General illumination is completely out.

No general illumination: trace the 6.3 V AC

Cause: Open GI fuse, burnt GI connector, or a failed GI relay or relay driver.

Checks

• Meter on AC volts. At the transformer, measure between the yellow and yellow/white wires: about 6.3 V AC.
• Measure the same at the two-pin GI input plug on the power supply board.
• Measure across the GI fuse on the power supply.
• Measure at the nine-pin .156 inch GI output plug next to the input plug.
• If GI is present at the input but not the output, suspect the GI relay (see the relay entry).

Fix

• Replace the open fuse, burnt connector or failed relay found.

Line voltage is present at the transformer primary; measure only on the low-voltage secondary wires.

Systems

• Williams System 7
• Williams System 3

Related tips

• Burnt GI connectors on the power supply (System 7, Hot Tip, Lucky Seven)
• No lamp matrix voltage
• Burnt GI plugs on the power supply board
• How the lamp matrix runs 6.3 V bulbs on 18 V
• All matrix lamps out, everything else works
• Score displays weak, dim or dead with fuses good
• Lower the display voltage to 91 V to extend display life
• Display supply low (under 90 V) after the resistors are changed

Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Two, Clay Harrell (pinrepair.com)