How the MPU sees the 43 V solenoid supply

The unfiltered +43 V from the rectifier board enters the MPU on J4 pin 15 and is dropped by a 2k resistor near J4 to about 21.5 V at TP3. A zero-crossing detector looks for the ripple on that line; if it is present the board gives its final LED flash.

Systems

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

Related tips

• Using a PC power supply as an MPU bench supply: which wires
• Where to clip a bench supply onto the MPU
• Testing an MPU in the game with only J4 connected
• MPU board test points
• MPU board test point voltages
• Why the MPU has ROM jumpers
• Telling a -133 MPU from a -35 or -17
• Converting a -133 MPU to a -35

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