Part of a digit, such as the top of a 0 or 7, is missing or stuck on, and only on some numbers or letters.

Segment missing or stuck only on certain characters

Cause: A failed 4049 (or 14049) CMOS hex buffer on the display board (U7-U11 or U15-U18 depending on board). Schematics sometimes mislabel them 4050.

Checks

• Red meter lead on the chip ground (pin 8), meter on diode test.
• Black lead on pins 2-7, 9-12 and 14-15: each should read 0.6 to 0.7; lower means a bad chip.

Fix

• Replace the bad 4049; the chips are static sensitive, so ground yourself.

Systems

• Williams System 11
• Williams System 11A
• Williams System 11B
• Williams System 11C

Related tips

• Display board chips shown as 4050 are really 4049
• Same segment missing across one 16-digit display
• Switch matrix connector pins and their chips
• A ball resting on one switch stops others in its column
• Characters missing from a display
• One segment lit on many characters of the top display: unsoldered leg on a driver chip
• Check the segment resistors before buying a UDN7180
• High-voltage fuse blows once the display connector is plugged back in

Source: Repairing Williams System 11 Pinball 1986 to 1990, Part Three, Clay Harrell