The MPU resets or freezes during play, typically when flipping, or sometimes while idle.

Game resets or locks up when the flippers are used

Cause: The +5 V dips: missing or broken flipper coil diodes, a worn C23 filter capacitor, or poor connections on SDB J3, the rectifier board J3 or MPU J4 (page 63).

Checks

• Each flipper coil needs two 1N4004 diodes, band toward the 43 V feed; tug them to find loose or broken ones.
• Measure AC volts across C23 on the solenoid driver board with the game on: over 0.25 V AC means it has failed, over 0.15 V means it is going.

Fix

• Replace damaged flipper diodes.
• Replace C23 with a 10,000-15,000 uF 20 V capacitor (15,000 uF if the game has more than two flippers).

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35

Related tips

• Globetrotters reboots during play (GI stays on)
• Flippers stop working after a few minutes of play
• Coil locks on at power-up or stays on
• No coils work at all
• Solenoid selection: four MPU lines decoded by a 74154
• Flippers weak or intermittent from the solenoid driver board
• Flipper weak, dead, or machine-gunning
• Flippers dead though fuses and EOS switches are good

Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Three, Clay Harrell (pinrepair.com)