Well shit – it seems as though something else went wrong with this scope while I was slogging though the ROM problem. It’s not the display board. We’re not even getting to the point where that’s being checked. It seems to quickly go into this vegetative loop pretty much immediately. I put back the old (bad) ROMs to at least see if I could get the ROM error pattern again, but nothing – just all lights lit.
Did some probing on the chip-select lines on the ROM card, and it’s clear that it’s going through some quick little loop, but it’s not always the same loop on each restart. I’m hoping to find a way to reset the CPU w/o cycling power. I think I can short TP1200-1 to ground, but waiting for someone to confirm so I don’t invariably nuke something else.
I confirmed that all power supply rails are spot-on.