No power to Back to the Future motherboard

[rkeefer]

Hey everyone,

So I've posted a couple times on my work repairing and getting my Back to the Future pinball machine back in working order.

A few months back, I replaced the mainboard with a new one from Rottendog, and then more recently, replaced the old Plasma score display with an LED. Other than some cosmetic things, everything was working fine.

Went to turn it on this weekend. Plugged it in (always have it unplugged except when playing it), and hit the power switch. The playfield lights up, but it never says, "Back to the Future" like it does in working order. Took off the backglass and am seeing the motherboard's trio of lights are all off. There's obviously some power to the unit as the whole playfield is lit as normal, but the motherboard is either dead or not receiving power. All cables seem to be seated properly.

Any ideas on how to proceed?

Thanks,
Ryan

[pinballdaveh]

It sounds like a power supply board problem. And usually it’s the 5 volt section that goes out. The first thing to check is fuses f1 & f2 both are 7 amp fuses, if either fuse is blown bridge rectifier DB1 is bad and needs to be changed. If both fuses test good the next step is C2 100uf 25v electrolytic cap. This is what I call a coupling cap and when it goes bad it takes down the whole 5v section. When this cap goes bad it leaks it’s acid onto the circuit board and corrodes the caps connection traces. Desolder and remove cap, inspect and repair traces if needed, clean any residual acid, solder in new cap even if old cap wasn’t leaking. The location of this cap being right next to the heat sink didn’t help the life of this cap either.