Make a System 3-6 driver board work in any game

Steps

• Locate resistors R204-R211 in the upper right corner of the driver board (1000 ohm on System 3, 330 ohm on System 4-6).
• Replace them with zero-ohm jumpers. System 7 boards already have this.
• The modified board then works in System 3 to 7 games; an unmodified System 3-6 board in a System 7 game will miss switch closures.

Systems

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

Related tips

• Switches missed in a System 7 game with an older driver board
• Driver board switch matrix resistors R204-R211
• System 6 and System 7 driver boards can be swapped for testing
• Switch column or row fault on the driver board
• System 6: game stuck in tilt with switch 1 closed: cracked pins in the 2J2 plug
• Switch matrix column test at 2J2 with Leon's code
• Switch matrix row test at 2J3 with Leon's code
• Board or playfield? The switch test jumper method

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