Quoted from CalBinduur:There does appear to be some corrosion on the main board. How should I go about fixing that?
You have to unsolder affected components and clean the board.
Directions here (it's a Williams board, but you'll get it):
https://pinside.com/pinball/forum/topic/vids-guide-to-bulletproofing-williams-system-6
Here is Bally info:
https://pinside.com/pinball/forum/topic/vids-guide-quick-bally-driver-board-repair-bulletproofing