Route of the +12 V supply to the MPU and 5 V regulator

The unregulated 12 V leaves rectifier J3 pin 8 on an orange wire, enters the solenoid driver board at J3 pin 12, loops back out on J3 pin 11 to the MPU, and also feeds the 5 V regulator through the large filter capacitor C23. Some boards have a jumper between SDB TP1 and TP3 to keep this loop from being broken.

Systems

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

Related tips

• No +12 V at the MPU: check the rectifier board
• MPU LED lit solid: check +5 V first
• Solenoid driver TP5 under 13 V
• MPU locks with LED on because of C23 or Q20 on the solenoid driver board
• Solenoid driver TP1 below 5 V with TP5 normal
• Replace filter cap C23 on every game
• Testing C23 for AC ripple
• Replacing filter cap C23

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