"double striking" problem

[Avery Todd]

List,

   I had a problem yesterday with our Hamburg D. One of our piano faculty
gave a recital last night and had been talking to me about notes "double
striking" (his term) on very soft playing. It was a VERY soft, but
noticeable, double strike on some notes (hammers not going into check and
bouncing back into the string), primarily notes in the middle third of the
piano.
   I was able to solve the problem to his satisfaction but I don't *really*
understand what caused it in the first place so it was pretty much a trial
and error kind of thing. I just now talked to him on the phone and
everything worked out very well. He said he felt very comfortable with the
piano. So, I guess all's well that end's well, but I need to understand
what causes this type of thing so I can prevent it from happening in the
future.
   It's very difficult to explain this problem in words, but here are some
questions and some things I did to try & solve the problem.

   What really controls how much upward "throw" (I can't think of a better
term) the hammer does when adjusting the repetition? Is it strictly how high
the hammer catches on the backcheck? Does the amount of let-off or drop
affect it also? I know that key dip would also affect where the hammer is
caught in relation to the strings, of course.
   The repetition spring tension is what "seemed" to be causing the problem.
I had to play at least mp to get the hammer to go into check but most
hammers had very little lift at all. Certainly not strong enough to cause
any kind of bounce at the top. I even had to increase the tension the
smallest amount possible to ensure repetition without getting a bounce back
on very soft playing. I brushed the hammer tails with a suede brush to get
any residue out the grooves on the tails to try to improve checking.
   Also the backchecks. They're Hamburg backchecks, about 1-1/2 yrs. old,
having been replaced when new hammers, shanks and flanges were installed. I
regulated them as high as possible without getting a rub on the hammer tail

on a hard blow. This also seemed to help some.
   I even increased the aftertouch a little by raising the hammer line
slightly, trying to insure that the jack was completely escaping from under
the knuckle. Yes, I lubed them, too. I did notice that the knuckle leather
is starting to get slightly loose around the core, so I guess I'll have to
do that correction as soon as I can put the piano out of commission for a
bit. Could that looseness have any affect on this?
   I normally regulate with a very close let-off, especially on our
performance pianos and have never had a problem. This time, I increased the
let-off and the drop, very slightly, to see if this would help stop the
hammers from bouncing back into the strings but I would like to put the
let-off back at 1/16" without having this problem recur. The bedding was
also checked.

   Sorry this is so long but I'm trying to give all the information I can
in hopes someone can explain this to me. I hope I've explained it clearly
enough for you to understand what I'm talking about. Any comments to help
me understand the "whys and wherefores" of this problem would be greatly
appreciated. Thanks.

Avery

_____________________________________
Avery Todd, RPT
Moores School of Music
University of Houston
713-743-3226
atodd@uh.edu
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