A GI string doesn't light though its driver board plug isn't burnt and the fuses are good.

One GI string dead with good fuses and connector

Cause: A poor contact in the GI connector from the transformer in the cabinet bottom (yellow wires), or bad traces to the driver board GI pins.

Checks

• Unplug and replug the transformer GI connector a few times.
• Check the driver board traces to the GI header pins.

Fix

• Clean or repin the connector and repair any broken traces.

Systems

• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• GI dead but the driver board connectors look fine
• One GI string dead though the board and fuse test fine: burnt connector in the cabinet
• One GI string dead after a restoration: unkeyed J115 plugged in upside down
• GI string still dead after replacing J115: broken through-hole, add a jumper
• Two GI strings still dead after fixing a burnt J120: broken pads and traces
• All backbox lights out with good fuses: burned J120 GI connector
• GI string dead after connector replacement: broken traces
• Pop bumper and target GI lamps dead: a previous owner's added socket shorting

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