Quoted from crashpad:
I eventually plan on tearing it down and doing a complete restore as a learning experience, but since I have the time to tinker for now, will take that option of trying to fix the board first. Thanks for the recommendation.
Just visualize what is going on..
Normally a coil has a hot side tied to high voltage (+43 VDC in this case)... and the ground size is driven by a switch or transistor to connect to ground. When the circuit is complete to ground.. coil fires.
The ground side in this case, is driven by the flipper switch. But because we want the game to be able to KILL the flippers by CPU control.. instead of just going coil -> flipper switch -> ground... it goes coil -> SDB -> switch -> ground. The relay on the SDB acts as an extra switch to break the path... nothing more. So the signal comes in one connector, goes through the relay, and back out the other connector. The path passes through the SDB purely so the flipper relay can break the path when the game wants.
So if that path through the headers and relay is broken.. or the relay never closes.. the path isn't complete. You could even test this with continuity between the two header pins.. the one in, and one out for the same flipper. Then trace step by step to where you see the path break.