The famous grey screen black frame issue.

[oyshals]

Hello!

I'm new to this forum, based in Norway, and have recently pulled my old Amstrad CPC 6128 out of storage (very poor storage).

I am having the grey screen, black frame issue, and have been reading up on it on this forum, and watching some videos by Noel's retro on Youtube. BTW, no beep when holding "DEL" button.

I have checked the following:

I tried the trick with removing PAL/FAL chip and forcing second RAM bank, as described in this forum, with no change.
Is it possible that I'm having issues with both banks?

I've tested the clocks with a multimeter, finding the clocks to the CRTC to be 1Mhz, the Z80 to be 4Mhz and the Gate Array (40010) to be 16Mhz as described in another thread in the forum. All seem to be correct.

I checked voltages on all chips, and they are currently at 4,6V, maybe a little low? I am using the original monitor as power supply, and tested the plug and found approx 5,1V.
However, while testing the voltages on IC114, IC115 and IC116 (gate array) I found the voltage to be only approx 2,6V. I'm not sure if i tested the correct pins on these though.

I also used a thermal camera on the board. With the camera, I could see that the temperatures were quite even on almost all chips, approx 30 degrees C. However, the Z80 showed a temperature of up to 60 degrees C (maybe over, depending on my camera), the PAL/FAL and the CRTC showed approx 40 degrees C.

I've ordered a new Z80 as these were quite cheap on Ebay, and found it "nice to have". Considering buying 16 new RAM chips with slots.

Today I fiddled a bit with the power supply plug from the monitor (5V), due to bad contact, maybe due to a bit corrosion in the plug, and sometimes the the grey field in the screen turned to small blue and yellow squares. This made me a bit suspicious towards the power supply also. Maybe I should buy an external one?

As far as I know, the computer has never been subject to over voltage. It as only been used with power supply from the monitor. However, it has been stored in a pretty rough enviroment, but there arent any visual corrosion.

I am hoping for tips on things I can check while waiting for my new Z80?

I've also seen that some of you have this "component" you plug in on the back of the computer (M4??) with a program to test the RAM banks. Is this unit with the program for sale somewhere? In that case I would appreciate a link.

I apologize for a messy post, and maybe some bad english.

I have some education and experience in soldering and house/ship electrics, but my experience in electronics is somewhat limited. However, I'm a fast learner.

I would really like to get this repaired, as it was a christmas present from my parents in 1988/89, and now I want to show it to my step-children.

Any tips would be appreciated, thank you!

[RetroCPC]

Try to bypass the CPC6128 Power Switch (just jumper it on the PCB) - I suspect the reason for your low voltages...

[oyshals]

Try to bypass the CPC6128 Power Switch (just jumper it on the PCB) - I suspect the reason for your low voltages...

Thank you for your answer, I tried that now, but sadly the same result. After removing the connector for the on/off swictch, I also measured the connector on the circuit board, and it read 5,1V, so the voltage issue seems to be "later" in the circuit.

[WacKEDmaN]

blue pixels on gray screen suggests one or more 74LS153 MUX chips is bad...

the "component" youre looking for is Bryces Lower ROM board..with diagnostics loaded into the rom...
it wont help with gray screen!.. it may show the memory as all bad even tho its fine and its the MUX chips causing the problem.. 

[oyshals]

blue pixels on gray screen suggests one or more 74LS153 MUX chips is bad...

the "component" youre looking for is Bryces Lower ROM board..with diagnostics loaded into the rom...
it wont help with gray screen!.. it may show the memory as all bad even tho its fine and its the MUX chips causing the problem..

Thanks, I located those chips. Are there any way to test these chips, by voltage, frequency (hz) or temperature?

The blue and yellow squares appeared only once by fiddling with the power supply plug, so the issue seems to be the grey screen with black border.

[WacKEDmaN]

even without the blue pixels.. just the gray screen... id still say one of the MUX chips...
theres no way to test them on board (unless you have an oscilloscope to actually check each line to make sure they are all outputing and the signal levels are correct)...

i forgot to mention, you can burn the diagnostics rom to a new eeprom, and replace the CPCs original ROM with it to test, instead of using the lower rom board..

[oyshals]

even without the blue pixels.. just the gray screen... id still say one of the MUX chips...
theres no way to test them on board (unless you have an oscilloscope to actually check each line to make sure they are all outputing and the signal levels are correct)...

i forgot to mention, you can burn the diagnostics rom to a new eeprom, and replace the CPCs original ROM with it to test, instead of using the lower rom board..

Ok, thanks, I will have to look into that then!

[Rabs]

Try to bypass the CPC6128 Power Switch (just jumper it on the PCB) - I suspect the reason for your low voltages...

Thank you for your answer, I tried that now, but sadly the same result. After removing the connector for the on/off swictch, I also measured the connector on the circuit board, and it read 5,1V, so the voltage issue seems to be "later" in the circuit.

Have you found the drop in voltage? When I use a new CPC for the first time, I use a current limiting bench power supply just incase there are any shorts. I find the service manual and the circuit diagram very useful.

[oyshals]

Not yet, off to work now, will continue tomorrow. Is 4,6v to low for the computer to run? I'm also curious about the IC114, 115 and 116 which I measured to 2,6v, there might be something there. I just hope it isn't IC116, the gateway arrey, i understand that one is irreplacable!

I have a lot of reading to do in the manual, but correct voltages has been difficult to find...

[Rabs]

Not yet, off to work now, will continue tomorrow. Is 4,6v to low for the computer to run? I'm also curious about the IC114, 115 and 116 which I measured to 2,6v, there might be something there. I just hope it isn't IC116, the gateway arrey, i understand that one is irreplacable!

I have a lot of reading to do in the manual, but correct voltages has been difficult to find...

I guess 4.6V is ok and the fact that you have measured a clock from the GateArray indicates it at least is running. I Think VDD pins are 6 and 25 on the GA. ICs 114 and 115 are standard 74 series so should be pin 20 for Vcc and 2.6v sounds too low but check your connection onto the pin just incase.

[oyshals]

Not yet, off to work now, will continue tomorrow. Is 4,6v to low for the computer to run? I'm also curious about the IC114, 115 and 116 which I measured to 2,6v, there might be something there. I just hope it isn't IC116, the gateway arrey, i understand that one is irreplacable!

I have a lot of reading to do in the manual, but correct voltages has been difficult to find...

I guess 4.6V is ok and the fact that you have measured a clock from the GateArray indicates it at least is running. I Think VDD pins are 6 and 25 on the GA. ICs 114 and 115 are standard 74 series so should be pin 20 for Vcc and 2.6v sounds too low but check your connection onto the pin just incase.

Thanks, I'll focus on that area tomorrow while waiting for parts. I will also be ordering som MUX chips as @WacKEDmaN suggested, as they were very cheap. Thanks for the help so far everyone!

[oyshals]

I found something. One of the capacitators for one of the ram chips looks damaged. Som of the insulation on top of it looks like it has snapped off. Could it be blown, and could it be that simple? 

[WacKEDmaN]

I found something. One of the capacitators for one of the ram chips looks damaged. Som of the insulation on top of it looks like it has snapped off. Could it be blown, and could it be that simple? 

possibly...if its shorted... remove it and see if ya voltage drop changes... ya should be able to test it fine without the capacitor.. its just for smoothing power...

[oyshals]

I found something. One of the capacitators for one of the ram chips looks damaged. Som of the insulation on top of it looks like it has snapped off. Could it be blown, and could it be that simple? 

possibly...if its shorted... remove it and see if ya voltage drop changes... ya should be able to test it fine without the capacitor.. its just for smoothing power...

Thanks! I'll give it a try tomorrow!

This might be a stupid question, it's been over 20 years since i had electronics at school; is it possible to measure of there is a shortage in a capacitator with a multimeter? It would be nice if I could check all of them if there's an issue with the one...

[WacKEDmaN]

you can check for shorts with multimeter.. continuity mode.. and probe both legs.. 
you cant test capacitors in circuit (to check its capacitance is correct)..you need to remove it and then test it ...if your multimeter can measure capacitance!
if you try to check capacitance with caps on the board, you are actually measuring the total capacitance of the board...eg all the caps and any other parasitic capacitance

[oyshals]

No luck, no shorts on the capacitators. I watched a troubleshooting video by Noel's Retro last night, he had 4,8 volts and said that was a bit low. I only have 4,6 volts, so I'll keep looking.

[Rabs]

No luck, no shorts on the capacitators. I watched a troubleshooting video by Noel's Retro last night, he had 4,8 volts and said that was a bit low. I only have 4,6 volts, so I'll keep looking.

It is a little low but you indicated you have a clock out of the GA. The 2.6V is more of a concern. Are you still sering this? I recently had a grey screen (total RAM failure). Turned out to be a dry joint on the power header connector on the PCB.

[Rabs]

As a rule with any new CPC  I always check for a stable voltage across the PCB. Also I clean the power switch and reflow power connectors to eliminate any dry joints both external and the internal PCB header connector.

[oyshals]

As a rule with any new CPC  I always check for a stable voltage across the PCB. Also I clean the power switch and reflow power connectors to eliminate any dry joints both external and the internal PCB header connector.

The 2,6v is no longer a concern, turns out i measured the wrong pins. So the voltage is now the same all over, 4,6-4,7 volts. I will find my soldering gear to try reflowing, I didn't think of that. I started to suspect that the old power supply in the monitor was starting to fail, and couldn't handle the load anymore (sometimes happens on larger electrical plants).

My Z80 is a bit hot though, approx 60 degrees C, but I'm thinking thats not abnormal for a CPU?

Still seeing clock signal from GA.

[Bryce]

60°C is too high for a Z80.

Bryce.

[oyshals]

60°C is too high for a Z80.

Bryce.

Thanks! I have a new one in order, also waiting for my paycheck to order new RAM, MUX, CRTC and GA, just in case. Better safe than sorry. Hopefully it's the Z80...

[Rabs]

My Z80 is a bit hot though, approx 60 degrees C, but I'm thinking thats not abnormal for a CPU?

Check if the Z80 is alive. I would check for clock input, pin 6, and then for activity on M1, pin 27.

[oyshals]

My Z80 is a bit hot though, approx 60 degrees C, but I'm thinking thats not abnormal for a CPU?

Check if the Z80 is alive. I would check for clock input, pin 6, and then for activity on M1, pin 27.

Thanks! I'll look into that, last I checked, I found the clock to be correct, the M1 I will check tomorrow.

Another stupid question: Can I do the "piggyback" thing on all kinds of chips, or is this only for the RAM?

[Rabs]

Another stupid question: Can I do the "piggyback" thing on all kinds of chips, or is this only for the RAM?

Sorry, not something I have experience of. I would always check the signal data when attempting to make a diagnosis as to whether an IC is faulty.

[eto]

Another stupid question: Can I do the "piggyback" thing on all kinds of chips, or is this only for the RAM?

As far as I understood, it depends on the type of defect. If the defect results in a short, piggyback will probably not work.