Sep 282011
 

This is the last of RCMs C64s that was in need of repair.
I spent many hours on this one.
First issue was the screen was just black. I originally went down a completely wrong route with this as my logic probe was showing missing signals the MC4044 however looking at these pins with the scope showed they were indeed there just at a level the probe didnt like.

Turned out that the previous owner had got the Kernel ROM and the Char ROM switched! Switching these back brought up a screen full of coloured garbage.
With my homemade test cart inserted I eventually was pointed to a potential problem with RAM U9. Knowing how inaccurate this test cart can be I wasnt totally convinced but I pulled it anyway and it was indeed faulty.

I replaced this and everything came back normal minus a cursor. I switched the 2 CIA chips and the cursor returned. Fitted a new CIA and ran the tests again. Everything now passes except for the SID chip but I already knew this was dead.

Im glad this one is fixed, I put a lot of time into it.

 Posted by at 5:38 pm