Shortly after power-up, the wiring to a chain of three lamps on the upper playfield heats up, smokes and starts to melt, even with the bulbs removed. No fuse blows.

Wires to a few upper playfield lamps get hot and melt: bent lamp socket tab

Cause: A bent tab on one of the lamp sockets in that chain was shorting.

Checks

• Switch off at once and unplug the lamp connector at the power board until the fault is found.
• Inspect every socket in the hot chain, especially any that flickered before, for a tab bent against the bracket or another lug.
• Check the fuse rating on that lamp supply: wires should not be able to melt without a fuse blowing.

Fix

• Straighten or replace the faulty socket and replace any heat-damaged wire.

Overheating wiring is a fire risk; do not leave the game on while this fault is present.

Systems

• Williams System 7

Games

• Black Knight

Related tips

• Burnt GI plugs on the power supply board
• Where the GI relay is on early System 6/7 games
• GI relay on Blackout, Scorpion and early Black Knight
• Solenoid fuse F2 blows at power-up as every coil fires: CPU blanking circuit
• Upper playfield half-moon wire guide keeps popping out of its holes
• 2.5 A solenoid fuse keeps blowing: check it is a slow-blow fuse
• Won't start a game: blown fuse, then trough switches not seeing all three balls

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