Hi All,  Another point to consider when calibrating the Tempo indicator
is the tension of the forward roll brake.  Also, call me foolish, but
I have always believed that the manufacturer spent more than a little
bit of time designing the air motor and air motor governor so that the
Tempo was relatively linear across the entire range of operation.

Realistically speaking, how hard would it be to experiment with
different graduated openings until one was found that accomplished
the desired result.  With that in mind, it seems reasonable to me that
a preliminary starting point would be to insure that the tempo slider
is just closing the opening at a Tempo of '0' (on the indicator) and
fully opened at the maximum Tempo.

Also, consider that when the system was brand new, there was almost no
lost motion in any of the linkage between the Tempo Lever, the Tempo
Indicator, or the tempo slider.  Consider also the condition of the
governor spring.  It was carefully selected to perform relatively
accurately across a certain range of operation (vacuum levels).  It's
entirely possible that it got over-stretched at some point in its life.
Or, it just got weaker as it aged.

In the pursuit of perfection, nothing can be overlooked.

Musically,
John A Tuttle
Player-Care.com
Brick, NJ, USA