MPU LED stays on even though the MPU parts test good.

MPU locks with LED on because of C23 or Q20 on the solenoid driver board

Cause: C23 on the solenoid driver board no longer filters the rectified 12 V well, or the +5 V regulator Q20 has failed.

Checks

• Measure +5 V at the MPU chips; look for low or noisy readings.

Fix

• Replace C23 (see the voltage regulator board upgrade in part one) and Q20 if the 5 V is bad.

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
• No +12 V at the MPU: check the rectifier board
• MPU LED lit solid: check +5 V first
• Only about 1.6 V at solenoid driver TP1
• Solenoid driver TP1 below 5 V with TP5 normal
• Solenoid driver TP5 under 13 V
• Testing C23 for AC ripple
• Replace filter cap C23 on every game

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