The MPU LED comes on and stays on; the game does not boot.

MPU LED lit solid: check +5 V first

Cause: A lit LED proves +12 V is reaching the board, so a missing or low +5 V supply is a common cause before any chip fault.

Checks

• Measure DC volts at MPU TP5 to ground; it should read 4.8 to 5.1 V.

Fix

• If +5 V is low or missing, work back through the solenoid driver board 5 V regulator and its C23 filter capacitor before replacing MPU parts.

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 locks with LED on because of C23 or Q20 on the solenoid driver board
• Route of the +12 V supply to the MPU and 5 V regulator
• A missing test point voltage: what to check first
• Solenoid driver TP1 below 5 V with TP5 normal
• What each MPU LED flash proves
• Flicker but no first flash
• Good MPU stays LED-on with a PC power supply

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