One pop bumper fires on its own, always at the moment a flipper is pressed. Opening the bumper's skirt switch so far that it cannot close does not stop it. Here it also opened the upper zipper flippers early.

Pop bumper fires on its own when a flipper is used: replace its switch capacitor

Cause: The small capacitor (and its diode) fitted across that pop bumper's switch had failed, so electrical noise from the flipper firing was read as a switch closure.

Checks

• Open the pop bumper switch fully to rule out a real switch closure.
• Inspect the capacitor and diode soldered across the pop bumper switch; clip one leg of the capacitor to test.

Fix

• Replace the capacitor and diode on that pop bumper switch.

Systems

• Bally MPU AS-2518-35

Games

• Medusa

Related tips

• Some games add a sixth switch strobe ST5 (switches 41-48)
• Pop bumper or slingshot fires by itself; random tilts
• Switch reads stuck closed, or a pop bumper chatters
• Flipping scores 10 points: a rebound switch squeezed closed by new rubber
• Hitting switches in one matrix row tilts the game: bad capacitor on the tilt bob
• Tilt doesn't work and closing it fires a pop bumper: dirty slam tilt switch
• Several playfield switches drop out together: worn insulation shorting the start button
• Pop bumper and both outkicker switches dead: bare wire touching the coin door

Source: Pinball Repair help group!! (Facebook group)