Bulb, socket and wiring test fine but the lamp stays dark.

Single matrix lamp out with good bulb and wiring

Cause: The lamp diode is open.

Checks

• Check the diode with the meter diode range.

Fix

• Replace the diode.

Systems

• Williams System 11
• Williams System 11A
• Williams System 11B
• Williams System 11C
• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• Testing lamp matrix columns with a test socket
• Testing lamp matrix rows with a test socket
• How the lamp matrix drives a bulb
• One GI string is out
• A few unrelated lamps never light
• One controlled lamp out while the rest of the matrix works
• One lamp dead though socket and wiring are good: diode on the lamp board
• System 11 lamp matrix

Source: Pinball Machines: How They Work & Troubleshooting, Norbert Snicer