Quoted from coop:the MPU was original and it was thrashed at the bottom with battery acid but it has worked for a couple of years with no issues so I assumed it was still good.
It's actually an acid that is used to remove the alkaline corrosion. This topic discuss the issue. Your example really helps illustrate how important it is to neutralize the issue early on, or have a similar outcome as you did.
https://pinside.com/pinball/forum/topic/stripping-battery-corrosion-with-a-strong-acid
Thanks for posting your solution!