A solenoid driver transistor fails again soon after being replaced.
A cracked coil diode keeps killing its driver transistor
Cause: The 1N4004 diode on the coil is cracked or has a broken lead, so the coil's turn-off spike reaches the driver transistor.
Checks
- Under the playfield, hold each coil diode by its body and pull gently; a cracked body or broken lead shows at once.
- Test suspect diodes out of circuit (see the diode test entry).
Fix
- Replace bad diodes with 1N4004s, band to the coil's power lug.
- Then replace the driver transistor.
Systems
Related tips
- Which way a coil diode goes
- Which way a coil diode goes
- Testing a coil, switch or flipper diode with a meter
- Testing the diode on a coil
- Always check the coil diode when a driver transistor has failed
- Testing a coil diode on the driver board
- Why WPC coil diodes are on the driver board
- Testing coil diodes on the auxiliary power driver board
Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Two, Clay Harrell (pinrepair.com)