Test a 2N4401 pre-driver with a meter
Steps
- Meter on diode test.
- Red lead on the centre leg.
- Black lead on each outer leg: expect about 0.4 to 0.6 V.
- Tested in the board, it can read bad when the 74xx chip driving it has failed, which is common.
Systems
Related tips
- Diode-test the TIP120/TIP102 and 2N4401
- One or a few coils lock on at power-up
- Solenoid driver test at 2J9 and 2J11 with Leon's code
- Flipper relay does not click on in test
- Flippers dead, or still live after game over: the flipper relay
- Replace the pre-driver along with a failed driver
- How a coil or flasher is switched: PIA to TTL to pre-driver to driver
- CPU and driver board transistors by position
Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Two, Clay Harrell (pinrepair.com)