The backbox general illumination never works while the playfield GI is fine, and fuses test good.

Backbox GI out, playfield GI fine: sticking relay on the backbox lamp board

Cause: The relay on the backbox lamp (insert) board sticks. It freed up when the board was taken off, then stuck again as soon as attract mode switched the backbox lighting.

Checks

• Check the GI fuses and the burnt-looking transformer and GI connectors first; most Williams GI faults are connectors.
• Watch or tap the relay on the backbox lamp board while attract mode flashes the backbox GI.
• The manual shows wires to a relay connector on the power supply that real games don't have; ignore that.

Fix

• Clean the relay contacts and deburr its armature, or replace the relay.

Systems

• Williams System 11A

Games

• Pinbot

Related tips

• System 11 general illumination
• Part of a lamp row or column stays off
• Single matrix lamp out with good bulb and wiring
• One side of the GI stays dark, GI relay held off: bad MOSFET on a replacement MPU
• Whole game flickers off and on, then dies: burnt ground pin at the power supply board
• System 11 lamp matrix
• Testing a lamp diode in place

Source: Pinball Repair help group!! (Facebook group)