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1920 Gibson "A" Mandolin
Shoring up a Sagging Top
© Frank Ford 2007; Photos by FF
|Here's a sight nobody likes to see! It's a fine old Gibson mandolin whose top has slowly collapsed under the load of string tension pressing downward on the bridge. Somewhat wider grain and a thin spot in that area have obviously contributed to the problem.|
|I made this little tool just for reaching in the soundhole of this style mandolin and jacking the top back to its original position.|
|Here's an exploded view of the tool.|
|After swabbing the inside of the top with a rag satruated in hot water, I cranked the jack to restore the top to its correct form. I left the jack in place for a week, and after I took it out the top remained in good shape.|
I taped a pen to my dividers and traced the contour of the top on a piece of 3/4" thick plywood as a first step in plotting the interior shape so I could make a brace to fit. Because the top had a somewhat springy feel on the treble side, I decided to make my brace long enough to span the entire width of the top.
|Using the Hacklinger magnetic gauge, I measured the thickness of the top at intervals across where I'd be placing a reinforcement brace.|
|Transferring the measurements to my traced contour line, I was able to make a pretty fair estimation of the exact interior contour. The arrows point to the thin spot where the bass side of the top had failed.|
|I wanted my new brace to be strong yet flexible and light, so I split some spruce to get the straightest grain alignment with no runout. That would ensure a brace that couldn't split under any load.|
|A laminated brace would allow me to establish the contour while keeping the grain alignment so I'd be able to make a strong brace that wouldn't be overly stiff. I worked the spruce down to just about 1/32" thick strips.|
|Once I'd trimmed my plywood form, I laminated layers of the thin spruce.|
|Here's my "glue-lam" cut to length and ready for installation. The idea was to have the brace strong, flexible and very light so it would have as little tonal influence as possible.|
|Getting a glue covered brace lined up inside is a difficult enough prospect when working through the soundhole of a guitar, let alone such a small hole as on this mandolin. I glued a pair of magnets onto the back side of the brace to help me with the job.|
|Because the brace was thin and springy, and contoured through its length, I was able to feed it into the instrument.|
|Then, with two matching magnets on the outside, I gently lifted the brace toward the top, and it snapped right into position.|
|A few strips of veneer protected the top as I clamped the brace.|
(OK, not "viola" but "mandolin") No sign of the old sagging problem, even with full tension (.011 - .040) strings.
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