I have a Monte Carlo and the first thing I would do the ground mods . the second thing would probably be get a chip quick kit and remove the daughter board, the solder joints that go bad are below the brown pin connectors on the underside of the board. the pins gottlieb used on that board are to big for the main circuit board and the traces are almost hair thin that's why in my humble opinion chip quick is the only way to fix the problem without destroying the traces. youtube has videos on how to use chip quick. More then likely C1 2 and 3 on the display board are bad ....c2 and c3 are supposed to read 10uf I bet they are not even close , I d be willing to bet C1 doesn't read 470uf either. its common for the driver board resistors to be slightly brown mine looked like that but they read correctly I replaced them anyway. Tackle the MPU first and then work the sound card ....The mpu is probably not sending the reset to the sound board thus the solid light. Reflow the connections on the 5V power supply and make sure the resistor can be set so it outputs 5vdc usually set mine (R3) to 5.01 vdc. if your mentor has a eprom programmer have him verify the eproms . I bought one because I have three sys80B games, they run about 120 dollars. Its a great player you get it running and it you will definitely find it entertaining

just an idea of how I do the mods ,seems to work good. The terminal strip in the head allows for easy board removal

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yep the 5 v supply is hard to see but it is soldered directly to the ground trace on the 5v supply circuit card and there is a wire that goes directly down to the main power supply .....you can use the yellow braid at the bottom right of the head if all the connections are checked and tight

It’s a fun game. Not fast but challenging</>
mine is, The guy I bought it from put hot flipper coils and led lights in. I just wish the damn ball would stop hitting the glass ...mine flies but i have no intention of changing it. He put green led's in the bumpers which is a really nice effect

I have mine set on 3 balls and the 10 mil shot on for every 40 switches cause its a blast when you make it and the machine lights up like a Christmas tree