So.... after a bunch of tracing down wires, voltages, ghost 'shorts', it turns out that we (buddy helped me find it) did have one busted diode - replaced with 1N4004 - on the first drop target set, had a few wires that re-soldered as they were hanging on by a thread, replaced every diode on every coil, but with Soleniod Drive 1-8 connected (aka 2J11 per the excellent Firepower schematics), each and every time the 2.5A fuse F2 would light up and burn out.
Testing traces, continuity of wires and coils, burning about 20 2.5A SB fuses (I knew this was going to be a slog so I bought a bunch of them) - and like a big baby throwing tools on the floor not once but twice - I pulled the Solenoid/driver board off. Examined each pin of the 'replaced' 40 pin female .156 pitch connector - they look like they were 'new old stock' parts, with original molex single contact points. Cleaned those contacts and more importantly (I think) cleaned the contacts (round pin - original) on the reworked MPU board. Plugged the *ast*rd back in, powered on, and the damn game started up. Sound. Displays. Lights, both GI and computer controlled, in attract mode, started working.
Several reboots, one reboot the displays didn't work, disconnected/reconnected master display 1J5 from MPU board, reboot, displays work. Then rebooted again after a bit of game play, and sound wasn't working.
Disconnected/reconnected 10J1-10J3 from Sound board, reboot, still no sound. Disconnect/reconnected 10J1-10J3 again and sound worked as expected.
Root cause I do believe now is that 40 pin .156 pitch connector off of the MPU board. The other items - diodes, wires, cleaning coils and sleeves - I'm certain helped. I KNOW the 8A circuit protection from the bridge rectifiers (the right - or lamp rectifier was replaced btw) helped to save my game.
Not done yet, but the game works now. And I'm very thankful to all who helped me.
@mitch @schwaggs @zacaj @vid1900 @cheddar @barakandl
Now I know why it's hard to part with a game you've spent a good deal of time with I must have 60 hours in this damned thing.