tkc8800

Technology and retro computer blog

Commodore 64C repair

A couple of months back I picked up this C64C for $10.  I suspected it wouldn't work but the case and keyboard were in very good condition, so if nothing else I though it would make a good parts machine.  On powering it up I got a very snowy picture, no colour and no basic prompt, but the typical C64 border could be seen, so there was an indication that something was working.


    


I took it apart and tried re-seating all the removable chips, there wasn't many as most were soldered.  Re-seating chips made no difference.  I eventually tried inserting a cartridge and that worked, although the snowy video was still there.  So at first I figured the problem must be the ram.  I bought some replacement ram chips and tried the piggy back trick but it made no difference.  I've got a C64 breadbin that I use for parts and most of the major chips in that machine are socketed so they can be easily removed.  I piggy-backed U3 (Basic ROM - 901226), from the spare machine and I got the Basic screen and prompt.  I entered some Basic commands and loaded a game using the SD2IEC and everything worked fine.  I de-soldered the bad 901226 and replaced it with a 24 pin socket and put in the replacement Basic ROM.


             

There was still the problem of the snowy video.  After doing some reading online I simply adjusted the pot near the VIC chip and this fixed the snowy video and colour.  See green pot in the top right of the square in the pic below.


           


There's still one problem with this machine, the audio output is very low.  I only get very faint sound through the TV.  I've checked the SID chip it's not that, so I suspect it may need some capacitors replaced.  Another C64 resurrected from the dead.


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