6.3 V AC reaches the power supply GI input but not the GI output plug.

GI in at the power supply but not out: check the GI relay

Cause: The GI relay contacts have failed, or the relay is stuck energised because its driver transistor or circuit on the driver board has failed.

Checks

• Unplug the relay coil connector: 3J7 on System 7 power supplies, or the black/red harness plug on Blackout, Scorpion and early Black Knight.
• If the GI now lights, the relay is being held on by a failed driver circuit, or the relay itself is seized.

Fix

• Replace the relay or repair its driver transistor circuit.

Systems

• Williams System 7
• Williams System 6 / 6A

Related tips

• Replacement GI relays
• Test the power supply with its outputs unplugged
• Match and player lamps light at random in play: faulty replacement driver board
• No lamp matrix voltage
• Lamp matrix column test at 2J5 with Leon's code
• Lamp matrix row test at 2J7 with Leon's code
• How the lamp matrix runs 6.3 V bulbs on 18 V
• Never leave a locked-up game switched on

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