TP5 reads 13 to 16.5 V but the +5 V at TP1 is under 5.0 V.

Solenoid driver TP1 below 5 V with TP5 normal

Cause: A board drawing too much on the 5 V line, or a weak Q20 regulator on the solenoid driver.

Checks

• Power off, unplug MPU J4, all lamp driver and auxiliary lamp driver connectors, then power on and read TP1.
• If TP1 is now 5.0 to 5.2 V, reconnect the MPU, then the lamp driver, then the auxiliary lamp driver, checking 5 V after each.

Fix

• Repair the board that pulls the voltage down.
• If no board is to blame, replace regulator Q20.

Systems

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

Related tips

• Only about 1.6 V at solenoid driver TP1
• MPU locks with LED on because of C23 or Q20 on the solenoid driver board
• Logic 5 V reads over 5.25 V on Bally/Stern: the R49 boost resistor
• Replace filter cap C23 on every game
• Testing C23 for AC ripple
• Replacing filter cap C23
• MPU LED lit solid: check +5 V first
• 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)