>> I have a complete service manual for the 2001 PET you can use.
My hunch is there's a screen problem, not a problem with the computer itself. When the video first went blank, there was the odor of something burning, I'm told. Maybe as simple as a cap in the monitor. So chasing down bugs among the motherboard might be a waste of time. I'd prefer swapping in a known-good screen, and/or swapping our screen into a known-good identical version of the 2001, and * then * start chasing the problem.