I burnt an EPROM with the beep test at slot 3 on the 27512 and installed with A15=vcc. Worked fine.
Next, I thought I would check A15 with the scope to see what activity there was. I looked at the expansion connector and found the wire which runs A15 to the 2.4 EPROM had come un-soldered. It was glued in place so it looked fine, but the connection had broken away!
It must have happened while I was handling the PCB de/soldering all the caps. So after all that, it looks like putting the caps in backwards didn't break anything. Phew! It was just me knocking a wire out.
Hehe, all good news and some fun experience tracking down what was wrong. There are actually 3 more caps to replace next to the modulator / power, then I have a fully electrolytic re-capped machine.
I hope some more non-working EP's come up on eBay, I think I could get the hang of repairing them with all of Zozo's tips in this thread alone...