Testing a lamp diode in place

Steps

• Game off, meter on diode test; no need to remove the bulb or desolder.
• Black probe on the banded end: expect 0.4 to 0.6 V. Reversed: no reading.
• Any other result: replace the 1N4004. Diodes on #555 lamp boards test the same way. An open diode leaves its lamp dark.

Systems

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

Related tips

• Lamp socket diodes and 555 lamp boards
• System 11 lamp matrix
• An entire lamp row or column is out
• System 11 lamp matrix basics
• Testing lamp matrix rows with a test socket
• A single controlled lamp out
• One lamp dead though socket and wiring are good: diode on the lamp board
• Quick lamp row test: ground the row driver tab

Source: Repairing Williams System 11 Pinball 1986 to 1990, Part Two, Clay Harrell (pinrepair.com)