Robbie Rhodes writes about the 3 bar linkage in Duo-Art grands and the
non-linear effect on knife valve movement.  Does anyone know the effect
of adjustment of the simpler upright duo-art linkage and the correct
starting position?  I have played around with it trying to compress and
expand the dynamic range and figured I would wear things out before I
figured it out.

The service manual warns to not adjust the leather nuts on the pull-down
rod as it is set at the factory.  This plus readjustment of the zero
setting screw would change the dynamic curve.  Unfortunately the factory
is no longer available to consult for the original setting or for advice
on the changing of that setting.