The general illumination and backbox lamps are dim, and readings at the interconnect board look far below the expected 6 V AC.

Dim GI and backbox lamps: GI relays swapped for boards that were miswired

Cause: A previous owner had replaced the GI relays with aftermarket solid-state GI boards that were not properly connected to the GI power lines. (Readings taken to ground were also misleading; across the transformer's yellow and yellow-white pair it gave a normal 6.7 V AC.)

Checks

• Measure GI voltage across each yellow / yellow-white pair, not to ground.
• Check the interconnect board GI connectors and headers (J6, J7, J9, J10) and fuses F1 to F4 and their clips.
• Check all three GI relays (one in the backbox, two under the playfield) or whatever has replaced them.

Fix

• Wire the GI relay replacements correctly (or refit relays), and repin any burnt connectors.

Systems

• Williams System 11C

Games

• Earthshaker

Related tips

• LED GI goes dim during play: poor header on the interconnect board
• Right flipper dead after a coil cooked: burnt trace on the interconnect board
• Flash lamp dead on an early System 11 game
• Rebuilding a burnt J6 GI connector: striped wires on the first four pins
• GI out on Banzai Run and later: burnt interconnect connector 2J6
• Roulette wheel not working: worn J15 connector on the interconnect board
• System 11 backbox boards and supplies
• The interconnect board (Banzai Run and later)

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