Did you ever solve this?
-mof

"Solenoid Expander Board" -- what is its role?
-mof

I already installed a new "red" SEB a few years ago, and it cleared up some issue (I forget what)

Here's the problem I am now having:

1. Play balls 1-2-3 no problem on rest of playfield
2. Shoot any ball into cave, and it always registers, "mermaids trapped" (switch 5 seems good) -- BUT -- sometimes it fails to raise 3 targets and say "dive again" and feed a new ball.

Clues:

1. sometimes right lower flipper has chatter
2. sometimes 2x scoring mode happens randomly?
3. last night I pressed the right flipper switch lug with the wire on it, and I saw "3" in switch test -- which is the top trough switch... WTF

-mof

Got some tips/ideas already here: https://pinside.com/pinball/forum/topic/fathom-owners-club/page/9#post-4460869

https://pinside.com/pinball/forum/topic/bally-fathom-shop-job#post-1716042

My past self already solved this, I'll go check in with him and see what's up

https://pinside.com/pinball/forum/topic/tech-fathom-cave-saucer-3-green-targets-dont-rise-after-locking-ball

Ok - so if I hold the switch down with a pencil - it works -- very strange, I suppose it is the switch -- will have to take a closer look -- tougher working on components way up there - especially when PF is on the hinge.

-mof

I cleaned the switch, and bent it up a little. It's all fixed for now -- but I still don't get how...