Testing a lamp diode

Steps

• Meter on diode test; neither the bulb nor the diode needs removing for a socket diode.
• Black lead on the banded end: expect 0.4-0.6. Reverse the leads: no reading.
• Any other result means replace the 1N4004.
• For diodes on 555 lamp boards, unplug the board and remove the bulb first.

Systems

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

Related tips

• Diode orientation on #44/47 lamp sockets, and 555 sockets
• Testing lamp-row drivers with a test lamp
• Testing lamp-column drivers with a test lamp
• Test a lamp socket diode in place
• Lamp diodes sit on small boards at each socket
• Testing a coil diode
• Testing the diode on a coil
• Test a 1N400x diode on a coil, lamp socket or switch

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