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MMD > Archives > July 2012 > 2012.07.20 > 04Prev  Next

Hobart M Cable Player Piano Problems
By John A. Tuttle

Hi All,  Nevin Engle writes in 120719 MMDigest:

> When I connect to a vacuum source, the stack is tight when the
> holes from the pouches are covered, without the top on.  When the
> tape is removed, all pneumatics close and everything is tight.
> After the top is attached, only half of the holes in the tracker
> bar can be uncovered and have the pneumatics close.  Some close
> rather sluggish and a few not at all.

The first thing that comes to mind is the size of the hole on the stack
verses the size of the hole in the tracker bar.  Typically, the hole on
the stack is twice as large (or larger) than the size of the hole in
the tracker bar.  So, the amount of air that can 'flow' into the pouch
is far greater than it is when the tracker bar comes into the picture.

Why is that important?  Because while we have air flowing into the
pouch to inflate it, we also have a bleed that's trying to suck the air
out from under the pouch.  And, when the hole supplying the air to the
pouch is 12-15 (or more) times bigger than the bleed, it overcomes the
bleed with little-to-no effort.

Putting the tracker bar back into the picture, we see that the maximum
possible air flow has been cut in half.  And, depending on the size of
the bleed, that 12-15 ratio has been reduced to 6-7, resulting in a
pouch that may struggle to inflate enough to activate the valve.

We also have another force working against us, and that is the
number of valves that get slightly activate (as evidenced by Nevin's
statement, "Some close rather sluggish and a few not at all."), which
reduces the vacuum level in the chest.  Ultimately, the combination
of the 'struggling' pouches and the lower vacuum level decreases the
maximum possible 'air flow' through the tracker bar (and tubing) to
the point where notes simply fail to operate.

The question now becomes, "Is there a solution to the problem?"  In
this regard, my initial response is 'No'.  Practically speaking, you
can't change to design of the system.  However, we also note that Nevin
said he changed the pouch material.  So, he changed the design of the
system.  Everyone knows that tan pneumatic skin is heavier and less
flexible than zephyr skin.  So, all forces being the same, the zephyr
skin pouch will inflate faster and easier than the leather pouch, even
if the leather pouch is well sealed.  So, my final opinion is that the
solution to the problem is to replace the leather pouches with zephyr

However, realistically speaking, how often are you going to be playing
more than 15-18 notes simultaneously?  Probably never.  So, stop
worrying about the fact that you can't get all of them to come on all
at once.  Or, return the system to its original design...

John A Tuttle
Brick, New Jersey, USA

(Message sent Fri 20 Jul 2012, 19:18:26 GMT, from time zone GMT-0400.)

Key Words in Subject:  Cable, Hobart, M, Piano, Player, Problems

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