The current state of Firepower- after fixing a couple of broken connections and replacing all (ALL) of the fuses, most of which were horribly mismatched, Firepower boots! Sort of. One of the coils is most likely shorted, as evidenced by the knock you hear when you flip on power, the horrible smell, and the immediate failure of the solenoid fuse. The previous owner might not have noticed the problem because they had a 20 amp fuse in a 2.5 amp rated slot; however, they might have noticed if they had left the machine on for any period of time because the coil would’ve definitely caught on fire.
So now I have to fix it. There are a few things that could be wrong. The coil itself could be shorted- I’ll know this because I’ll measure resistance across all of the coils, and see if one is significantly lower than the others. There is a diode that runs across the two terminals of the coil. I should probably know what it does, but I don’t. I do know that sometimes the diode shorts, so if the coil isn’t shorted maybe it’s the diode. If that’s not the problem, then one of the transistors on the driver board (which controls current to the solenoids) could be fried, so I’d test all of those next.
If that doesn’t turn up anything, well, that’s bad news. The two remaining problems are that a chip on the MPU or driver board could be bad- at which point you might as well buy something like this. Or the 40-pin connector could be bad, which is common on these games. The MPU (game logic) and the driver board (controls power to the lamps/solenoids) sit on top of each other, and are connected by a 40 pin connector which is super fragile. You can see it in the below image- it’s the connector between the top (MPU) and bottom (driver) boards on the left.
So anyway, a solid couple of hours of ruling out various things that could be wrong, starting from the most localized problem (a bad coil, which would only affect one mechanism) to the most global (a bad chip or 40-pin connector, which would break whole game systems). Not too different from debugging software!