The probing questions posted by Bernt Damm regarding the purpose of
the "Play-Don't play" sections of the Duo-Art test roll may have no
answers.  There are many things modern collectors and rebuilders have
been unable to find answers for in Aeolian's Duo-Art piano.

Some mysteries have been solved, by deduction, to a degree of certainty
that satisfies most of us.  Without going into laborious detail, here
are some of the facts giving basis to those beliefs.

1.  Soft pedal (half-blow) compensation exists in several different
     schemes.
2.  Stack construction and pneumatic size is widely variable.
3.  Individual note strikers were regulated for even play (see below).
4.  Some rolls do not conform to the "Play-Don't Play" scheme of
    test roll #3.
5.  Several sizes of stack valves exist.

What must be remembered is that the Duo-Art expression system does not
compensate well for the loss of stack vacuum when multiple notes are
being played.  That compensation _must_ be achieved by roll coding.

Production standards simply cannot guarantee equal vacuum loses in
parallel situations in every system produced.  For that reason, a
method to compensate for production variables and lost motion within
the awkward mechanical knife valve linkage had to exist.  Hence a test
roll used to set each gap of the accordions.  Indeed, anyone who has
recovered these accordions and has failed to mark each seemingly
identical adjusting/stop block (3 per gap), has found they are not
interchangeable, but vary due to production inaccuracies.

The stack pneumatics are made with small phosphor bronze springs
embedded at the hinge of the tenor and treble notes.  After rebuilding
several original stacks, I realized that I always found some of these
springs that apparently had worked out and were loose at one end.  Upon
further investigation, I discovered that every pneumatic with a loose
spring also had a scratch mark on the movable leaf.  I called another
rebuilder and asked if he too, could make that observation.  He did
and it was the same.

This spring, which cannot be seen once the stack is assembled, was
apparently regulated by technicians using a special tool that could
reach the hidden hinge and dislodge one end of the spring.  In using
that tool, the bottom, movable leaf became scratched.  Stacks so
scratched have been found in several areas of the country.  From that,
we deduce the factory made the adjustments.  I'm still searching for
the service manual describing this procedure, if one exists.

The Duo-Art was a mass produced, home entertainment device.  Modern
rebuilding techniques can bring these instruments to amazing levels of
perfection.  As collectors, we really delve into these instruments.
But still, we tend to get stuck in a rut of dissecting the mechanism in
terms of static charts and figures.

Dynamically, the mechanism does much more.  An example of this, is the
situation when one side of the stack is exhausted by both the theme and
accompaniment at the same time.  How did the editors get around this
and other oddities?  They simply listened to the roll and changed the
coding until it sounded right.  Existing master rolls show numerous
corrections, proving this trial and error method.

There may not be any more manuals and written standards of production
to find.  But these examples show that both piano regulation and roll
production were a subjective process with human ears being the
benchmark.  Through deduction, we can figure out some of the process.
As they say in London's Underground, "Mind the Gap".

Bob Taylor