The microprocessor board is the lesser of your potential problems. Tricky is that the 3 ROM version of that board uses mask ROMs. There are no pin-equivalent EPROMs for those. I went down a rabbit hole with one of my 5342A, assuming that a ROM was faulty. In the end it wasn't.
Worth checking is the dipswitch on the microprocessor board. It is used to enable signature analysis mode. I found stories from people who had a bad switch, obviously that prevents the firmware from running correctly. I replaced the switch in mine, just because.
Wilko
