While I am in no way a professional repair person, I have gained some experience with repairing Commodore 64s over the last couple of years doing it and thought it would be a neat idea to collect some of my resources here. The C64 is a great machine to start getting into repairing because it is incredibly well documented and there’s a lot of great sources to get information from for it these days.
I’ll expand whenever I have the time.
Here’s some handy resources to get you started with C64 repair:
The Pictorial C64 Fault Guide is here: https://www.pictorial64.com
Ray Carlsen’s troubleshooting and repair guides are here (mostly they can be used without any special tools/test equipment): http://personalpages.tds.net/~rcarlsen/cbm/c64/
You can get the .bin files for various C64 diagnostics cartridges from World of Jani (in case you want to make your own or try in an emulator): http://blog.worldofjani.com/?p=164 Basically, there are two main variants: the Dead Test which works without any additional hardware and overrides most of the chipset to still give you a reading (screen flashes for indicating RAM faults) and the Diagnostics cartridge which is meant to be used with a diagnostic harness on mostly working C64s to test all the ports. It also can be useful without any extra hardware but it will indicate some faults if you don’t have the (correct) harness connected (see below).
Here’s Sven Petersen’s page with info about the Diagnostics test harness, all files to make your own are available on his GitHub (for use with the C64/C128 Diagnostics Carts): http://tech.guitarsite.de/c64_diag_harness.html
Sven also made a very handy reference guide with oscilloscope screenshots from a working C64 to compare the readings with machines you are working on: http://tech.guitarsite.de/c64_scope.html