Keep going Rob - Process of elimination.
It does not seem to be a stuck switch. I've seen similar problems before. What your need to do is take out the tens and hundreds score wheel and tighten the switch stacks with a flat head screwdriver (with the game off). Clean the contacts on each reel stack with a business card.
Start a game
Do all reels reset to zero ?
Does the ball get served and no score added ?
Hit a tens switch - what happens ?
When it gets to ninety, hit another tens switch - What happens ?
Hit a hundreds switch - What happens ?
When it gets to nine hundred, hit another hunderds switch - What happens ?
