damper after ring?

[hbj@cableinet.co.uk]

I hope the following will launch some useful discussion.
This morning after a rather urgent call from a favorite customer I went
to look at her Steinbach under-damper over strung.
This piano has a case of ringing on after dampers return.
Not specific notes, or individual dampers failing, but a slight echo
effect.
For example when striking individual notes in the bass, you get a
secondary partial singing on, usually an octave above the fundamental.
I have isolated the offending strings within the same break of strings,
by placing a hand over the group of strings, and stopping the singing
within the break
from either the string itself, or one of its neighbors.
Yet, the dampers come home clean, and the strings are blocked.
But I think this must be due to the possibility that the dampers aren't
quite meeting the node point of the string.
I found by resetting a few of the dampers that made this condition most
apparent I was able to get rid of most of the singing on, but not all of
it.
Several things occurr to me.
As the piano ages, and the strings get older is it possible that the
node point changes to a point where the dampers will begin to have less
of an effect?
In the cases I found, I had to raise individual dampers a fraction of a
centimeter and the effect seemed to be cured.
What about the expedient of raising the height of the action standard by
a fraction to raise the general level of the dampers in
question?
Short of going back to basics, and replacing all of the dampers, or at
least rebuilding all of the damper levers with slightly longer damper
wires and repositioning all of the dampers, I can't see any other
solution.
How can one find the "right" node point.
Seems to me this will be a real pains taking procedure.
I know that one quick thought will suggest that the dampers are worn,
and should be replaced.
But, I think that the dampers need to rather than be replaced, will most
likely need to be repositioned to find this node point. I have seen very
old pianos with very poor dampers with no singing on what soever.
I am nterested in any thoughts
Thanks
regards



--
Henry Brugsch
Editor Piano Technician's Quarterly
http://www.airtime.co.uk/forte/brugsch.htm


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