One matrix lamp stays dark with a good bulb.

A single lamp does not light

Cause: A worn socket (shrunken fibre insulators let the contacts corrode), an open or missing 1N4001 diode on the socket, or a break upstream in the daisy-chained row or column wire (which also kills lamps further down the chain).

Checks

• Check the socket contacts and lugs.
• Check the socket diode.
• Check for a break in the daisy chain.

Fix

• Replace the socket or diode, or repair the wire.

Systems

• Williams System 3
• Williams System 4
• Williams System 6 / 6A
• Williams System 7

Related tips

• Fit a remote battery holder
• Updating pre-Ice Fever reset boards to protect RAM Z5
• How the lamp matrix runs 6.3 V bulbs on 18 V
• No lamp matrix voltage
• GI lamps and matrix lamps
• All matrix lamps out, everything else works
• Light a lamp row by grounding its transistor
• Lamp flickers or is out with a good bulb

Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Three, Clay Harrell (pinrepair.com)