Coils are weak.

Weak coils from a browned 3J3 connector

Cause: The 3J3 solenoid power connector on the power supply has overheated and turned brown, adding resistance.

Checks

• Inspect 3J3 for discolored pins and housing.

Fix

• Replace both the header pins and the housing pins at 3J3.

Systems

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

Related tips

• No solenoid voltage
• Replace the 3J6 logic power connector
• Burnt GI connectors on the power supply (System 7, Hot Tip, Lucky Seven)
• No coils fire: check the 28 V solenoid supply
• Overnight test to separate connector faults from power faults
• Pull the coil and lamp fuses for the first power-up
• Burnt GI plugs on the power supply board
• Flippers dead: fuse and coil voltage

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