[Coco] seeking for help...

[Gene Heskett]

On Tuesday 12 February 2019 13:21:26 Camillus wrote:

> Hi, coco users,
> Anyone can give some hints on how to repair a coco2 that has bad sync,
> did all the classic chip swapping to no avail. Scope images are close
> to what I found in coco2 service manual, but the device I have is a
> Korean made, so no correct schematics. The coco powers up with a
> scrambled screen, I can see the square and some color stripes on the
> right side. Sometimes it comes up ok, even with BASIC logo, but after
> switching some ram and other Ic's the Basic does not show. Don't know
> I made things worse. Anyway got lost a bit so maybe some hints? Thanks
> in advance

Two candidates. That big filter cap (look at the 5 volt line with a scope 
looking for dips below 5 volts at 2x the line frequency) and bad memory, 
again best checked with a scope, look for one chip with a different 
waveform at its data i/o pin, its probably bad if markedly different 
compared to the same pin on the others.

Cheers, Gene Heskett
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