Main fuse F6 blows, or there is a melted red disc near the line cord entry.

Blown line varistor VR1

Cause: The line varistor VR1 has absorbed a surge and shorted.

Checks

• Look for a red disc that has melted, still attached to its two wires.

Fix

• Replace the varistor.

Line voltage (120 V AC) is present on the rectifier board and its J2 connector; unplug the game before touching it.

Systems

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

Related tips

• No voltage at any rectifier board test point: burnt mains wire to the transformer
• Backbox GI out and fuse F6 blowing: burnt rectifier J3 plug and a shorted bridge
• Game totally dead after running a vacuum from the service outlet: power cord

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