EMs are the easiest things to work on. Once all the switches are cleaned and adjusted they're usually good to go for a long time as long as you play them occasionally. Gottlieb Ax and Bx relays are typically the most finicky aspect of those games if things don't work right. Getting the reset circuit working so that all the bonus steppers and score reels zero out will stop the score motors from continuously running.
Once you understand how an EM works, then you can pretty much troubleshoot any EM from any manufacturer as they're all pretty much the same. Bally, Gottlieb, Williams, Chicago Coin, Sonic, Zaccaria, etc. The biggest difference is how well the schematics and documentation are done. I like Gottlieb and Williams manuals/schematics the best. Bally's I don't care for and I'm working on a Sonic with no schematics or manual; so I can't comment on theirs, but the game is working great now even without them.
