- Jeff and anyone interested in c64 repair,
There was a laminated 8 1/2" x 11" card called the Commodore Diagnostician by Ian Perry that I use from time to time. Just bare in mind it's for Australian C64's so you have to transpose some of the chip IDs to their American equivalent (PAL --> NTSC). I found a version of this resource on the web:
This is sure to help get a person started with possible points of failure on any C64.
A few of my own tips
1) The c64 power supplies with no load will be well above load specs, that is normal..so if you test an unplugged supply bare this in mind.
2) Usually it's best to have a known working drive, cassette or modem, and video hooked up to the system when you're testing. Some diagnostics expect you to have a system with peripherals attached.
3) The blink codes from the disk drive will tell you a lot (when attached to the computer)
4) Try your c64 with a cartridge attached to the system. If it works with a cart and does not work without, that tells you the BASIC PLA or something in the circuit is bad
5) Most important - have a working C64 to compare with your sick one. This is extremely important if you have no schematic or docs about the computer to go by. Systematically compare and contrast probe points to locate where the two are different.
6) Consider this like Collosal Cave/Adventure. Get some graph paper and make a map of what you find, as if the motherboard were a cave complex...
Hope this helps you too.