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Double-top classical repair
http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10137&t=49947		 Page 1 of 1
Author:  Paul Micheletti [ Tue Nov 14, 2017 12:02 pm ]
Post subject:  Double-top classical repair
Hi Folks,

My guitar instructor has a double top that has an issue. He bumped against the top and heard a crack noise. Since then, there are a few notes that have a sitar buzz resonance. This is a guitar made by Cervantes, and does not have a high resale value.

I looked inside the guitar in the region of the top where the bump occurred, and cannot see anything that looks like a cracked or lifted brace. The internal bracing is made of very tiny spruce lattice in about a 1" square pattern. It is clearly a double top because the inner top grain does not match the outer top grain, and it has a heavy Smallman style plywood top support ring.

I'm suspecting that the damage that causes the buzz is an internal delamination between the layers of top, that is only excited at certain frequencies. Has anyone successfully figured out how to fix such an issue on a guitar on their bench?

I have an idea to use a pair of small rare-earth magnets inside and outside of the top. The goal is to locally squeeze the top back together so that there is no resonant movement between the layers. The idea is to place the pair of magnets in the middle of each square of lattice and see which ones act like a clamp to tighten up the top so that the buzzing at that frequency stops. If a location is found, inject some epoxy into that area through a small hole from the inside and reglue that area. But I'm looking for anyone who has gone through this first to say whether they have succeeded in this type of endeavor or not.

Thanks,
Paul M.
Author:  DanKirkland [ Tue Nov 14, 2017 6:39 pm ]
Post subject:  Re: Double-top classical repair
Usually in these instances a buzz is a loose brace. Have you checked everything with the guitar strung to tension?

I've had multiple repairs like this and for each one it was a loose/cracked brace that was vibrating at a very specific frequency.

Delamination usually doesn't happen unless the guitar has had contact with some hardcore solvent. Not saying it's impossible but delaminating anything usually happens at an exposed as well.
Author:  Paul Micheletti [ Tue Nov 14, 2017 7:25 pm ]
Post subject:  Re: Double-top classical repair
Thanks Dan. This was a well cared for instrument that just took an unfortunate bump. I looked visually for loose braces, but didn't see any. I've only briefly inspected this at the owners home. The guitar was strung to tension, and there were no obvious openings under the bracing. There also have been no solvent issues which would be noticeable with the thin french polish on the top.

Those tiny lattice braces are much smaller than anything I've dealt with before. They look incredibly delicate. There could be a crack that I just couldn't easily see. If I can convince my instructor to let me look deeper, I'll wiggle everything about looking for a subtle separation of braces before I start looking deeper for possible top layer separation.
Author:  DanKirkland [ Tue Nov 14, 2017 7:42 pm ]
Post subject:  Re: Double-top classical repair
Paul Micheletti wrote:
Thanks Dan. This was a well cared for instrument that just took an unfortunate bump. I looked visually for loose braces, but didn't see any. I've only briefly inspected this at the owners home. The guitar was strung to tension, and there were no obvious openings under the bracing. There also have been no solvent issues which would be noticeable with the thin french polish on the top.

Those tiny lattice braces are much smaller than anything I've dealt with before. They look incredibly delicate. There could be a crack that I just couldn't easily see. If I can convince my instructor to let me look deeper, I'll wiggle everything about looking for a subtle separation of braces before I start looking deeper for possible top layer separation.


Good plan, in all likelihood it's a cracked brace that's just doing a really good job of hiding from you.

If you're careful, when you have the strings off the guitar you can *very* lightly press on the top and sometimes a cracked brace can become apparent since it won't flex with the top in the same manner as the other braces. Be careful if you do that though cause it's very possible to push just a bit too hard and cause more damage.
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