So I was playing my Bally Playboy with no problem, and then I turned it off to replace a bulb, and then turned it back on. I got a nasty sound from the speaker for a seconds and then turned it off. After turning it on again, I didn't get that nasty sound anymore, but the game won't start. I haven't taken the backglass off to look at the MPU (but will do so, of course). The electronics are AllTek (MPU, Solenoid, Lamp) and NVRAM (Sound).

I should say that I have been having an intermittent problem with the game not starting at power-up. I had had a bad display the last time, and after I removed it and reset the MPU, it worked. I was getting 2 then 4 flashes of the MPU when it was not working. Before removing this bad display, I had been having a problem with the pin not starting up on the first turn-on, but it would start up after doing this a few times. The current displays (#4 is just missing now) are all working fine.

Did a quick test with the connectors (J4 always being plugged in), and it doesn't start up when J1 is plugged in. Interestingly, when J2 & J3 are plugged in, there is the start-up melody, but the timbre was different than usual.

The 20A fuse for general illumination was blown, so I replaced it.

With only J4 plugged in, it powers up, but with the sound timbre different, but the credit button does not start a new game. Also accidentally, I noticed (i.e., with only J4 plugged in) that the coin door, which sometimes comes loose by a few mm, if pushed all the way back to where it should be causes the general illumination lamps to get slightly dimmer, and the coin slot label lamps to get significantly dimmer - and this behavior is observed even when all J plugs are in. This comports with the fuse blowing.

I shall investigate the general illumination wiring with respect to the coin door

OK, I replaced the coin label lamp sockets, and everything has been working fine!