Scattered bulbs stay dark in lamp test.

A few unrelated lamps never light

Cause: Dead bulbs, bad sockets, broken wires at the socket, or cracked joints on a lamp board's header pins.

Checks

• Swap in a known good bulb.
• Check both wires are soldered to the socket.
• On lamp boards, check the header pins for cold joints.

Fix

• Replace bulbs or sockets and resolder wires or header pins.

Systems

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

Related tips

• Some tachometer (RPM) lamps won't light: failed lamp diodes
• 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
• One controlled lamp out while the rest of the matrix works
• One insert lamp dead with a good bulb and socket: diode on the lamp board
• How the lamp matrix drives a bulb
• No lamp matrix lamps light

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