Duo-Art plays too loud despite being regulated correctly

Two things come to mind that might cause the problems experienced by
Bernt Damm's customer, and both can afflict a piano that is regulated
correctly according to the test roll.

A common fault on Duo-Arts is that the springs returning the accordion
pneumatics to their zero position are not strong enough.  This means
that once the regulator is moved away from it zero point it never
returns there!  This is not covered by the test roll, so it's quite
possible for a piano play far too loud despite having a well-set zero
level and correctly-gapped accordions.

There is a very easy test for this: cover the tracker bar with a roll
leader, tempo zero, and turn on the suction supply.  The regulator
pneumatic should move up the specified half-inch or so to set the zero
level suction.  Now pull the accordions shut by hand, and let them go.
The regulator should rise and then return to exactly the same position
it started from! If it fails to return to its initial position, then
the return springs are too weak.  Strengthen them until they do their
job.

These springs have to be disconnected for some work on the expression
box, and it's easy to under-estimate the required strength.  They have
to work against the friction of the regulator knife valve, which
increases as the pump suction supply rises: a setting which works with
the suction off may not be when the instrument is playing.  The
condition of bearings and bushings also has an effect.

Another problem that can cause theme suction to be too high is blocking
the small bleed into the theme regulator -- this is (on UK instruments,
anyway) a hole drilled through the narrow end of the regulator, and
covered with a felt disc.  It allows the theme level to decay faster
than residual rapidly.  If blocked a high suction level can persist
longer than required and make a later themed note too loud.

Julian Dyer