Random resets that got worse until the game reset almost every time the left flipper (or both flippers) was pressed, and when firing coils from diagnostics.

Resets when the flippers are used: arcing flipper coil, then bridge rectifier BR21

Cause: Two faults: the left flipper had the wrong coil fitted and a loose winding on it was arcing to ground, and bridge rectifier BR21 on the I/O power driver board was failing.

Checks

• Inspect the flipper coils for the correct type and for loose or damaged windings touching the frame.
• Reseat the ribbon cable between the CPU and driver boards and the power plugs, and check the board's mounting screws.
• Test the bridge rectifiers on the I/O power driver board.

Fix

• Fit the correct flipper coil.
• Replace bridge rectifier BR21.

The driver board carries high-current DC from the transformer; unplug the game before working on it.

Systems

• Sega/Stern Whitestar

Games

• Twister

Related tips

• One flipper flips but has a weak hold and drops when hit: bad transistor at Q16
• Whitestar flipper weak or dead after a MOSFET swap: fake part, test with TPL1
• F7 blows at power-up: flasher socket tabs touching, which also shorted Q27
• After cleaning the cabinet most coils are dead and the display goes dark: reseat J16
• Whitestar: I/O board blanking LED flashes, lamp rows 1 and 3 dead: data bus short
• Won't boot after a board swap, CPU LEDs dark: loose 5 V wire in the CPU power plug

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