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MMD > Archives > February 2002 > 2002.02.26 > 12Prev  Next

Groaning Noise in Expression Regulator Unit
By Craig Brougher

I misread what William Meyer intended to say in regard to his noise
in a Welte upright system, so in light of that, I would like to pursue
the problem further.  If you have a groaning in the unit, that is
caused by feedback oscillation, and always by a leakage.  Here's what's

If you can feel the oscillation in the governor, (which, by the way,
doesn't necessarily mean the problem is in the governor) something is
causing the governor pneumatic, hence the governor valve, to open and
close quickly enough to create the oscillation you feel.  That means,
either the control circuit is trying to close the valve more and when
it gets too close, something causes it to drop back, _or_, vice versa.
I will opt for the first scenario.

Let's suppose that your stack is perfectly airtight (which it should
not quite be, but let's suppose it is.)  Let's say new patent leather
valves were used in its reconstruction.  Because it looks to the
governor as an airtight seal, the vacuum pressure between the governor
valve and the stack quickly becomes closer to the pressure in the pump.
That is often going to be quite a bit greater than the intensity being
programmed, so since the expression vacuum is fed back in the Welte to
balance the expressions, the governor valve opens too abruptly and
overshoots!  Then it instantly compensates for the overshoot and that
creates an oscillation.  The leakage here is during the overcompensation.

What you might try to do is introduce two individual leaks to air in
your stack.  But make them "proportional leaks" by using a small felt
covered poppet and valve hole that seals its hole tighter as the vacuum
increases.   the overall leakage at 6" should be equivalent to a 3/32"
hole at each end of the stack.  Anywhere in the line you can put these

Now try that and see if your oscillations stop.  When you use the type
of leather that came off the piano in the first place, this problem
should not happen.  But then again, it can also be elsewhere.  It
doesn't have to be in the stack.  Anywhere in the expression loop
circuit, which involves the stack is suspect too.

The other thing I would ask is, if this changes it but doesn't fix it,
let me know which kind of governor valve you have.  There were three
types.  Is yours the plug type, the pallet type, or the round valve type?

Sorry about the misunderstanding regarding pneumatic leather.  No
excuse.  I just didn't read it carefully, and what is so funny is that
I reread it again to make sure I didn't misread it!  Ridiculous.

Craig Brougher

(Message sent Tue 26 Feb 2002, 19:30:33 GMT, from time zone GMT-0800.)

Key Words in Subject:  Expression, Groaning, Noise, Regulator, Unit

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