One general illumination string stays dark. The fuse is good, continuity looks fine and the triac parts test good, but only about 2.8 V AC appears across that string's output pins at J121.

One GI string dead though the board and fuse test fine: burnt connector in the cabinet

Cause: A burnt 9-pin GI connector in the bottom of the cabinet, previously hacked with cut wires and tape, wasn't passing current properly.

Checks

• Follow the GI wiring from the power driver board down into the cabinet and inspect every connector for scorching or bodged repairs.
• Measure the GI voltage at each connector along the run to find where it drops.

Fix

• Replace the burnt connector pair with a new housing and pins and undo the hacked wiring.

Systems

• WPC-Security (WPC-S)

Games

• The Shadow

Related tips

• GI string dead after connector replacement: broken traces
• One GI string is out
• All GI out (playfield and backbox) with good fuses and plugs: reseat the CPU ASIC and ROM
• No GI at all
• General illumination strings
• Continuity checks after replacing GI header pins
• Lamp matrix faults with all lamp transistors good
• Both flippers dead in game but fire in coil test: loose old wire splice at J116

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