Gottlieb Bank Shot (Gottlieb, 1976) knocker "sticking"

[njPinWiz]

On a Gottlieb Bank Shot (add-a-ball), when the score is achieved to award a WOW, the knocker sounds & then gets "stuck"...humming noise occurs. When ball drains, the knocker returns to rest position. Interestingly when a WOW is awarded for anything other than score value the knocker functions properly. Any ideas as to what might be causing this "sticking" and what is the solution? Thanks.

 

[faralos]

is the knocker a solenoid?
the rod may be hanging up inside it due to the tip
'mushrooming' I'd replace the solenoid if that is the case
if that rod hangs up inside it is keeping the relay in an active position
hence the buzzing sound
and you may burn out the solenoid from it or worse take out a board

 

[njPinWiz]

Thanks Faraols, will check that out. I've also discovered that it might not be a problem with the solenoid on the knocker, but wtith the relay switch on the solenoid on the WoW stepper. Someone else had suggested that. Any more thoughts appreciated.