Double top repair ideas

Construction and repair of Classical Guitar and related instruments
Paul Micheletti
Amateur luthier
Posts: 504
Joined: Sat Jan 16, 2010 12:48 am
Location: San Diego, CA

Double top repair ideas

Post by Paul Micheletti » Mon Nov 13, 2017 6:16 pm

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. Similar to what Alan does with blue-tac only instead of just adding weight it also clamps the top surfaces towards each other. 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.

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