As most MMD subscribers know, I have been transcribing Duo-Art rolls
to make computer files for the PowerRoll and for MIDI players of
all sorts.  I have been working on some new music and I came to the
disappointing realization that the music sounded better on my piano
synthesizer module than it did on my Steinway Duo-Art piano.  Was
something wrong, or had the old technology met its match?

I had a couple of rolls that were particularly bad and I knew something
was wrong.  I narrowed it down to the expression box which had been
rebuilt before I bought the piano.  It was truly tired and needed a
rebuild.  I decided to do a top quality job with all original materials,
etc., etc., blah blah blah.

When I got the expression box apart I decided that it was time for some
improvements.  The box was made of a very soft wood, which I had not
seen before, that made rebuilding more trouble due to stripped screws
and damage caused by disassembly.  The previous rebuilder had done a
fair amount of damage that was repaired with automotive body filler.

I did all the usual patching that I always do, milling out the bad wood
on the vertical mill and splicing in new wood, but I decided that this
should not happen again to the next rebuilder, especially as I hope to
live long enough to be that person.

I installed machine screw inserts for the knife valve seat blocks to
insure that they would stay in place and drilled the box for screws to
hold it together, rather than using glue.  The new pouch leather is not
likely to last as long as the old, and I know it will not be 70 years
before the box needs to be split apart again.

I made a gasket out of a closed cell sponge tape that is used to keep
water out of metal roofing.  It is supposed to last for many yeas
without getting gummy.  I believe that it is made of polyethylene,
guessing from the feel of the material.  It squeezes down to a very
thin gasket without squeezing out of the seam.  If it should ever fail,
it is just a matter of removing 13 screws to replace it, not splitting
the box with any of the previously used methods.  It adds just about
the same amount of travel to the theme valves that a couple of sandings
of the box had previously taken away.

I used 4.5" "drywall" (tek) screws that hold much better in soft wood
than tapered wood screws.  They are completely inside the box and
invisible once it is reassembled.  Now that I had modified the box to
make it easy to disassemble without damage, I decided to make some
rather drastic changes.

The knife valve and regulator pneumatic are the heart of the expression
box.  Any sticking causes missed expression calls, as everyone knows,
but it does more than this (as I was to discover later).  I did not
want to have to stretch the return springs to their limits to make the
box respond (as I have had to do to keep older rebuilds working), so I
installed ball bearings on the inside ends of the main shafts and left
the outer bearings original, as they are not as prone to wear and a
seal is needed there.

I installed bushings of a high density PTFE compound into the large
bearing of the knife valve and made washers of this same material for
thrust bearings where they were originally bushing cloth.  I left the
small bushing in the knife valve original and rebushed it.  This
bushing needs to have a little give or it can cause the knife valve
to cock.

When I installed the box, and corrected the usual number of crossed
tubes and reset the zero intensity levels, I was amazed at the results.
This piano now has a sensitivity and fluidity that I have never heard
in a Duo-Art before.

I am sure that a certain percentage of new pianos sounded this way and
that some careful rebuilders today may achieve the same results, but I
believe that my methods are an improvement in terms of reliability in
the current context of worn components and unavailability of original
materials such as the pouch leather.

I believe that most of the improvement is due to the smoothness and
lack of play in the knife valve.  I now realize that it makes subtle
compensations in vacuum within a very short time when working properly,
and when it is not doing so it causes a subtle deterioration of the
music which is difficult to quantify but which prevents the piano from
being truly musical

I would recommend to fellow rebuilders that they pay very careful
attention to the details of the knife valve whether they copy my
improvements or not.  These boxes are now old enough that nothing can
be taken for granted.  Every little part needs to be made perfect for
them to perform properly.  Old bushings may not be left, and a single
loose screw can be a disaster.

I now want to listen to all of those rolls that I never liked before.
I have tried several and realize that it was the piano, not the roll
that was bad.

Spencer Chase
http://nt.mcn.org/spencer/