Recently Josh over at RetroTV1 Tech purchased a C64 that the seller said was in working order. He wasn't able to get it working so he sent it to me to see if I could repair it. He has put up a video showing what he did to try to get the system to work.
The system was very clean and in good condition.
The paper shield had some sort of corrosion on it which I cleaned up before returning the system.
The inside of the unit was also very clean. Josh had swapped the PLA chip with a modern replacement since that is a common point of failure but this did not resolve the problem.
I unsolder the lower shield to get a look at the back of the board which looked clean and had no obvious evidence of prior repairs.
When powering the unit up with the dead test cartridge it flashed the screen with a code the indicated a bad RAM chip. Since the RAM chips were soldered in I wanted to rule out everything else before replacing the RAM chip. So I started by swapping the PLA and the SID with a known good unit. I also tried swapping in the original PLA. I was getting very strange results during this process. It took a while but I eventually figured out what I was doing wrong.
Here is a picture of Josh's C64 with the locations of the PLA and SID.
Mine had a slightly different PCB rev and you can see here that the positions of the PLA and SID were revered. The chips have the same number of pins so I was mistakenly putting them in the wrong sockets!
With that worked out I was quickly able to determine the Josh's C64 had a bad SID and that his original and replacement PLAs were fine. Whatever was wrong with the SID was appearing as a RAM failure. I happened to have a spare SID in my chip collection, no idea where I got that from because I never did any C64 repairs, so I put that one in and that got the dead test working. Even though this SID allowed the system to work, I was getting really bad sound out of it so we ended up getting a modern replacement SID. We chose the ARMSID which has a really nice sound to it.
Next step was to remove the dead test cartridge and boot the system normally. When I did this I got just a black screen. I did some research and the most common cause is a bad Kernel ROM. I swapped that with the one in my system and confirmed it was the cause.
Here are a few pictures of that build process. You can also buy a PCB for this that makes the process a lot easier.
This adaptor worked ok, but Josh decided to get a modern replacement since it would be more reliable.
With all these problems worked out, the final step was to replace the electrolytic capacitors in the system. This is a great site for cap kits, the provide the kits and good instructions on how to replace them. When you buy a kit be sure to get the one that matches the version of the PCB your system has.
The system has quite a few electrolytic caps that need to be replaced. Be sure to read the instruction on the web site since the replacements are not all identical to the originals.
Josh will eventually be doing a video to show of the repaired system.
This is an excellent write up! Thank you for fixing my C64! Such a thorough job!
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