OP

Quoted from newbieinKC:

I recently noticed that my knocker doesn't seem to knock any more. My machine would knock when I completed the saucer after hitting all of the other targets (sinking the Eight Ball), and also when I hit the replay score.
When I do the solenoid test, it knocks every time in proper sequence with the other solenoids. I can't find a corrupt register value to explain why it doesn't work. Not a huge deal, but the knocker is a little bit like a party popper on New Year's Day and not having it takes a little bit of fun out of the game.