One thumper (pop) bumper never fires when the ball hits it, although the coil and driver are fine.

Thumper bumper does nothing: switch has a capacitor but no diode

Cause: The bumper's skirt switch had only a small capacitor across it and was missing its matrix diode, so the MPU never saw the switch close.

Checks

• Look at the components soldered across the bumper switch: it should have a diode, and may also have a small capacitor.
• Check the bumper in switch test, and fire the coil in solenoid test to confirm the driver works.

Fix

• Fit the correct diode to the switch with the band facing the same way as on the other bumpers; remove the capacitor if it is the only part there.

Systems

• Bally MPU AS-2518-17

Games

• Mata Hari

Related tips

• Wrong coil fires or a coil never fires
• Displays strobe wildly in one game only
• Drained ball sits in the outhole until the switch is pushed by hand
• Coil diode and wire orientation when replacing a coil
• No playfield coils work
• Use the solenoid test to find a coil that does not fire
• Check for power at a coil and fire it by hand
• No coils work at all

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