A switch shows as stuck in switch test although its contacts are open, or a pop bumper fires several times per hit.

Switch reads stuck closed, or a pop bumper chatters

Cause: A leaky or shorted capacitor across the switch; Bally's caps were often damaged at the factory and also snap off from vibration.

Checks

• In switch test, clip one end of the capacitor. If the number clears, the cap is bad; if not, reconnect it and check the switch diode the same way.

Fix

• Fit a new 0.047 uF 50 V non-polarized ceramic capacitor; any voltage rating works since the circuit is only 5 V. The cap helps the game catch quick hits, so replace it rather than leaving it off on pop bumpers.

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35

Related tips

• Pop bumper or slingshot fires by itself; random tilts
• Flipping scores 10 points: a rebound switch squeezed closed by new rubber
• Pop bumper fires on its own when a flipper is used: replace its switch capacitor
• Pop bumper and both outkicker switches dead: bare wire touching the coin door
• Switches register with the wrong numbers
• Tilt doesn't work and closing it fires a pop bumper: dirty slam tilt switch
• Bank shot lane scores by itself now and then: replace the switch diode
• Random game faults when a switch without a diode sticks

Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Three, Clay Harrell (pinrepair.com)