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MMD > Archives > September 2003 > 2003.09.03 > 06Prev  Next


Leaks in Airmotor System
By Pete Knobloch

Michael Hamilton has a Pratt-Read player and is complaining about
tempo changes due to pumping.  He also asks if the roll brake plays
a part in the tempo.

This description doesn't give enough information to diagnose the real
problem.  As far as the hold-back brake, it should not affect the
stability of the tempo.  If it has been oiled, which should never be
done, then that could be a problem.  As Robbie eluded to, try holding
the brake off of the drum and see if the problem goes away.  I don't
think that it will fix it.

Dirty oiled brakes tend to cause the roll to slow down at regular
intervals that have no relationship to the pumping speed.  Bad air
motors also exhibit this same problem.  If the problem has to do
with the speed slowing down at the end of the roll, then that can be
accountable due to a dirty brake or the tension adjusted too high on
the brake.  It can also be associated to a leaky air motor or bad cloth
on a regulator.

Let me assume that you mean that when the pump pressure is high, the
motor runs fast and that as the vacuum level drops, in between pedal
pumps or when playing softly, that the motor slows down.  I also have
to assume that the upper sections of the player are in good condition
and that it is fairly airtight.  This includes the air motor.  Since
the problem is speed regulation, the motor is probably OK.

When the player is in the rewind mode, a valve on the pump is turned on
that feeds extra vacuum to the motor which speeds up the rewind speed
during re-roll.  If the valve is leaky, it will cause this symptom.
On the Pratt-Read player, this valve is connected to the wind motor
with a "T" that goes between the motor and the regulator.

You can remove this valve from the system by pulling the large vacuum
hose going to the upper end of the pump and the "T" and use masking
tape to plug the open ends.  Now play a roll.  If the problem goes
away, then you have a leaky valve at the top of the pump.

If there is still a problem, start looking at the regulator.  While
someone else is pumping a roll at a normal tempo, squeeze the regulator
pneumatic with your hand while the roll is playing.  This won't hurt
the regulator.  All it does is close the knife blade in the regulator.
The wind motor should almost come to a stop or even stop while holding
the regulator closed.  If it doesn't stop, you have located the
problem.

The knife blade may not be closing all of the way because of a large
adjustment screw found at the bottom center of the regulator.  Take
a screwdriver and start unscrewing the stop screw (counter-clockwise)
while the roll is playing and you squeezing the regulator closed with
your hand.  The roll motor should slow down as the regulating screw is
coming out.  Right at the point that the wind motor stops is where you
want this adjustment screw set.

Now release the regulator with your hand and continue playing.  If
it doesn't speed up like it did previously, then you have fixed the
problem.  At this point you should verify that the motor doesn't stop
when you pump hard.  If it does stop, turn the regulator screw in
(clockwise) 1/4 turn and verify that it doesn't stop under hard pumping.

If the problem is still not fixed, you will have to rebuild the
regulator.

Note: if the pneumatic cloth on the tempo regulator is stiff, rebuild
the regulator anyway.  And as far as the leak at the tempo linkage that
you describe, this shouldn't be so loud that it can be heard.  There
will always be some leakage around these areas even when rebuilt
properly but you should never hear it.  I don't think that this leak
is your primary problem.

Pete Knobloch (Tempe Arizona, USA)


(Message sent Wed 3 Sep 2003, 18:49:10 GMT, from time zone GMT-0700.)

Key Words in Subject:  Airmotor, Leaks, System

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