The left pop bumper fires weakly and slowly while the other two are strong. The coil matches the others in type, voltage and resistance, the sleeve is new, and the skirt switch has been cleaned and gapped.

One pop bumper weak and sluggish though coil and switches are fine: replace Q77

Cause: The pop bumper's driver transistor on the CPU board (Q77) was failing; it still tested good in circuit.

Checks

• Compare the weak pop's coil resistance, voltage and switch with the good ones to rule out mechanical and coil problems.
• The three pop bumpers are driven by Q73, Q77 and Q79 (TIP102-type); compare their behaviour, or briefly ground a transistor's tab to check the coil fires at full strength.

Fix

• Replace Q77 with a new TIP102 even if it measures good.

Systems

• Williams System 11B

Games

• Space Station

Related tips

• Quick check of all TIP122/102 drivers before first power-on
• CPU and driver board transistors by position
• Use TIP102 when replacing a TIP122
• A coil has power but never fires
• Supercharger diverter dead even in test: open TIP102 at Q81 that tested good
• Same coil driver transistor keeps blowing after replacement: counterfeit TIP102s
• Gottlieb pop bumper driver transistor keeps burning out: check the coil's diode
• Use a TIP102 in place of the pop bumper board's 2N6057

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