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Well, I'm making progress on this. I did manage to drive the pins that were extruding out of the bottom of the cap back up ever so slightly so I could get a tight fitting glue joint. I sanded off the old layer of glue, which wasn't much, and reglued the cap on. I had to drive three screws through the cap into the bridge to clamp as I could figure out no other way to clamp the bridge. After it sits for a day I will remove the three screws, then I will restring that section and tune it. Hopefully, at this point my piano will sound like a piano again and not like a dog barking while playing the piano. I'll let you know how it turns out.

Last edited by Pianolance; 07/03/14 02:37 PM.

Knabe 5'2" Louis XV Walnut circa 1927
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I told you about screws, seem to me.

Very agréable. Is it because I am not.... ?

Last edited by Olek; 07/03/14 02:55 PM.

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Originally Posted by Olek
This is much wrong.

I cannot say much.

Is it from a reputed brand? Incompétence, I would say.

You are right -- there was a lot wrong with the design of that bridge.

There was no "dogleg" (or offset) in the curve of the bridge across the scale break. The tensions on the treble side were reasonable but on the tenor side they were very high. Upwards of 90 kgf if memory serves.

For many years it was worse but at some point in that model's design history someone had made an attempt to improve the scaling without building a proper bridge. They did this by simply offsetting the pins on each side of the scale break as can be seen in the last of the photos. As you can see the bridge pins right next to the scale break are very close together.

Bridge failure was fairly common on pianos using this scale. During the late 1980s I redesigned that scale and one of the battles I had with management was over the bridge; I wanted to modify it by putting a dogleg in there so the scale tensions could be equalized across the break but that made the bridge a little more expensive to build.

In the end we compromised -- I got some of the offset I wanted but not all of it. They were still able to make new bridge design on the same equipment. At best I can say that the new design is considerably less bad than the original. Which is not to say it was great.

ddf



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Yes Olek, I used your suggestion and appreciate the input. Screws were the only way I could figure out how to clamp the cap onto the bridge. There was absolutely no way to clamp it without them. I thought about putting the piano on its back and trying to clamp it with weight such as a sand bag, but I didn't have enough room to do that. I used three screws. I pre drilled everything and then wiped off all of the wood chips created by the drill bit. I drilled the cap on my drill press and then drilled into the bridge with a hand drill. I plan to leave the screws in place if they don't end up interfering with the strings, but I might have to remove them when I restring. That remains to be seen. I appreciate the help.


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I have seen quite a few Hamiltons with this. The bridge cap glue joint fails. The fail can be only a few notes or an octave in span. If you're lucky it won't happen directly behind the key bed.


Paul E. Dempsey, RPT
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Originally Posted by mupianotech
I have seen quite a few Hamiltons with this. The bridge cap glue joint fails. The fail can be only a few notes or an octave in span. If you're lucky it won't happen directly behind the key bed.


The pins are suppose to go within the bridge and not stay "half installed" that way.
Yet normal bridge pins tend to raise in time, those ones may have pushed on the cap because of seasonal wood motion under them.


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