After putting the game back together (with an NVRAM fitted), setting the maximum credits adjustment to 00 doesn't give free play.

Free play won't take on System 7: do a factory reset first

Cause: The stored settings needed a factory reset before the adjustment would work.

Checks

• The DIP switches on the System 7 CPU only matter when the board is used in older System 3 or 4 games; they don't affect this.

Fix

• Do a factory reset by setting adjustment 50 to 45, then set adjustment 18 (maximum credits) to 00 for free play.

Systems

• Williams System 7

Games

• Black Knight

Related tips

• Game powers up in audit mode instead of attract mode
• Reset or lock-up: which points where
• Usual causes of lock-ups and resets
• Re-jumper a System 7 CPU from Defender/Hyperball/Star Light to a standard game
• CPU never starts: check the reset line at IC1 pin 40
• Intermittent lockups with both CPU LEDs on
• Damage after the black and white plugs were swapped
• Score displays dead but digits glow: bad wire in the CPU blanking plug (1J3)

Source: Pinball Repair help group!! (Facebook group)