This is an old post, preserved for reference.
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This is something I've been meaning to get around to for a while, and testing the Commodore 128 has brought it to the front. Last year I built up a couple of Commodore 64 Diagnostic cartridges, the C64 Dead Test and the standard C64 Diag.
The first appears to be reverse engineered from an original test harness. This is slightly difficult to follow as all the wiring goes back to a 26 pin connector on the user port adapter, so you have to trace it through that. The second source also includes ROM images and manuals for the diagnostic cartridges. Zimmers.net contains a slightly different schematic and also the ROM image. Credit to all those guys for providing the schematics.