After years of neglect, I decided to play around with my bride (The Machine Bride of Pinbot). When I turn the power on I hear a loud zap sound that is coming from the back box (presumably the speakers?). It is loud! Zap! After the zap, the machine boots up as normal, however, it displays the Factory Settings Restored message on the DMD. When I try to use the direct switches on the coin door to adjust the settings, none of the the four switches work. None of the wires are disconnected, burnt, or show signs of problems. I unplugged all of the molex connectors from the board on the left wall under the playfield and that did not help. I opened the back box and reset all of the plugs and ribbon cables and that did not help. I removed the J205 connector from the mpu and attached a wire to the ground harness of the back box and touched each pin of the connector. Nothing.
The mpu LEDs light up as if it is a working board (LED 20 blinks and LED 21 stays lit, while LED 19 goes out after start up). I removed the mpu board from the back box and gave it a visual inspection. The sticker on the board has a date from 1994, so it is a replacement mpu board (TMBOP was built in 1991). I did notice some greenish white crust on some of the legs of the chips directly under the battery, which I lightly scrapped off. All the pins of the chips are firmly soldered in place by the factory. No repairs were ever done on the board. Other than the light crust on the chips' legs, the board looks like new! The batteries inside are new and there is no sign of previously leaked batteries. A few years ago the machine worked fine and I managed to get all of the high scores on it!
What is causing that loud zap sound on start up? Would replacing the mpu board resolve this issue? Thanks.