Diode-test the lamp pre-drivers (2N6427/MPSA14)

Steps

• Game off, meter on diode test.
• Column pre-drivers are the even positions Q62 to Q76; row pre-drivers are the even positions Q46 to Q60.
• Red lead on the middle leg. Black on the right leg: 1.0 to 1.3. Black on the left leg: 0.6 to 0.8.

Systems

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

Related tips

• Test a 2N6427 (MPSA14) lamp matrix transistor with a meter
• Lamp and switch matrix transistor positions on the CPU board
• Lamp column still dead after a new TIP42
• Lamp matrix still wrong after replacing transistors: upstream parts
• Diode-test the lamp column and row transistors
• Diode-test the lamp 7408 and 7406 chips
• Photo: MOSFET lamp drivers with jumper wires
• How the lamp matrix runs 6.3 V bulbs on 18 V

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