The MPU will not run, or the game resets heavily, though the 12 V and fuses look fine.

MPU shuts down or resets: broken C23 ground return wire

Cause: The negative side of SDB capacitor C23 has its own white/brown ground wire from SDB J3 pin 10 back to the rectifier board (usually J3 pin 17). If it is open, the 5 V regulator and MPU shut down.

Checks

• With the game on, jumper the left (negative) side of C23 to any backbox ground; if the game comes up, the white/brown return is at fault.
• Rectifier J3 ground pins (1-4 and 14-20) are often moved around on older games, so follow the wire rather than the pin number.

Fix

• Repair or re-pin the white/brown return wire and its connector pins.

Systems

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

Related tips

• MPU locks with LED on because of C23 or Q20 on the solenoid driver board
• Adding a ground jumper at C23
• Ground mod for the solenoid return at J3 pins 23-24
• Testing C23 for AC ripple
• Replace filter cap C23 on every game
• Replacing filter cap C23
• 12 V at the rectifier board but not at the MPU
• No +12 V at the MPU: check the rectifier board

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