[ Bob Maniaci wrote in 120722 MMDigest:

> On treble crescendo, it rises smoothly to 9 inches then jumps to
> 16 inches and continues to rise.  On decrescendo, the treble smoothly
> drops to 11 inches and holds.  It will not return to 6 inches.

Hi Bob,  I'll take a stab at this.  It sounds like one of the treble
intensity valves, probably #6 but possibly #4, has a blocked bleed,
causing its pouch to be overly sensitive, or there is some slight
leakage in the pouch tube which puts it right on the edge of inflating.

When the vacuum increases as the treble crescendo is closing, that
increase is also present in the valve body above the pouch.  If the
bleed is unable to carry away the (possibly increasing) atmospheric
signal under the pouch, the point will be reached (9") where the valve
will trip and the vacuum will jump to 16".  After the crescendo
"catches up", the vacuum will continue to whatever maximum is available
from the pump.

During decrescendo, the valve is now locked on, and the vacuum will not
drop below what you note as 11".  The reason it does not go back to 9"
is probably there is a bit of mechanical "hysteresis" in the linkages
and precise repeatability from both directions is difficult to achieve.

In this state, the only way to reset the valve is, as you note, by
turning the motor off and back on.  When you block 7T open, you send
a constant cancel signal to the valve so that it sever "sets" during
the crescendo excursions.

Observe the crescendo unit on opening.  If it still returns to its full
open position (indicated by the pallet valve "hunting"), and the vacuum
remains at (anything) above 6", then almost certainly one of the
intensity valves is locking-on spuriously.

If you are lucky and a clogged or partially clogged bleed is the
culprit, you may be able to clear it by applying a strong vacuum at
the valve pouch nipple directly.  No amount of tracker bar pumping will
clear these bleeds because they go through the expression cut-out unit
which acts as a pneumatic diode, preventing extraction of any bleed
blockage.

If the bleed is clear, then there is slight leakage somewhere between
the valve and the tracker bar.  It could be a leaky gasket in the
expression cut-out unit, or if you are still operating on the original
lead tubing from the tracker bar, there may be corrosion pinholes
anywhere along its length sufficient to trip the valve under increasing
vacuum from the crescendo unit.

Good luck and let us know what happens.

John Grant