I ran into an interesting situation recently with the Black Hole Pinball I restored (documented on this site).
The owner reported that the machine would turn on and light up, but would not start a game or have any sounds in attract mode. It would reduce a credit when you pressed the start button though.
I noticed an 8 Amp slo-blo fuse was out and put in a circuit breaker to test it out. I tried a 5 amp breaker, and that did revive the game, but the game would blow this when it was resetting all the solenoids, so since the next highest value I had was a 20 amp (I forgot my fuses), I tried that.
After playing the game for a while, thinking things were probably ok, I noticed some smoke from a burning coil. Well, that’s why the fuse blew I guess!
It was a non-computer controlled kicker, and I figured one of the activation switches had gotten bent permanently closed or something, so while I was under the playfield I replaced the coil, the diode, and the capacitor on the coil and got everything ready to put the playfield back to check the switches above the playfield. As I was ready to put the playfield down, I noticed that I could actually see the contact of one of the two switch under the playfield. It almost looked like the swtich was bent such that the head of the switch was actually under the playfield instead of going though to the top of the playfield. It turns out that the switch had actually broken in two, and the top part had fallen down into the switch itself and shorted the switch such that the coil was locked on.
I replaced the switch and the game was as good as new.