Non checking upright. The backcheck felt should not be curved unless the wooden block is curved as well. If the block is straight then replace the felt. THe letoff rail has a strip of felt glued to the back side to act as a jack movement limiting rail, or liberty rail. If this rail is too close to the jacks the butt can bounce off the top of the jack causing non checking. If it is further back the jack can become wedged between the rail and the butt and can cause the action to not return to rest position. The theory of backchecking on an upright is that the backstop will have an arc of such and such a radius and the angle of the backcheck will be such that the backstop will be wedged into and held by the backcheck. So the angle of the backcheck to the horizontal is critical to functioning. Likely in this case is that the backcheck needs to be tilted back slightly at the top then adjusted so it catches at 15 mm. The wire should act as a spring to help retain the backstop under a hard blow. If there is any lose of mechanical integrity such as wires loose in backcheck or wippen or broken glue joint at backstop shank and butt checking will not work. Check the glue joints carefully. If all else fails try some 220 (very fine) sandpaper lightly on the buckskin to raise a nap. Poor checking can be caused by poor regulation. You indicated that the letoff was way off. Regulate the action completely first then determine in the problem remains. Newton
This PTG archive page provided courtesy of Moy Piano Service, LLC