Check every coil resistance before first power-up
Steps
- Set the meter to its lowest ohms range.
- Measure across each coil's lugs (on three-lug coils, from the common lug to each other lug).
- Expect 2.5 ohms or more; lower means a burnt coil or a shorted driver transistor or diode.
- For a low coil, lift the wire off one lug and measure again: still low means the coil or its diode is bad; now normal means the driver transistor is shorted.
- Cut one leg of the 1N4004 diode and recheck to rule out a shorted diode.
- Replace any bad coil before repairing its transistor, or the new transistor will fail on the first fire.
Systems
Related tips
- Check for power at a coil or flashlamp
- Coil overheated: check its resistance
- Which way a coil diode goes
- Which way a coil diode goes
- Check the coil diode after replacing a driver
- Old Williams SA/SG 23-850 coils and their AE/AL replacements
- Coil outputs available on System 3-7
- No solenoid voltage
Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part One, Clay Harrell (pinrepair.com)