Segments or whole digits are missing, nothing displays, digits strobe slowly, or segments bleed together.

Alphanumeric display segments or digits missing

Cause: Missing segments: a UDN-7180 (sometimes with its 74LS374). Missing digits: a UDN-6118 (sometimes with its 74LS240). Blank: fuse F301 or no plus/minus 90 V. Slow strobing: the 90 V has sagged to about 30 V. Bleeding segments: a ribbon cable on backwards.

Checks

• Swap the display driver and display boards with known good ones to isolate the fault.
• Check F301 and measure the plus/minus 90 V.

Fix

• Replace the failed driver chip, repair the high-voltage supply, or turn the ribbon cable round.

The alphanumeric display supply runs at about plus and minus 90 volts DC; do not touch it while powered.

Systems

• WPC Alpha-Numeric

Related tips

• WPC display and DMD controller board versions
• Alphanumeric display board numbers
• WPC alphanumeric display board high-voltage checks
• Alphanumeric WPC games: weak or dead score displays
• Display shows only a few broken left digits after board work: ribbon cable reversed
• Display shows random periods and shaky digits after its ribbon was unplugged

Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part One, Clay Harrell (pinrepair.com)