A segment stays lit all the time.

Segments stuck on

Cause: Usually a solder splash or bridge on the master display board; less often a shorted UDN7180A.

Checks

• Inspect the board closely for bridges.

Fix

• Clear the bridge, or replace the chip.

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

• A segment missing on a score display
• Bottom segments missing and master display resistors burning: shorted display glass
• Score displays dead but digits glow: bad wire in the CPU blanking plug (1J3)
• Four digits missing on the player 2 and 3 displays: bad driver IC12 on the master display
• Master display board versions
• All score displays dead
• One score display dead
• UDN display driver chips

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