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MMD > Archives > April 2001 > 2001.04.13 > 01Prev  Next

Duo-Art Expression Box Adjustment & Springs
By Sam Harris

Spencer Chase wrote:

> Modern springs are not rated for initial rate and can be much lower
> than the original springs.  This can force the springs to reach their
> elastic limit before the top is finished.

I realize how important it is to find springs with rates as close to
the originals as possible.  Here is the method I used to determine how
close the new springs are.

1.  Removed the springs from my Steinway XR for testing.

2.  Using a digital fish scale each spring was stretched 3" and the
pounds-force were noted every 1/2 inch.  (To extend the spring 3" is to
stretch them 1" further than the actual working parameters.)

3.  After measuring the two old springs I measured the two new springs
and charted the results.

The chart can be viewed at this link.  If it fails try it over again and
it will load the second or third time:

This is a graph of the raw data and you can make your own assessments.
My assessment is that the springs are so close there is no way I would
spend any great amount of effort or money to wind new springs when
these stock springs match so closely.  Keep in mind that the last 1" is
beyond the actual working parameters, and when you look at the spring
rates from 0" through 2" you will see they vary little.  In fact, who
knows, but the old springs may have been just like these new springs,
but have changed because of aging and stressing.

This test was to measure the springs throughout their working length
(actually just a little further) to see how close the new ones were to
the old ones.  I did other measurements like wire size and coil diameter
before ordering the new springs.

It should be noted that any _miniscule_ changes these springs might
cause can easily be worked out by thoroughly charting the box on the
bench.  With all the different springs we see, even on the grands (not
counting the uprights), I believe that these new stock springs, which
are so close to the old originals, are a great replacement.

Craig Brougher stated about Pete Knobloch's Duo-Art box:
> In this case, reverting to "the weakest springs" did not really
> solve his problem either."

I had a problem with the Accompaniment vacuum being too week at step 15,
so I decided to tighten the bonnet on the spring to see if that would
bring it up.  It is my opinion through this experience that adjusting
the springs will make only very SMALL differences at the higher levels
and affect greater the lower levels.  I set the springs just tight
enough not to rattle and left them alone.  The levels were roughed in
using other methods.

A special thanks is in order to Al Pebworth, Craig Brougher, Pete
Knobloch, and Spencer Chase for providing me a vast array of expert
advice in my first expression box restoration.  Craig, you really know
your stuff and I appreciate all the detailed and understandable advice
you have given.  Thanks for being patient and kind.  Al, thanks for
sending the set of springs to test and for sharing your expression box
charts with me.  You guys are the best.  I would not have taken the job
if I had not known you guy's were there to help.

The Steinway expression box was installed yesterday and it sounds
great.  You won't believe it but the only adjustment needed was raising
the Accompaniment about 1/2" H2O.  The only problem is the pump vacuum
measures 50+" H2O at the pump and only 30" H2O from the modulator which
was set to normal!  The leather belt allows too much slip and cannot be
adjusted any more.  When it was explained to the owner that the contrast
between ppp and fff would be much greater, he was not interested in
raising the fff levels any more than they were.  He liked it and was
pleased with the outcome.  I would have wanted the full dynamic range,
but he was satisfied and enjoyed listening to the four or five rolls we
listened to.

Have a great Easter weekend everyone!

Sam Harris - Greenville, North Carolina

(Message sent Fri 13 Apr 2001, 16:18:33 GMT, from time zone GMT-0400.)

Key Words in Subject:  Adjustment, Box, Duo-Art, Expression, Springs

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