All playfield GI is out or flickers while the backbox GI works.

Single-pin backbox GI connector burns (System 3 to 6)

Cause: All playfield GI current passes through one single-pin connector in the harness between the backbox GI fuse and the playfield, and it overheats.

Checks

• Find the single-pin connector in the cabinet-to-backbox harness and look for browning or a loose fit.

Fix

• Replace both halves of the connector, or splice the wire with a properly rated joint.

Systems

• Williams System 3
• Williams System 4
• Williams System 6 / 6A

Related tips

• Lamp matrix wire colours and connector pins

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