During play a smell and smoke come from under the playfield: one pop bumper coil has overheated after the bumper kept pulsing on its own.

Pop bumper keeps firing until its coil smokes: skirt caught under playfield protector

Cause: The pop bumper's skirt (wafer) had slid under the edge of a clear playfield protector and stuck tilted, holding its switch closed so the bumper fired over and over.

Checks

• Look at each pop bumper skirt to see that it sits level and moves freely, especially where a playfield protector is fitted.
• After a coil overheats, check its driver transistor and any wiring or board joints that got hot.

Fix

• Replace the burnt coil (and driver if damaged), free the skirt, and trim or remake the protector around the bumper so the skirt can't slide under it again.
• Members also suggested fitting a separate small slow-blow fuse in each special solenoid's drive so a jammed switch blows a fuse before the coil cooks.

Systems

• Williams System 6 / 6A

Games

• Firepower

Related tips

• Pop bumper auto-fires and its spring jumps off: the metal yoke is missing
• Pop or sling works in the game but not in test, or the reverse
• Pop bumper or slingshot locked on
• Game keeps resetting: replace the 5101 RAM (an NVRAM works)
• Pop bumpers do not show in switch test: pull the ring down to test them
• Coil overheated: check its resistance

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