Finding a short that blows GI fuse F5

Steps

• Clip a resettable circuit breaker across the F5 clips instead of feeding it fuses.
• Unplug rectifier J1 (playfield) and J3 (backbox), keep J2 (cabinet), power on. A trip means the cabinet wiring, usually the coin door lamps.
• Otherwise plug in J3 with J1 still off. A trip means the backbox GI; look for the head ground braid touching a socket behind the insert.
• Otherwise plug in J1 with J3 off. A trip means the playfield GI.
• On the playfield, unsolder one feed wire from a lamp string at a time (keeping any doubled wires joined so later strings still get power) and power up until the trip stops.

Systems

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

Related tips

• Find a short that blows the 20 A GI fuse
• No general illumination after a new rectifier board, fuses good: check the bulbs
• No feature lamp voltage at TP1 (-18 board)
• How Bally controlled lamps are powered and switched
• None of the controlled lamps light
• Dim controlled lamps and a buzzing expander relay
• F3, F4 or F5 blows on the rectifier board
• Main solenoid fuse F4 blows (hard short)

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