+5 V regulator and MPU shut down, or the game resets heavily.

Dead MPU from an open C23 ground return wire

Cause: The separate white/brown ground return from C23 (solenoid driver J3 pin 10 to rectifier J3 pin 17) is broken or its pin is burnt.

Checks

• Temporarily jumper the negative (left) lead of C23 to a backbox ground; if the game comes up, the return wire is at fault.
• Note rectifier J3 pins 1-4 and 14-20 are all grounds and may have been moved around over the years.

Fix

• Repair the white/brown wire and replace its connector pins.
• Add the permanent C23 ground jumper described for the driver board.

Systems

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

Related tips

• MPU shuts down or resets: broken C23 ground return wire
• Route of the +12 V supply to the MPU and 5 V regulator
• MPU locks with LED on because of C23 or Q20 on the solenoid driver board
• Bally solenoid driver 5 V reads only about 4 V: swap the drop-in regulator

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