C070523-001

17/01/2021

This board came to me with white screen and the diag cart showed some RAM problems, however identifying and changing the chip on the at the relevant position didn't make the problems go away. The more this board was on the desk the more obvious it became that somebody'd already messed with it. All the HIGH TOS sockets were upside down for example. There were damaged components as well.
Continuing the tracking of the signals I found corrosion and cut track under one of the RAM latches. At some point the whole data bus went crazy with total random signals juming, so started to strip down and replacing almost every element that has something to do with the data.
At one point when completely removed the onboard ram there were some promising results with alternative ram, but sooner the image started to fall apart and the board bombed out. I put this board aside couple of times to have a think about what's next. At the beginning of this year after a new MMU and some other checks finally came back to life and it seems that now it is sorted for long term. Probably will keep this as test board since every key ICs are now in sockets.

This was a very tricky repair at then end, socketed almost everything, changed all bus resistors and many chips. Repaired tracks and somebody else's previous messing around.