Finally put together a Black Rose that had been sitting non-working for at least a decade with previous owner, so unknown condition. Rottendog MPU (looks new, but no way to tell how many years old it is), rest of boards seem original. Prior to turning it on I went through checking voltages, etc at every board, plugging in minimal things each time and rechecking, to make sure it was alright, and everything checked out. Connected new batteries, booted it up, got the factory reset message (expected), turned it off and on, adjustments were retained, everything seems fine. Checked coils, all fine, checked flashers, all fine, went to all lamp test to see what sockets were bad. While making a note of what lights weren't on, when I heard a very loud pop / bang from the backbox. Couldn't tell whether it was from the speakers or something else. Immediately shut the game off, opened backbox and did a visual inspection of all the boards, but didn't see anything. Checked various areas to see if anything was heated up, but didn't find anything. Checked all fuses, but they were fine. Turned it back on, everything still works fine, but I got the factory reset message again. Turned off and on, and the message went away. Don't know much about WPC, is there anything anyone could think of that could have caused it, or that I could check? I'm out of ideas besides 'ignore it until it happens again'...

No smoke or burnt smell (though I burnt something earlier on the machine next to it so if it was subtle I wouldn't have been able to tell). I sniffed around in the back box and no area stood out. Do you think a capacitor could make a sound like that with no outward appearance of damage? (they all looked normal)

All lights are working now, I fixed the only problem. Will do another coil+flasher test to see if one of those blew.