Test the power supply with its outputs unplugged

Steps

• Unplug every power supply connector except the two square ones, 3J1 (12-pin inputs) and 3J2 (6-pin ground from the backbox bridges).
• Turn the game on and measure each output at its header as below.
• Fix the power supply before connecting anything else.

Line voltage is present in the cabinet; unplug the game before working on it.

Systems

• Williams System 3
• Williams System 4
• Williams System 6 / 6A
• Williams System 7

Related tips

• What the power supply board takes in and puts out
• Swapping power supply boards between generations
• No +5 V on a System 3-6 power supply
• No +5 V on a System 7 power supply
• Early Black Knights use the System 6 power supply
• Power circuits in a System 3-7 game
• Parts of the 28 V solenoid supply
• No coils fire: check the 28 V solenoid supply

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