Rectifier board TP3 has 12 V but MPU TP2 has none.

+12 V at the rectifier board but not at MPU TP2

Cause: A burnt pin or broken wire in the orange 12 V line.

Checks

• Inspect rectifier board J3 pin 8 (orange) for burning.
• Follow the orange wire to solenoid driver J3 pin 12; it loops back out on J3 pin 11 to the MPU.

Fix

• Replace the J3 pin 8 header and housing pin; this pin burns often.
• Many boards carry a TP1-TP3 jumper on the solenoid driver to protect the loop.

Systems

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

Related tips

• 12 V at the rectifier board but not at the MPU
• Rectifier board J1 (to playfield) pins
• Rectifier board J2 (to cabinet) pins
• Rectifier board J3 (to backbox boards) pins
• Pre-Xenon rectifier board outputs by connector
• Pre-Xenon transformer to rectifier board wiring
• Replace browned rectifier header pins and housing pins together
• No +12 V at the MPU: check the rectifier board

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