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For pop bumpers, I usually look at the switch to make sure the contacts are clean and adjusted properly. Then, I look at the small pop bumper board. Swap it around with a known working pop bumper board in your game and see if the same pop bumper still refuses to work, or if the problem follows the board, and causes a different pop bumper to stop working (while at the same time, causes the original pop bumper to start working).

There are several different symptoms of malfunctioning displays. What exactly are the symptoms?

There are some common problems and solutions on pinwiki: http://pinwiki.com/wiki/index.php?title=Gottlieb_System_80#Display_Problems