In my last post I repaired the mainboard of an old Commodore 64 I had retrieved from the attic. After replacing quite a few components, the board was working again for most practical purposes. One issue remained to be investigated though: under certain circumstances, some characters on the screen would change their color seemingly at random.

Random characters changing their color.

This was already visible in the diagnostics output but turned up again later in a graphics adventure I tried. Besides the VIC chip itself, the static color RAM U6 or the 4066 quad switch logic IC U16 seem likely causes for this problem. Continue reading →

Commodore 64 “breadbox”.

Recently, I retrieved a Commodore 64 “breadbox” version from my parent’s attic that was given to me by a friend in the early 90s. As I recall, it wasn’t fully functional even back then. It would halt as soon as a program tried to play any sound. But when I connected it now and powered it up, all I got was a black screen.

The broken ASSY NO. 250407.

I took it apart completely, even the keyboard, in order to give the case and all the mechanical parts a good cleaning. Then I went on to see what I could do to fix the “ASSY NO. 250407 REV.B” motherboard that I found inside. Continue reading →