Audit or bookkeeping values change to nonsense.

Garbage in the bookkeeping totals

Cause: The CPU ran with unstable +5 V and wrote over the U8 memory ("run away"). Usually a weak Q20 regulator or C23 on the solenoid driver board, or a leaky VR1 zener on the MPU.

Checks

• Check +5 V stability at the MPU.
• Check Q20 and C23 on the solenoid driver board and VR1 on the MPU.

Fix

• Replace the faulty part(s), then reset the bookkeeping.

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35
• Stern MPU-100
• Stern MPU-200

Related tips

• MPU VR1 is an 8.2 V zener; the manual's "1N9598" is a misprint
• MPU LED locked on with no flicker: likely causes
• Testing the MPU reset circuit at U9 pin 40
• MPU boots only some of the time
• What the MPU VR1 zener does
• Game locks up entering bookkeeping after a battery change
• Get the MPU booting before you change any ROM jumpers
• Resetting replay values and high scores

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