old 80C31 circuit not working. EPROM data retention issue?
well, this was some time ago, but I did resolve the issue, and thought I'd come clean.

the EEPROM was fine. I read the data and compared the code with a good machine.

Eventually the issue turned out to be the HC273 latch. I can only imagine that the 1's at the input were just not visible on my scope - I should have checked with a logic analyser perhaps, or just changed the latch anyway (which I did eventually - and might have done sooner if it wasn't for needing to order it).

The upside was that the PROM reader I bought (50 bucks from China) also had a function to test 74 series chips, so having desoldered it I could at least confirm it was duff before ordering a replacement. Useful gadget!

Anyway case closed. 

(In older electronics, these HC/HCT chips are quite prone to failure as static protection then wasn't as good as it tends to be now. The fact that the chip faces the front panel might also have made me suspect.)