Socket colour tells the bulb voltage (Williams)

Swap a suspect bulb into a known-good socket to test it. White sockets take 6.3 V bulbs; black sockets are for the 12 V flasher bulbs.

Systems

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

Related tips

• Single matrix lamp out with good bulb and wiring
• Bulbs to keep on hand for WPC games
• New GI sockets stay dark despite 6-7 VAC: GI wire soldered to the isolated diode lug
• How the lamp matrix drives a bulb
• One insert lamp dead with a good bulb and socket: diode on the lamp board
• Upper GI fuse blows after a pop bumper rebuild: solder blob inside a new socket
• Diode orientation on #44/47 lamp sockets, and 555 sockets

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