The vertical up-kicker (VUK) stopped working in the middle of a game and does not fire in coil test either. The coil, its fuse and its drive transistor test good, and grounding the drive transistor makes the coil fire.
VUK stops firing, even in coil test: bad pre-driver transistor on the CPU board
Cause: Since the coil fires when its driver is grounded, power and the coil are fine and the turn-on signal isn't arriving. Here the transistors on the booster board were fine, but one of the pre-driver transistors on the main CPU board had failed.
Checks
- Fire the coil in coil test; if it fails there, the switch is not the cause.
- Ground the drive transistor briefly; if the coil fires, work back toward the CPU board along that coil's drive line.
- Test the pre-driver transistor for that coil on the CPU board in diode mode and compare it with its neighbours.
Fix
- Replace the failed pre-driver transistor on the CPU board and retest in coil test.
Games
Source: Pinball Repair help group!! (Facebook group)