Had black screen on power up.
Tested the PLA, this was fine. The VIC has getting really hot, replaced and system powered up but only showed 796 bytes of RAM free.
Using a homemade diagnostic cartridge, I found that a RAM at U11 was to blame. I pulled this and tested it but it passed. I set the tester to “PASS LOOP” which will rerun the RAM tests as long as the outcome is a PASS. This chip was now showing failures after a few cycles.
I replaced the RAM and retested, all RAM is available once again.