Diode-test the TIP120/TIP102 and 2N4401

Steps

• Game off, meter on diode test.
• TIP120/TIP102: black lead on the metal tab (or centre leg), red lead on each outer leg in turn: 0.4 to 0.6 each.
• 2N4401: red lead on the centre leg, black lead on each outer leg in turn: 0.4 to 0.6 each.
• A bad reading means the transistor (or the gate chip feeding it) is bad; a good reading is about 95% reliable in circuit.
• Replace a bad TIP120 with the tougher TIP102 and replace its 2N4401 at the same time.

Systems

• Williams System 3
• Williams System 4
• Williams System 6 / 6A
• Williams System 7

Related tips

• One or a few coils lock on at power-up
• Solenoid driver test at 2J9 and 2J11 with Leon's code
• Quick test for shorted TIP122/TIP102 drivers
• Test a TIP120/TIP102 solenoid driver with a meter
• Flash lamp stuck on and melting plastics on L/R reversed games
• A coil or flasher comes on and stays on at power-up
• Quick check of all TIP122/102 drivers before first power-on
• CPU and driver board transistors by position

Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Three, Clay Harrell (pinrepair.com)