A GI fuse blows again as soon as it is replaced.

GI fuse keeps blowing: isolate driver board or wiring

Cause: A short in the GI string wiring, or less often on the driver board.

Checks

• Unplug the GI connector for that string (J119, J120 or J121; J105 or J106 on WPC-95), fit a new fuse and power up.
• Measure about 6.3 V AC at the driver board pins. WPC-S and earlier: string 1 J120 pins 1 and 7 (F110); string 2 J120 pins 2 and 8 (F109); string 3 J120 pins 3 and 9 (F108); string 4 J120 pins 5 and 10 (F107); string 5 J120 pins 6 and 11 or J119 pins 1 and 3 (F106).
• Voltage present means the board is fine and the short is in the playfield or backbox wiring.

Fix

• Trace the wiring short (see the GI short-finding entry).

Systems

• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)

Related tips

• Lamp matrix fuse F114 (F106 on WPC-95) keeps blowing
• Test GI outputs on the driver board
• GI string dead after connector replacement: broken traces
• Testing lamp-row drivers with a test lamp
• Testing lamp-column drivers with a test lamp
• Where GI power flows on the driver board
• GI string still dead after replacing J115: broken through-hole, add a jumper
• WPC lamp matrix and GI driver parts

Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part Three, Clay Harrell (pinrepair.com)