A single CPU-controlled lamp never lights.

One controlled lamp out while the rest of the matrix works

Cause: Socket lamps: a loose or corroded socket, or a broken daisy-chained wire upstream. Board-mounted lamps: cracked header-pin solder joints on the lamp board, a cracked diode on the board (common with 555 bulbs), or a bad IDC connector.

Checks

• Check the bulb and socket.
• Trace the wires back to the previous socket.
• On lamp boards, inspect the header pins and diodes.

Fix

• Replace the socket, repair the wire, resolder the header pins, or replace the diode.

Systems

• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• No lamp matrix lamps light
• All matrix lamps stay on
• None of the controlled lamps work, or they light in the wrong pattern
• Part of a lamp row or column stays off
• Single matrix lamp out with good bulb and wiring
• A lamp row very bright or dead
• A few unrelated lamps never light
• One insert lamp dead with a good bulb and socket: diode on the lamp board

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