*** For MMD with associated technical diagram ***

In MMD 031116, Bernt Damm described his UK Duo-Art stack with
a crude channel carved to connect the bleed exhaust channel of
the note at the stack split point to the other side of the stack.
He assumed this was a factory fault that had been bodged up and
fixed it.  Actually, this was standard construction, and these
stacks should be left exactly as built!

The problem arises from the design of Aeolian stack that has two
staggered rows of pouches with the bleeds installed in the pouch
boards and exhausting into the stack.  The stacks with separate
bleed rails (as normally found in grand pianos) are not affected
because the suction supply to the bleeds is separate from the pouch
board.

The stack divider runs diagonally across the stack between the
pouches.  In one direction, this results in all the bleeds exhausting
into the same side of the stack as the pouch they operate.  However,
if the stack split is shifted by one position either way, the divider
has to run on the opposite diagonal.

Unfortunately, the board layout isn't symmetrical, so reversing
the divider results in one of the bleeds next to the split point now
exhausting into the opposite half of the stack to its pouch.  If
this is left uncorrected, the note will be operated merely by the
difference in suction levels on either side of the stack.  (I've
produced a diagram to help visualise this.)

The fix adopted at the factory was to block the inappropriate exhaust
channel and gouge a new channel along the edge of the pouch board to
join up with the adjacent exhaust into the correct side of the stack.
It looks a very crude bit of work, and I'm sure numerous rebuilders
have "corrected" it then wondered why their instrument doesn't work
(if they ever notice).

The difficulty faced in the factory was that stacks were designed
with a 44:44 or 40:40 split for Themodist instruments, and when so
divided all is well.  When the Duo-Art coding shifted the split by
one note to 39:41, the pouch board design couldn't cope.  A redesign
that inverted the two rows of pouches would have worked, but then the
stack would be wrong for Themodist instruments -- and the factory had
to produce both.

The simple fudge of joining the 'wrong' exhaust channels to the
correct side of the stack can be seen as a quick and effective
shop-floor fix to a problem arising from a ill-considered usage change
imposed from on high (anyone working in the software industry today
will sympathise).  The people who built them weren't idiots and
understood what they were doing, even if modern rebuilders don't!

So, in this case the original build was functionally correct, if
inelegant, and should be left alone.  Connecting the bleed exhaust
to the wrong side of the stack will give rise to false notes, and
modifying the stack split point gives an instrument incapable of
correctly playing the rolls it was build for.

Julian Dyer

 [ Thanks, Julian, I'll place your article and image at
 [ http://mmd.foxtail.com/Tech/bass-treble.html  -- Robbie