On an electromechanical game a relay pulls in and stays in after its coil should have dropped out, even though the return spring is strong and nothing binds. Pushing it in by hand doesn't make it stick.

EM relay pulls in and sticks despite a strong spring: clean it or add a thin spacer

Cause: Either residual magnetism in the core and armature, or old lubricant and dust that turns sticky when the coil warms up, holding the armature against the coil.

Checks

• Turn the game off: if the armature still stays down, it is mechanical stickiness rather than the coil being energized.
• Inspect the face of the armature and the top of the coil core for residue or a worn dimple.

Fix

• Clean the armature face and coil core with isopropyl alcohol or naphtha; smooth any worn spot.
• If it still sticks, put a very thin non-magnetic spacer (a small piece of electrical tape or thin card) between the armature and the core so it can't seat fully.
• As a longer-term fix, replace the steel screw that holds the relay coil with a brass or stainless one, or replace the relay.

EM relays and their wiring can carry line voltage or high-voltage AC; switch off and unplug the game before working on them.

Systems

• Electro-mechanical

Related tips

• Game won't start though credits and reels reset: bad joint on the latch coil
• Credits register but the start button does nothing
• Start button does nothing but the start relay works by hand: dirty inertia switch
• Push relays in by hand gently and straight
• Player unit sticks: clean off old lubricant, don't add more
• Game resets but the outhole relay never fires: feed wire hanging by a strand
• 10-point relay sticks on every time 10 points score: adjust the relay's own switch
• Score motor runs on at game over: end game relay not pulling in

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