Some game features do not work or work erratically.

Features misbehave with the backbox ground strap loose

Cause: The braided ground strap behind the backglass is not fastened under its wing nut.

Checks

• Check that the ground strap is on the wing nut and tight.

Fix

• Fasten the strap and tighten the wing nut. On Firepower and later, also put the white-with-red playfield ground wire under the same wing nut.

Systems

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

Related tips

• Keep the backbox ground strap tight
• Loose wire at the backbox ground braid screw is the MagnaSave ground
• Individual controlled lamps dead with good bulbs: dirty backbox harness connector

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