With the playfield propped up, the general illumination works and the fuse holds. As the playfield is lowered into the cabinet the GI goes out and the GI fuse gets red hot or blows.

GI fuse glows and blows only when the playfield is lowered: bare GI wire on a screw

Cause: A run of bare GI wire under the playfield was touching the screw that holds a wooden trim piece on the playfield, shorting the GI to ground once the playfield settled.

Checks

• Lay the playfield upside down and follow the bare GI wire runs, looking for any place they touch screws, brackets or the cabinet.
• Recheck any lamp socket you have just reinstalled, since moving parts can shift the wiring.

Fix

• Move the wire away from the screw and insulate the spot (electrical tape or tubing).

Systems

• Williams System 6 / 6A

Games

• Gorgar

Related tips

• Group of GI lamps dark after an LED swap: broken bare bus wire at a staple
• Match and player lamps light at random in play: faulty replacement driver board
• Playfield GI fuse blows after a playfield swap: solder blob shorting a socket

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