Amstrad CPC 464 - Screen with Garbage

[jarcher1980]

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!

[gerald]

Most likely a RAM issue.
I'would first re-seat ROM / RAM on socket

[jarcher1980]

Most 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.

[jarcher1980]

I've seen some 4164 ram chips on ebay and the speed is 100ns. Would this be fine with a CPC 464? Thanks!

[gerald]

Do 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.

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.

You need working RAM for this. The delete trick only hint you of a bad video connection.

[Bryce]

Do 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.

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.

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.

[jarcher1980]

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!

[Bryce]

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.

[jarcher1980]

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.

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.

[SerErris]

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

[jarcher1980]

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

Thanks for the tip. 😊

[jarcher1980]

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.

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.

[gerald]

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.

[jarcher1980]

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.

Thanks gerald. Could you explain what you mean by 'mux'. I do have the pcb schematic for the 464 to hand. 

[jarcher1980]

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.

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

[jarcher1980]

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.

[Bryce]

Check and clean the power socket, but the power switch may also need to be opened and cleaned.

Bryce.

[eto]

but 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).

[jarcher1980]

Check and clean the power socket, but the power switch may also need to be opened and cleaned.

Bryce.

I will try that, thanks. 

[jarcher1980]

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

Perhaps it is the power socket after all.

[Bryce]

It should always be 5V at that position. Either the socket is still very dirty or there is a break in the circuit.

Bryce.

[jarcher1980]

It 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.

[Bryce]

If the voltage is fluctuating it would suggest that the powersupply is struggling to supply the required current. What PSU are you using?

Bryce.

[jarcher1980]

If 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?

[Bryce]

Anything below 4.75V could make the computer unstable.

Bryce.