Cpc 6128 yellow screen

[Simojohannes]

Hi

I have a problem with my old cpc6128. When power ON (power light works), appears only bright green screen on GT65 monitor. When power OFF in cpc, monítor has a dark green screen.

Keyboard doesn't respond or make any sound by pressing DEL or other buttons. Motherboard looks fine, but temperature of RAM chips IC123-IC126 and IC131-IC134 rises way too high.

This fault occurred years ago, so I can't be sure anymore if there has been possibility to over-voltage. I used CPC with MP-2 modulator back then and I remember that the picture on TV screen was a bit of too blurry and the modulator got heat before it died. So I don't know could it been caused also this damage?

Do I have any realistic chance to fix this computer or is it just a waste of time?


Sj

[Bryce]

Hi and welcome,

First off, yes it is repairable.
If DEL makes a beep, then the OS is running ie: the RAM is fine, CPU is fine, the Gate Array is fine and most of the AY is working. Over-voltage is also unlikely. Check the brightness control on the GT65, then check the monitor socket on the CPC for dry joints or dirty terminals.

Bryce.

[Simojohannes]

Hi and thank you!

Sorry my bad writing. The situation now is that only signs of life in cpc is working power light and blank bright screen. Pressing DEL or any other button doesn't respond any beep sound.Visually motherboard looks fine. For some reason, 4 lowest RAM chips in both memomy banks overheat. Power supply is ok.

So I was wondering that could this kind of problem be solved by just replacing those overheating memory chips, or is it clear (considering these symptoms) that there is also other damages in CPU, AY etc.?

Sj

[Bryce]

Without doing further testing it is difficult to say. The damaged RAMs are enough to stop everything else from working, whether the other chips are good or not.

Bryce.

[Simojohannes]

Ok thanks! 

That's what I wanted to hear. Now I have motivation to learn to find out what's wrong. Looks like there's lots of good information in this forum so hopefully I manage to get further with this project.

[Bryce]

Let me know what knowledge and equipment you have so far, so that I can guage what test etc I can suggest.

Bryce.

[Simojohannes]

I have basic skills in digital technology, thanks to former studies of electrical engineering. On the other hand I don't have experience about fault finding and repairing these "old" CPCs.

Today I work in a whole different industry and I have only basic tools at home:-multimeters-jointing tools
-power supplies and other old and new pc stuff

However it's of course possible to buy some necessary equipments if needed and the price is reasonable. There could be use for those also later.Probably the easiest way is to find working CPC but that's not the point. 

Sj

[Bryce]

A RAM issue is most likely. Are only the bottom 4 chips of each bank getting hot? Or are IC127 - IC130  and IC119 - IC122 also getting hot?
If you are able to replace chips, then you'll need to source 16 RAM IC's and sockets and swap all of these out.


Bryce.

[Simojohannes]

Only those 8 (IC123-IC126 and IC131-IC134) are getting hot.

So next I'll source new chips and sockets and swap them. I was wondering does it have to be exactly the same chip model (M3764-20RS) or are there also other chip models which work fine?
Sj

[Bryce]

3764 are compatible, but you shouldn't mix RAM types within a system, so you would be better to get 4164 IC's. They should cost about the same anyway and are easy to find.

Bryce.