How the lamp matrix runs 6.3 V bulbs on 18 V

The lamp supply is a steady 18 V DC, but the driver board pulses it so each lamp is on about a third of the time, roughly 6 V on average. The supply has only three parts: a backbox 35 A 400 V bridge rectifier, a large 30,000 MFD filter capacitor and fuse F3 on the power supply board.

Systems

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

Related tips

• No lamp matrix voltage
• All matrix lamps out, everything else works
• Lamp matrix column test at 2J5 with Leon's code
• Lamp matrix row test at 2J7 with Leon's code
• Lamp matrix wire colours and connector pins
• Fixing the burnt lamp matrix resistors R149-R156
• Tester LED check of the lamp columns
• Tester LED check of the lamp rows

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