I have a problem with my Theatre of Magic. During gameplay, it constantly registers balls dropping in the trunk, thus engaging multiball. It also randomly pops new balls.

I recorded the mrs when playing a game: https://photos.app.goo.gl/8Rm5JXLqH4R8fEj8A

In switch test it can also be reproduced, by selecting single switch > subway opto, and then just engaging the flippers. You can see it registers the opto about half of the times. First I thought about cleaning or replacing the optos but then I found this topic and decided to test with the other optos, eg in the trough. They also register sometimes during the flipper-test, which explains the random new balls popping.

I have found a topic with a similar problem (https://pinside.com/pinball/forum/topic/tom-really-odd-issue-with-trunk-during-multiball) and here it turned out to be bridge rectifiers.

Unfortunately, while I can use a DMM, as soon as components need to be replaced, I am out. So I prefer to test a few things, even swap boards out, if someone could just point me in the right direction. Unfortunately, this is my only WPC-S title. I do have WPC and WPC-95 machines, but I am not sure the driver boards are interchangeable.

On a hunch, I cleaned and reseated the connectors on the opto board (since that board is right next to the flippers, it could very much influence the readings). Two test games in, still no problem!