also not in the CPC):
https://www.youtube.com/watch?v=riNLmstDAVA&t=1273s
Quote from: Nworc on 13:10, 25 April 22Out of curiosity I wanted to find out how low you can go on the 464, using a laboratory power supply, measured using a Voltcraft VC404 at the connector (voltage drops by 0.03V when turning the machine one, voltage measured under load):
The 464 boots stable at a voltage of 4.02V and above.
Between 3.90V and 4.02V is the range of free demos, a different one per different voltage, boot message still visible.
Below 3.90V I could not get a boot message.
These figures will be different from machine to machine, but I think it's obvious to see how stable the 464 runs even if the PSU is not ideal.
In the demo voltage area once I experienced the clicking of the cassette motor relais, another time I could observe the vertical stripes shown in this thread, so it is possible to get the component to fail at a too low voltage, I mean that component which fails regularly in the board mentioned here.
I don't think I'd trust a Voltcraft VC404 to measure voltages accurately. Your stated results are way too low to be correct.
Bryce.
Quote from: Bryce on 08:08, 26 April 22Quote from: Nworc on 13:10, 25 April 22Out of curiosity I wanted to find out how low you can go on the 464, using a laboratory power supply, measured using a Voltcraft VC404 at the connector (voltage drops by 0.03V when turning the machine one, voltage measured under load):
The 464 boots stable at a voltage of 4.02V and above.
Between 3.90V and 4.02V is the range of free demos, a different one per different voltage, boot message still visible.
Below 3.90V I could not get a boot message.
These figures will be different from machine to machine, but I think it's obvious to see how stable the 464 runs even if the PSU is not ideal.
In the demo voltage area once I experienced the clicking of the cassette motor relais, another time I could observe the vertical stripes shown in this thread, so it is possible to get the component to fail at a too low voltage, I mean that component which fails regularly in the board mentioned here.
I don't think I'd trust a Voltcraft VC404 to measure voltages accurately. Your stated results are way too low to be correct.
Bryce.
Any thoughts on the diagnostic rom results? I'd be keen to hear your thoughts before I buy some more 4164 ram. Thanks.
Quote from: BryceI don't think I'd trust a Voltcraft VC404 to measure voltages accurately. Your stated results are way too low to be correct.
Bryce.
I know, the VC404 is cheap, but I like the simple design. However, the Hioki validates the result:
Quote from: jarcher1980 on 21:57, 25 April 22I'm starting to think the voltage may not be the issue. I have acquired an Amstrad Diagnostic rom and it's produced some interesting results. https://photos.google.com/share/AF1QipO5iI8JGaMO9iX7d_FN6-ZSRCcRjpGg9ddKLWcdA-2QAOBw_U1Qr9N4luPSxpFjDQ?key=eVhVb3VldG1NLTQ1bzlFeFIzVHpNZEUwb0pvTlNR (https://photos.google.com/share/AF1QipO5iI8JGaMO9iX7d_FN6-ZSRCcRjpGg9ddKLWcdA-2QAOBw_U1Qr9N4luPSxpFjDQ?key=eVhVb3VldG1NLTQ1bzlFeFIzVHpNZEUwb0pvTlNR)
The first image was the first test. It showed 3 good ICs at bit 2, 4 and 6.
I have experience of Spectrums, and I know these tests can show RAM faults when there's actually other problems. So, I deliberately moved the green (working) ICs to bit 0, 1 and 2 and they still showed green (see image 2). Does this suggest that these green memory ICs are actually good and the others aren't? I'm guessing so.
The thing is, I have a DRAM tester and I've checked all the original memory ICs and some new ones I bought, and they're all reported as fine.
I'm more confused now. :-)
Is it worth buying some more 4164 memory ICs, or could there be an another problem I'm missing?
Thanks!
I have acquired 10 new 4164 ram chips (KM4164B-10). I have a tester and they're all reported as good.
When testing the system via the Amstrad Diagnostics rom - none of these new chips are showing as green. Some others chips I've had for a while (which are exactly the same make/number/timings) are showing as green. I even moved these 'green' ones to different IC positions and they show green in those positions too.
Is the CPC very fussy about ram chips, or could something else be going on? I can't keep buying ram chips.
Quote from: jarcher1980Is the CPC very fussy about ram chips, or could something else be going on? I can't keep buying ram chips.
Hm, I think you already received a couple of ideas on which chips to test that deal with the RAM logic, but what I see for now is that you have checked the RAM chips, so how did the test results for IC112/114/115 and the address line multiplexers turn out?
Quote from: Nworc on 19:50, 30 April 22Quote from: jarcher1980Is the CPC very fussy about ram chips, or could something else be going on? I can't keep buying ram chips.
Hm, I think you already received a couple of ideas on which chips to test that deal with the RAM logic, but what I see for now is that you have checked the RAM chips, so how did the test results for IC112/114/115 and the address line multiplexers turn out?
I've checked for continuity between the RAM chips and IC115 / IC114 and IC112 and it's all good. I haven't sourced replacement ICs yet as I'm not sure that's the problem; although that will be my next step.
I don't understand why Amstrad Diagnostics (by llopis) would report green for certain 4164 chips in any ram position (IC117 to IC1124) but red for others—despite all of the ram chips passing a dedicated 'ram tester' check. Only 1 ram chip failed the test.
@jarcher1980 Can you post a Photo of your Ram tester Please
Thanks
Keep Safe
Ray
Quote from: Audronic on 23:38, 30 April 22@jarcher1980
Can you post a Photo of your Ram tester Please
Thanks
Keep Safe
Ray
Thanks. It's this one. It seems to be working as intended as it detected a known faulty chip.
https://www.ebay.co.uk/itm/124726020732?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=HFzPW3iNSgu&sssrc=2047675&ssuid=&widget_ver=artemis&media=COPY
@jarcher1980 Ok Thats the one I use and it seems to be a good choice.
I just found 14 (Fourteen) Faulty out of a purchase of 20 - 4116 Memory chips with that Dram tester
Thanks for the photo
Good luck with the Project
Keep Safe
Ray
Just checking a few things periodically. I found this site which suggests that the Gate Array (40007) VCC2 should be reading 5v—just like VCC1. Can anyone confirm if this is definitely the case, as I'm pretty sure the last time I checked it was reading around 3.3v on my system
https://www.grimware.org/doku.php/documentations/devices/gatearray
Thanks.
did you finally find the fault?
I have exactly the same symptoms.