[Mark1960]

I took a quick look at the schematic Alan posted. If my guess based on the symptoms was correct the places to look would be as follows.

LSD5
F10 pin 23 (on the 6316 character generator, fingers crossed that IC is OK)
F9 pin 15 (74ls373 very common easy to find)

SD5
F9 pin 14
F7 pin 13 (2114 RAM, getting rare now)
E7 pin 4 and 5 (74ls244, not too rare)

Less likely is BD5 as I think you would not get the start up message displayed
E7 pin 15 and 16 (74ls244)
E10 pin 15 and 16 (74ls244)
I11 pin 13 and 14 (74ls244)

If you aren’t able to get access to a scope and decide to try swapping parts then I would suggest starting with E7 and then F9 before F7.

It would be worth getting a few spare 74ls244 if not too expensive, as they seem to be a problem in PETs based on Colin’s PET marathon repair.

If you do decide to start removing ICs the I would recommend considering the pcb itself as the most valuable part, consider the ICs disposable. Remove them by cutting the legs close to the body of the IC, then remove the legs one at a time with a soldering iron, then clear the holes with desolder braid or a solder sucker. Take extra care with solder sucker, try to move the soldering iron tip away before you release the solder sucker, as the shock when it releases can hammer the soldering iron against the pcb and damage traces.