Author Topic: Amstrad 6128 - grey screen fix and keyboard not respond  (Read 745 times)

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Offline periband

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Hi I' new to the group (and some time playing with Amstrad computers)


I will explain my case.
I have an Amstrad 6128 which some time ago was pugged to wrong voltage by mistake.
After that appears the typical grey screen.

I have another fully working Amstrad 6128 which I can use to verify some components.

1st - Tested the cpu (z80) , Gate Array (40010),  PAL (40031) in the working 6128 and everything works
2nd - And take the same chips and placed to the non working 6218 and grey screen remanins.

So,  z80, 40010 and PAL seems to be OK

3rd  After that I was removed the PAL and I configured with some cables to test memory banks (only bank 0 and only bank 1).
With bank1 computer starts OK (the amstrad plc ... starting message appears and also the prompt)
With bank0 computer with grey screen.

4rh - After that with help of M4 board I tested the ram with Ramtest.bin and I discover that chip IC134 was not OK.

At that point I want to point that when I did the test with bank1 the computer was working (and also keyboard was working fine)

5- Then I  had unsolder and replaced the ram chip 134 with another socketed one.

After that change here is the situation:

6 After that I was removed the PAL and I configured with some cables to test only bank 0 and only bank 1.
With bank1 computer starts OK   (the amstrad plc ... starting message appears and also the prompt)
With bank0 conputer  starts OK  (the amstrad plc ... starting message appears and also the prompt)
After restoring the PAL also the computer starts OK.

So the problems with the memory seems has been fixed.

The problem now is that keyboard does not responds.
None of the keys works , also the joystick not responds. and no sound pressing DEL.

I have connected the keyboard to the working cpc 6128 and the keyboard works fine , so the problem is not the keyboard.

Seems I did some mistake in the process unoldering/soldering the ram , but now I'm not sure how to follow the problem.

I have seen lot of posts with problems in the keyboard and seems can be related to chips AY-3-8912A , 74LS145 , and 8255 (but all of them typing keyboard displays something at least).

With a Multimeter I have checked the voltages of AY-3-8912A , 74LS145 and 74LS132   (measuring +Vcc -GND in that chips)

For AY-3-8912A is 4,9 - 5v
For 74LS132 is 4,9-5v
For 74LS145 is aprox 0.5v (not sure if this is ok)

Is there any way to verify if this chips are OK? before to try to replace?
Maybe the problem is not in the IC's and some short-circuit in another point of PCB done during the soldering proces ¿?

At that point I'm not sure how to follow.

If somebody has experienced similar problem or can help-me to identify possible next steps I will appreciate. I'm litle lost now.   :(
Thanks in advance

Offline Bryce

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Re: Amstrad 6128 - grey screen fix and keyboard not respond
« Reply #1 on: 13:41, 28 November 19 »
It's about a 99% chance that the AY has failed. Not because you did anything wrong when swapping the RAM, it just failed when the over-voltage occured. Swap it out and you'll most likely have a fully working CPC again.

I'm pretty sure you've measured the voltage wrong on the 74LS145. Either the wrong pins or one of the power pins is disconnected. If it was really at 0.5V the CPC wouldn't power up at all.


Bryce.

Offline periband

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Re: Amstrad 6128 - grey screen fix and keyboard not respond
« Reply #2 on: 14:10, 28 November 19 »
Hi Bryce and thanks for the quick response.


I'll try to obtain new AY and replace it.  (And cross fingers expecting this fix the issue  :laugh: )
Regarding , 75LS145 is possible I had taken wrong measure ,(I will double check  if I measured the wrong pins ...
I'll keep you posted with the the results. And again thanks for the help.

« Last Edit: 18:57, 28 November 19 by periband »