Commodore 64 (Breadbin) repair

A rather sorry looking C64 early breadbin model, I've had this for a while and finally decided to have a look at it. First thing I had to do was replace the missing ram at U21 with a socket and 4164. After powering on there was garbage on screen, so still a bad ram or two to find.

This is where my previously made test cartridges came in handy (dead test and diagnostic), first I tried the diagnostic cartridge but the text on screen was too messed up to show anything useful. Next I used the dead test cart and the screen flashed five times, the manual states five flashes is a bad ram @ u10. So I removed u10 and replaced it with a socket and new 4164 ram, this fixed the garbage screen and the familiar blue screen was now visible but it isn't quite sorted yet.

There was a scrolling rainbow colour effect across the whole screen, doing a quick google for 'color' ram shows that the 2114 @ u6 is the colour ram. I removed and replaced this again with a socket and new ram and the colour problem is fixed. I thought this was the end of the repair but after letting the diagnostic cartridge run fully (it was getting stuck at color ram before) it showed that char rom was bad. This rom has the same pinout as a TMS2532, luckily I had one and the GQ-3X was willing to program it.

 

The eagle eyed will notice who ever looked at this previously marked all the working chips with a yellow dot. The character rom didn't have this mark, so who ever it was knew it was bad but obviously didn't have a TMS2532. :)