All the backbox general illumination is out, though the backbox flashers still work. The J120 connector on the power driver board is visibly burnt.

Backbox GI out while flashers work: burnt connectors at J120 and J121

Cause: Burnt GI connectors and header pins at J120 and J121 from the current drawn by incandescent GI bulbs.

Checks

• Inspect J120 and J121 and their header pins for discolouration and loose pins.

Fix

• Replace the connector housings and pins at J120 and J121 (Trifurcon crimp pins handle the current better than IDC), and replace burnt header pins on the board.
• Switching the GI to LEDs reduces the load and helps stop it happening again.

Systems

• WPC-Security (WPC-S)

Games

• Theatre of Magic

Related tips

• Trough eject and left sling dead together: reseat the CPU to driver ribbon
• Slingshot also fires an outlane magnet: ribbon cable not fully seated
• GI string dead with a scorched connector
• Repair a burnt GI connector on the driver board
• GI connectors on the driver board
• Check the AC power input connectors too
• Lamp matrix connectors on the driver board
• Resets from worn power connectors between transformer, driver and CPU boards

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