The display on the pinball machine still doesn’t work. Diagnosing this required getting into the backbox where all the circitry is. Getting into the backbox requires removing the backglass (with the pretty art drawn onto the glass). There is a lock holding the backglass in place.
We didn’t have the key for this lock.
So, today, I bought a drill, and my wife the master driller drilled through the lock, into the screw on the other side attaching the lock to the bar holding the backglass.
I now have access, and can start in with my new multimeter at diagnosing things.