Sticking Damper in Young Chang GS-150

[Marc Lanthier]

Hi,
 
The damper in the G5 region of a Young Chang model GS-150 small grand piano
gets stuck open if you hit the key pretty hard (it must be pushed back down
by hand). A medium hit on the key seems to jam the hammer for 1 second but
it returns to the down position. Even if I hit the key lightly I observed
some binging of the damper before it returned to the down position.
 
What could be the problem, does the action have to be taken out of the piano
for repair?
 
Thanks,
 
Marc
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