One function or the whole game fails; close inspection shows a blade with no contact on it.

A switch blade has lost its contact

Cause: The contact has fallen off the blade.

Checks

• Clean and inspect every switch; missing contacts and broken blades or wires stand out.

Fix

• Sand the blade and solder a new contact on; the other blade's pressure can hold it while soldering.

Systems

• Electro-mechanical

Related tips

• Replacement switch contacts for EM games
• Why EM switch contacts get dirty
• File EM switch contacts
• Game stopped working after filing switches
• Playfield switch damping blade shorting
• Neighbouring switches in a stack short together
• New to EMs? Do not adjust switches that work

Source: Repairing Electro-Mechanical (EM) Coin Operated Games to 1978, Part One, Clay Harrell (pinrepair.com)