A chattering flipper is usually caused by a bad hold winding on the flipper
solenoid. Almost all flipper solenoids have two windings--a strong, kick
winding, and a weak hold winding. The strong winding is a thick gauge
wire. The weak winding is a thin gauge wire. Check that the thin wire on
the solenoid is not broken or disconnected from the soldering lug. Also
check that the wires leading to the solenoid are all connected properly.