grounded:
Yaw Damp in modern aircraft is a calculation in the FCC that constantly corrects rudder position, if you saw the rudder in flight it would look like it was fluttering constantly. Even in the 727 yaw damp is more or less a function of the auto pilot.
The 727 didn't have a FCC, rather a lousy two-axis Sperry auto-pilot. The two yaw dampers were independent of that lousy auto-pilot in all respects.