Pinball machines have batteries.  A lot of people don’t know this.  The batteries maintain your settings and high scores in the game’s RAM.  These batteries must be periodically changed so that they don’t leak.  Here’s the consequences of not changing those batteries:

This the the MPU board from a Williams WPC-89 game.  It is ruined, and basically beyond repair.  You are looking at $170+ for a new board.

You can avoid problems by remotely mounting the batteries.  This is the best solution.  There are pre-made solutions to this, or if you are handy with a soldering iron you can install something similar yourself.  You can also use nonvolatile RAM to eliminate batteries. Finally, if you stick with batteries, use Energizer Advanced Lithium batteries.  From all information I’ve been able to find, lithium batteries do not leak.  They become inert as they age and die.

If you don’t remote mount your batteries or use another solution, no matter what, change your batteries regularly.  Some people recommend every year.  I think this is overkill for a home use machine.  But at least check them every year.  If you have a machine that is turned on for many hours during the day, replacement every year is probably a good idea due to the heat that builds up inside the backbox.

Change the batteries with the game turned on to maintain your settings and high scores.  If you change them with the game off, the RAM will lose power, and when you power up the game it will be set with the default factory settings.  You’ll then have to go in to the game’s menu and set it to free play and re-enter any adjustments that were made.

Another thing I should mention, if your game says “Factory Settings Restored” every time you turn it on, your batteries are either dead or missing.  Cross your fingers, hope that they haven’t leaked, open the backbox, and check.

Stern pinballs with the SAM board set, starting with World Poker Tour in 2006, do not have AA batteries.  They use a 3v lithium CR2430 “button” cell to maintain the game’s settings.  These batteries last for a very long time and generally do not leak.  Still, it doesn’t hurt to check every so often and replace it after a few years.

Here’s another look at the damage to the board.

Update, I figured I’d post some more carnage.