Two of the three GI strings on the backbox insert panel are out.

Two backbox GI strings out: the fuse was bad even though it tested fine in place

Cause: A blown fuse that read as good when tested in its clips.

Checks

• Take each fuse out of its clips and test it on its own; testing in place can read through the circuit.
• Then check the connectors, headers and the GI relay board's solder joints.

Fix

• Replace the bad fuse with the correct rating.

Systems

• Williams System 11C

Games

• Whirlwind

Related tips

• No drop target switches register: 12 V feed lost at a loose connector
• Interconnect board and flipper power board fuses
• LED GI goes dim during play: poor header on the interconnect board
• System 11 backbox boards and supplies
• Roulette wheel not working: worn J15 connector on the interconnect board
• Right flipper dead after a coil cooked: burnt trace on the interconnect board
• Dim GI and backbox lamps: GI relays swapped for boards that were miswired
• Upper right flipper only buzzes while the magnet holds the ball: failed opto-isolator

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