The backbox or playfield general illumination never lights.

Backbox or playfield GI always off

Cause: A blown GI fuse on the bottom panel, or the T relay energized or its contacts bad (it cuts playfield GI on tilt).

Checks

• Check the GI fuses on the bottom board.
• Make sure the T relay is not pulled in during game over and check its normally closed contacts.

Fix

• Replace the fuse or clean and adjust the relay contacts.

Systems

• Gottlieb System 1

Related tips

• None of the CPU controlled lamps light
• No CPU controlled lamps light
• How the Tilt and Game Over lamps are switched
• All CPU controlled lamps are dim
• Pop bumper or slingshot locked on or dead
• Spare diodes, capacitors and other parts for System 1
• What is on the System 1 bottom panel
• What the Game Over (Q) and Tilt (T) relays do

Source: Gottlieb System 1 Pinball Repair from 1977 to 1980, Clay Harrell (pinrepair.com)