[SiriusHardware]

Regarding the damaged ROM, fixing or replacing that is now the priority because the system won't work at all without a working set of ROMs. One way to try to rescue it is to put it into a conventional 24 pin IC socket and solder what's left of the damaged pins to the contacts on the top side of the socket, then plug the IC+socket into the original mainboard socket. If it is only the narrow part of the IC pin which has fallen off, cut a couple of complete pins off any spare or scrap IC and solder them to what's left of the broken IC pins. In the long term you will probably need to get a Hitachi or Texas 2532 EPROM programmed as a replacement. You won't be able to carry out any of the steps below until you have repaired or replaced that ROM.

Assuming the supply voltages turn out OK, possibly the next thing to do is to measure the voltages on the video RAM data pins.

While a meter will only show the average voltage, that may be enough to show whether one bit is stuck high or stuck low because the chips are permanently selected and in read mode for the vast majority of the time. Even better if any bit identified as possibly stuck coincides with the bit Mark flagged up as potentially stuck.

After that, in the absence of a scope it may be necessary to remove the video RAMs UF7, UF8, and socket and swap them over to see if that changes the nature of the fault. This is something which can be tried without buying any new parts (with the possible exception of IC sockets).

If that exonerates the video RAM ICs (ie, no change when swapped) we can remove the video RAM and force various states on the SD0-SD7 lines to verify the operation of the video latch UF9 and the character generator ROM UF10.

The above suggestions assume that BrackenFix feels able to desolder ICs from double sided PCBs without damaging either the IC or the PCB.

Ordinarily, we would pursue a more diagnostic approach first but in the absence of a scope we are already almost at the point where the most pragmatic approach is to chip-swap. I'm assuming (BrackenFix) that you don't have a logic probe otherwise you probably would have mentioned it.