Hi! I hope someone familiar with system 3 boards or boards in general could give me some insight on what might have caused this catastrophe :O
Short version:

I picked up a Lucky Seven (ss),cleaned it and made sure all fuses were replaced with correct value. It booted and seemed to work except the different reel combinations did not light the corresponding lights (only for a couple combinations), and dropping the targets did not award the value. I cleaned the reel circuit boards with some rubbing alcohol, without much change. Upon troubleshooting with the game in-play, suddenly i notice (and heard) the center reel coil was being locked on by itself. I pressed a switch to advance it, and it got unstuck. But at that time it was REALLY hot. I turned off machine and let it cool down. Then i turned on and started a game to trouble shoot. Now the lights were all wonky, blinking or being on when they shouldnt, and the coil at intervals locked on again. Thats when we started smelling burned electronics. Turned off machine, opened the backbox and HOLY CRAP, all of the...resistors (?) / Capacitators on the MPU board was literally black and melting, looks almost like it was fully burning!

Very severe, and i am kinda scared because I dont know what might cause this extremity?
Dont want to plug in a new rottendog board if the same might happen to it.
If you look at the pre toast photo, the column that burned seems a little brittle, so maybe they were already failing`?

The reel circuit boards should not affect the reel coil, as it only indicates what lights and value should be lit. Can there have been failing components on board from earlier, making it lock on by itself?

Any help appreciated!
Morten
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Yes thanks, my boards are on the way from you as we speak!

Thanks for the answers, it is helpful into understanding what is happening. I changed the title to "driver board" now,not MPU.
As there are severely burnt components, and also an early version of the board and the horrible interconnector is there (the game had been in storage since 1989, until i picked it up some weeks ago), I am thinking of just doing the safe thing and buy a new combo mpu/cpu board.

Any thoughts on these three, which seems to be the options?

PinballPCB:
http://pinballpcb.com/products/williams-system-3-thru-7-driver-board/ (there is also an mpu there,for a bundle)

Rottendog:
https://littleshopofgames.com/shop/boards/rottendog-boards/mpu327-williams-pinball-machine-system-3-system-4-system-6-mpu-and-driver-combo-board-rottendog/

Pinball Dreams all-in-one board:
https://www.pinball-dreams.com/flipper-pinball-flipperautomaten-modding-shop/verschiedenes/all-in-one-nw7.html?fbclid=IwAR2r3whhKWV9lasiLbNS0t3DRrAoN0_osFD5Dy9AhAfLDkp1K-aWHDE-YHY

Inkochnito I have the bridge and the solenoid board coming from you as mentioned, are those boards compatible with the reproductions above? I guess your boards are an added safety measure outside those boards in any case?

Just to wrap this up, (and continue with some other issues in a separate thread. I have swapped out all boards, and added some extra boards like the bridge rectifier board and super solenoid saver board. The machine now boots and nothing is burning

The original issues with the drop targets down not awarding the lit value and resetting, and the different score reel combinations does not lite the values it should. The drop target issue was caused by one switch making poor contact on the drop target circuit board.

So just the slot reel issue remains.

A followup on the slot reel issue, I have not fixed everthing. I took the slot reels apart, cleans it thoroughly with Mothers Mag polish, and also adjusted the double-switch where needed, and now it works

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