One or more digit positions never light.

Whole digits missing

Cause: The 4069 input chip or a UDN6184/UDN6118 on the master display board, or a 74154 on the CPU board (which would affect players 1 and 3, or 2 and 4).

Checks

• With high voltage removed, probe UDN6118 inputs (pins 1-8): no activity points to the connector or 4069.
• Probe outputs (pins 11-18): inputs active with outputs dead means a bad UDN chip.

Fix

• Replace the failed 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
• Four digits missing on the player 2 and 3 displays: bad driver IC12 on the master display
• Missing segments: check the UDN7180 with a probe
• Seven-digit scoring on six-digit displays for green-ROM games
• CPU LEDs flash and go out but the score displays stay dark
• Bottom segments missing and master display resistors burning: shorted display glass
• Ball and credit displays show only even numbers after header work
• Score displays dead but digits glow: bad wire in the CPU blanking plug (1J3)

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