Fuse F4 blows at power-on.

F4 still blows with the playfield coils out of circuit

Cause: Either a shorted or locked-on playfield coil, or something upstream: backbox knocker, coin lockout coil, the solenoid bridge rectifier or the varistor on the rectifier board.

Checks

• Pull the under-playfield coil fuse and fit a new F4: if F4 holds, the fault is a playfield coil.
• If F4 still blows, unplug rectifier board J1 and J3 and try again: if it still blows, the solenoid bridge or varistor on the rectifier board is at fault.

Fix

• Replace the shorted coil, its diode or driver, or the rectifier board bridge or varistor.

Switch off before changing fuses or connectors; the rectifier board has mains and high voltage.

Systems

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

Related tips

• Rectifier board fuse ratings (all Bally 1977-1985)
• Every flash but the last one
• Find the coil that keeps blowing the playfield fuse
• Playfield coil fuse blows at power-up: shorted outhole kicker coil
• Rectifier board J1 (to playfield) pins
• No 12 V at rectifier TP3 even with a good F3
• Rectifier board and playfield fuse ratings
• MPU shuts down or resets: broken C23 ground return wire

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