The 43 V circuit has an overvoltage varistor

A varistor in the 49 V AC coil circuit shorts above about 55 V and blows the fuse. Check it, as well as BR3, when F4 keeps blowing.

Systems

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

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