[ajgriff]

Quick update on progress such as it is. Seem to have cured the black bar problem by cleaning the connectors inside the monitor. Fingers crossed.

Checked the output voltage on pin 2 of the inverter at start up and it stays low for a couple of seconds before rising to a steady 3.7V. This is roughly in line with the SNLS04 datasheet so I think the MPU's timed reset circuitry is working properly. For reference I'm attaching a cropped image of the timer circuit and PDFs of the schematic plus parts list for this particular PET 2001 version.

The only other thing I've noticed is that each time the machine is started the random character screen is very similar but not identical. Don't know whether or not this means anything. Anyway next step is to remove the motherboard and start pin/socket cleaning.

Alan