Quote:
Originally Posted by dominicbeesley
Fair play. It was just a thought
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Probably worth mentioning anyway, for the benefit of anyone who may come along in future, searching for help with a similar problem .....
Anyway, replacing the column decoder IC appears to have fixed one fault (key presses detected as wrong column) only to show up another (one row responding only intermittently). So, welcome officially to the world of vintage electronics restoration!
This behaviour positively screams "poor connection" -- something is making contact, but only just, and sometimes coming apart and going back together. It could be a cracked PCB trace or a broken wire, a dry solder joint or a fractured crimp, but it's most likely to be a mechanical connection -- a socketed IC or a plug and socket connection. Try probing about with a wooden cocktail stick to see if you can find where brings it in and out of contact. The schematic diagram will show you which connection the faulty keys have in common, and which connectors it passes through on the way to which pin of which IC.
It seems, from watching a few restoration videos, that the IC sockets on Commodore motherboards have a bit of a reputation for unreliability, and are best replaced with modern, turned-pin versions.