Replace the TIP42 lamp column transistors with IRF9Z34N MOSFETs

Steps

• Find the eight TIP42 lamp column transistors (odd numbers Q63 through Q77), just left of the large R149-R156 resistors.
• Remove each TIP42 and fit an IRF9Z34N MOSFET facing the same way.
• The MOSFETs need almost no drive current, so R149-R156 stay cool. Old burnt resistors can stay unless they are open, or they can be replaced with wire jumpers or zero-ohm resistors.

Systems

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

Related tips

• MOSFET lamp column upgrade
• Test a TIP42 lamp column transistor with a meter
• Test a 2N6122/TIP41 lamp row transistor with a meter
• Test a 2N6427 (MPSA14) lamp matrix transistor with a meter
• Driver board transistors and what they drive
• Replace burnt 3 watt lamp resistors with 5 watt
• Spare transistors for driver board and power supply
• Test a 2N5060 lamp matrix SCR with a meter

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