System 3 CPU board reset circuit modification

Steps

• Take out C27 (capacitor) and the two resistors R30 and R40.
• Fit a new 10K 1/4 watt resistor (R96) from the left pad of R30 to the banded (top) end of diode ZR1.
• Williams made this change during Disco Fever production to make the reset more reliable; it is worth doing on every System 3 CPU board.
• With this change made, a System 3 board can also be bench-run without the unregulated 12 V supply.

Systems

• Williams System 3

Related tips

• CPU never starts: check the reset line at IC1 pin 40
• Restoring brightness on 4, 6 and 7 digit display boards
• Reset or lock-up: which points where
• Usual causes of lock-ups and resets
• World Cup will not boot after a CPU board swap
• Intermittent lockups with both CPU LEDs on
• "Adjust Failure" at power-on
• Identifying a System 3 to 7 CPU board

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