Need advice on repair of 1934 Gibson L-10

[tahoeguitar]

A customer brought in this 1934 Gibson L-10 for a repair estimate. Its main problem is the top has collapsed under the bridge area about 2/10". A straight edge placed on the frets with slack strings shoots over the bridge by about 1/16", when it should bump into the bridge about 1/8" below the crest. Accompanying the top collapse are a pair of cracks, one on each side, extending north from the F holes about 1-1/2 to 2" The area around the bridge is pretty flat across the grain, in fact slightly concave. I looked around inside with a light and mirror and I can see no broken braces, or any place where the braces have separated from the top. My initial impression is that the bridge area of the top has lost its arch through repeated exposure to heat (left in the car or attic) combined with downward string pressure on the bridge. Possibly the x brace junction has opened a tad? (It doesn't even have one of those little glued on fabric patches)

I was told that the top was OK when he got it and the cracks developed shortly after he tightened up the strings. . Even if I had seen loose braces or broken ones, I believe my repair plan would have to call for removal of the back since there are only the F holes to work and see through.

My initial repair plan is this:
1) Build a jig that will allow pressing the top back into shape. Cork padded "side blocks" will preserve the rim shape so the back can go back on exactly as it came off
2) carefully score the edges and remove the back binding and using light heat. Save it for re-use
3) carefully remove the back using a hot pallet knife
4) carefully remove the braces in the affected area (it's X braced) save them if possible
5) Re-shape the top to its original arched profile using low heat and slow pressure possibly a rounded metal heating caul on the inside
6) re-install old braces or make and install new ones, repair F hole cracks at this time
7) reassemble and touch up of course everything will be done with hot hide glue and nitro lacquer except binding reattachment.

For those unfamiliar with me I am a "journeyman" luthier who has built close to 50 flat top guitars and a smattering of other instruments, and I've been doing repairs major and minor for about 13 years. I'm completely confident and competent about all the steps above except for #5 so I'm looking for advice and answers to the following questions, hopefully from someone with experience on 1930's Gibson arch tops.

A) Is the flat area across the bridge abnormal or normal? I'm assuming abnormal
B) The top is nearly 1/4" thick and is solid spruce. What is a good technique for restoring a cross grain arch in the bridge area, which will also cross the center seam. I'm worried about causing further cracking in the bridge area and or opening the center seam. I've straightened bellied flat-tops before by judiciously using an LMI heat blanket and a flat caul, but they were 1/2 as thick as this.

I'm gonna try to post some pics now....

[tahoeguitar]

[Howard Klepper]

Larry, the photos don't look half as bad as your description of a "top collapse."

How does it sound? Are you sure no braces are loose or cracked?

If the answers are "pretty good," and "pretty sure" my advice is first to do no harm--your plan may be an overreaction. Then see about whether you can get anything thin between the braces and the top. If the braces are tight and the bridge is not resting on its base with high action, you may not need to do anything. It's not ideal to have any concavity, but I've seen it before on guitars that sounded and played OK. And the braces can be reinforced without taking the guitar apart.

[B. Howard]

I hate to judge things off pictures but I Do not see enough damage here to warrant removing the back of the instrument.

The top cracks at the F holes can easily be repaired from outside the box and once repaired should help hold the top back up under string pressure. FWIW the 30's Gibby jazz boxes I have worked on seemed to have rather flat spots on the top under the bridge when in good shape and may have actually been part of the way they were made during the period.

However in looking at your pics of the relation to fret plane and bridge height I Wonder if a neck re-set isn't a more appropriate repair.

[MC5C]

One thing I would do is ask "what would Frank Ford do?". He did this, with a similar repair: http://www.frets.com/FretsPages/Blog...2collapse.html

Not unlike your plan, to be honest. Take off the braces, fix the cracks (which should start to restore the arch), create a mold, let heat and pressure do the work, and re-brace. If it's X-braced I would add a transverse brace under the bridge area, probably behind the bridge across the widest part of the lower bout, to help sustain the arch.

Brian

[tahoeguitar]

Thanks for all the help folks. Brian's post with Frank Ford's plaster mold and hot sandbag approach is exactly what I was looking for.

Generally I agree with the "first do no harm" approach but this guitar can't be strung to pitch. Collapse is probably too strong a word but this top is down so far that the adjustable bridge has to be extended almost off the pins to get the strings high enough for playable action. And that is with the strings pretty much slack... just snug enough to get a line. The top will drop more when they are tightened to pitch. I did not tighten them as the customer reported the F hole cracks showed up when he tightened them.

[Frank Ford]

I wonder if the plaster mold is necessary for this repair - a stronger X-brace is probably necessary to hold the top up, and that in itself might restore the arch sufficiently by itself. You may be able to judge that as you go by dry clamping the ends of the x-brace and leaning on the top to guess its resistance to collapsing compared to how it feels with the old brace intact.

[tahoeguitar]

Thanks, Frank. Good advice. I'll try that first.