The game freezes (scores, displays and lamps stuck) during heavy play, or coils fire and scores count with no switch hit.

Game locks up or behaves strangely, worst in multiball

Cause: A broken or failed diode on a coil, most often a flipper coil, lets the switch-off spike back into the CPU board. Pop bumper and slingshot diodes also break.

Checks

• Look closely at the diodes on every flipper coil; a diode can test good and still be faulty.

Fix

• Replace every flipper coil diode with a new 1N4004 (or 1N4007), band to the power lug as before.

Systems

• Data East CPU Version 1/2
• Data East CPU Version 3
• Data East/Sega CPU Version 3b

Related tips

• Random resets or lock-ups during play: broken flipper coil diodes
• Test a 1N400x diode on a coil, lamp socket or switch
• STNG: diverter coils lock on during play
• STNG: under-playfield diverter coils lock on during play
• Reset or lock-up only when one device fires
• No score displays light at all
• Spare diodes for coils and switches
• Testing a coil, switch or flipper diode with a meter

Source: Repairing DataEast 1987 (Laser War) to Sega 1995 (Batman Forever), Part Two, Clay Harrell (pinrepair.com)