I repair a Canadian (made in USA) black screen Commodore C64c that was kindly donated to me recently. Turns out to be a simple but interesting fault (no spoilers)... I also replace a key stem that was broken off.
Here's some handy resources I refer to when troubleshooting and repairing C64s:
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