One GI string on the playfield (or backglass insert) is out.

GI string out on a game with under-playfield GI relays

Cause: Many System 7 games switch GI strings with 12 V DC relays under the playfield or on the insert board. The relay contacts are normally closed, so a dead string is usually a poor lug connection on the relay.

Checks

• Find the relay: five lugs, centre one unused. Two carry the GI wires (solid yellow or purple), two carry 12 V and the return to the driver board TIP122.
• Check the GI wire lugs for loose or corroded crimp connectors.

Fix

• Crimp on fresh lug terminals, or tighten the existing ones with pliers.

Systems

• Williams System 7

Related tips

• The solenoid L/R select relay and transistor Q29
• Kicker coil locks on again after a new TIP122: dead 7408 gate on the driver board
• F5 blows on a flip, then slings dead: shorted drivers and a loose coil power wire
• 50 V coils use PPB TIP36c transistors, pre-driven from the CPU board
• Multiplexed L/R (A/B) banks and wire colours
• Three-stage drive for 50 V coils
• Where the TIP122 drivers are on the CPU board
• How a coil or flasher is switched: PIA to TTL to pre-driver to driver

Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Two, Clay Harrell (pinrepair.com)