Testing a dead coil or flasher

Steps

• Power off. Measure the coil's resistance (flash lamps removed from the socket): 3 ohms or more is normal; open means a broken winding; near 0 means a shorted 1N4004 diode or coil.
• Power on in attract mode. Measure DC at each lug: 50 to 60 V for coils, about 20 V for flashers.
• Clip a test lead to ground and briefly touch the coil's lug on the unbanded diode end (for a flasher, the socket side not chained to the others). It should fire.
• Look up the device's driver MosFET (the Q number) in the schematic.
• Clip a lead to backbox ground and briefly touch the metal tab of that MosFET. The device should fire; if it does, the wiring is good and the MosFET or its drive is suspect.
• The same method covers devices on the aux driver board.

Systems

• Gottlieb System 3

Related tips

• Cue Ball Wizard: coils holding on weakly (BUZ72L MosFETs)
• Checking for power at a coil or flasher
• Flippers dead in a game
• Coil MosFET ratings and substitutes
• A whole row of eight switches dead
• Spare MosFETs for the driver boards
• Logic chips worth stocking
• What the A3 driver board drives

Source: Repairing Gottlieb System 3 Electronic Pinball Games from 1989 to 1996, Clay Harrell (pinrepair.com)