Hi Roy,  Player-Care.com has a fair amount of information about the
Simplex player mechanism. Just use the handy Search Feature on the main
page and type in the word "Simplex". Or you can go to the Search Page
(see below) and perform the search:

  http://www.player-care.com/atlas_99.html

You can also 'see' and 'hear' a well-working Simplex player at Gabe
Della Fave's web site. See  http://www.gabesplayerpianos.org/

There is also a plethora of information in the Archives at the
Mechanical Music Digest.  Just go to:

  http://mmd.foxtail.com/Archives/KWIC/S/simplex.html

All that said, one problem that is sometimes overlooked by people is
the trackerbar tubing in the spoolbox.  The Simplex players have
a transposing trackerbar.  So, between the trackerbar and the lead
tubing, there are over 80 pieces of rubber tubing.  In many cases that
tubing has never been replaced and it may be leaking significantly.

The indication is that, while the player will rewind a music roll very
well, once the system is put into 'Play' nothing happens -- everything
seems to just die!  That is because a large number of notes (perhaps
all of them) are trying to play at the same time, and while a well
rebuilt unit 'will' play all of the notes simultaneously, a unit that
has never been rebuilt will not.

Another problem could be roll tracking.  The older the unit gets,
the less forgiving it is about being "on-track".  That's because the
pouches are probably leaking and/or getting too stiff.

By the way, under normal circumstances (when the stack, or upper
section) is in good condition, you should not be able to hear the air
exhausting from the exhausters (foot pedals) unless you are quite
close to the flap valves.  In a unit with significant leakage, you
will hear the air exhausting because there is no music playing and
an excessive amount of air is being exhausted into the atmosphere.

If you have other questions about the Simplex mechanism, feel free
to ask.

Musically,

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