Measuring a coil after replacing its driver transistor

Steps

• Always do this after a transistor replacement: an overheated coil can lose its enamel, shorting turns together.
• Remove the wire from one of the coil's lugs.
• Set the meter to low ohms and measure across the lugs.
• Most coils read 5-15 ohms, a few up to 150 ohms or down to 3 ohms.
• A coil well under 3 ohms is effectively a short and will burn out its new transistor; replace it with the same type.

Systems

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

Related tips

• Why WPC coil diodes are on the driver board
• Driver transistor dies after fitting a coil that has a diode
• Checking for power at a coil with the game on
• Coil-not-working checklist
• Grounding test for WPC coils 1-8
• Spotting the +50 V wire on a coil
• No 50 V coils work
• Checking the coil supply at TP6

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