The game resets or freezes (displays and lamps hold their last state), mostly in hard play or multiball.

Random resets or lock-ups during play: broken flipper coil diodes

Cause: Most often a broken or failed flipper coil diode lets the switch-off spike reach the CPU board. Other causes: +5 V sagging below about 4.8 V when many coils fire, or a cracked diode on a pop bumper or slingshot coil (Fire! and earlier).

Checks

• Inspect each flipper coil diode; give every coil diode a gentle tug to find cracked leads or joints.

Fix

• Replace all flipper coil diodes with new 1N4004 (or 1N4007) diodes rather than testing them; they can test good when bad.
• Replace any cracked diode on other coils. A failed diode on a busy coil can also damage its driver and the logic feeding it, so test those too.

Systems

• Williams System 11
• Williams System 11A
• Williams System 11B
• Williams System 11C

Related tips

• Game locks up or behaves strangely, worst in multiball
• Resets from damaged flipper coil diodes
• Reset or lock-up only when one device fires
• Testing the diodes on a flipper coil
• Testing a coil, switch or flipper diode with a meter
• STNG: diverter coils lock on during play
• Test a 1N400x diode on a coil, lamp socket or switch
• STNG: under-playfield diverter coils lock on during play

Source: Repairing Williams System 11 Pinball 1986 to 1990, Part Two, Clay Harrell (pinrepair.com)