After changing the bulbs to LEDs, three always-on GI lamps in one corner of the playfield (beside the snake pit) stay dark, while every other lamp works.

Group of GI lamps dark after an LED swap: broken bare bus wire at a staple

Cause: These GI sockets are linked by a run of bare tinned wire stapled to the underside of the playfield, in series with other GI lamps further along. The wire had broken where a staple bit into it; working under the playfield to change bulbs can finish off a weakened spot.

Checks

• Swap a known-working bulb into one of the dark sockets to rule out the LEDs.
• Follow the bare GI wire from a working socket to the dead ones and check continuity along it, especially at each staple.
• Check the sockets for corrosion.

Fix

• Repair the break in the bus wire (solder in a short jumper) at every spot where a staple has cut into it.

Systems

• Williams System 6 / 6A

Games

• Gorgar

Related tips

• GI fuse glows and blows only when the playfield is lowered: bare GI wire on a screw
• Match and player lamps light at random in play: faulty replacement driver board
• A single lamp does not light
• How the lamp matrix runs 6.3 V bulbs on 18 V
• Tester LED check of the lamp columns
• Tester LED check of the lamp rows
• Lamp matrix wire colours and connector pins
• Photo: MOSFET lamp drivers with jumper wires

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