Using a logic probe on the display board safely

The display board carries 100 V, which will destroy a logic probe. Start the display test so the digits cycle, then pull the two high-voltage fuses on the power supply board; the logic stays running and the board can be probed safely.

The display supply carries about 100 V DC (plus and minus); keep fingers and probes off the master display board and power supply high-voltage section with the game on.

Systems

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

Related tips

• Master display board versions
• Which UDN6184 drives which display
• All score displays dead
• UDN display driver chips
• One score display dead
• Test MPS-A42 and MPS-A92 display transistors
• Find a shorted display glass that blows F1
• A segment missing on a score display

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