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Repairing a old Gibson archtop

If you have a string instrument of any kind that needs fixing, a mistake you made in building a new instrument that you need to "disappear," or a question about the ethics of altering an older instrument, ask here. Please note that it will be much easier for us to help you decide on the best repair method if you post some pictures of the problem.

Re: Repairing a old Gibson archtop

Postby Bill Raymond » Tue Mar 21, 2017 5:30 pm

Beate, I think you're on the right track re-forming the top. I think that you could use a spacer of the correct height to fit below the bridge position and a sturdy piece of plywood, clamp that firmly to the back as you use heat and moisture to "lift" the top at the bridge (you could even use multiple spacers, as needed). As for the back, I would listen for bubbles beneath the veneer layers as I ran my fingertips or nails across the back and then make a slit with a razor blade or other thin blade along the grain and work some glue into the pocket, then clamp. If you could make an inside mold of the arching of the back, that would be ideal, and a vacuum back should suffice to clamp. I've made arched backs of veneer with a vacuum bag and interior mold for my Maccaferri guitar--not as much arch as an archtop, but still...--and it works well.
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Re: Repairing a old Gibson archtop

Postby Bob Francis » Tue Mar 21, 2017 7:04 pm

Bill not to steal Beate's thread but what thickness veneer did you use?
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Re: Repairing a old Gibson archtop

Postby Beate Ritzert » Tue Mar 21, 2017 7:54 pm

You do not steal "my" thread. Actually i am the one who had stolen the original thread with a related problem. So feel free to ask ...
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Re: Repairing a old Gibson archtop

Postby Bill Raymond » Tue Mar 21, 2017 11:52 pm

I'm embarassed to admit I don't recall the thickness of the veneer. I used EI Rosewood veneer, probably 1/32 inch thick on the outside, African mahogany on the inside, probably also 1/32 and a core of poplar crossbanding veneer 1/32 thick, as best as I can recall. I checked the discussion in the old library and confirmed that I said there that total thickness after glue up came to 2.5 mm thick. Without digging out the remaining veneer pieces and measuring them, I'll have to say 3 layers of 1/32 inch. As a side note, John Monteleone, who worked with Mario Maccaferri, said that Maccaferri wanted his backs laminated in an arch, but Selmer wouldn't or couldn't do it, so they laminated flat backs and arched them over braces; the sides weren't contoured to match a radius, but the edges of the back lay on a plane (except for in the cutaway) and a "dome" was created by shaping the braces.
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Re: Repairing a old Gibson archtop

Postby Beate Ritzert » Wed Mar 22, 2017 3:57 pm

In this contribution i showed the bottom plate: viewtopic.php?p=48552#p48552

Reglueing the broken edges turns out to be more difficult than expected - not too seldom, all three pliers are torn apart, and i recognize that i need to open many of the damaged parts by about 2-3 times of the area its area.

Upon close inspection i noticed fine cracks densely covering the entire inner ply. Mhmm, well, that might explain the lack of a good tap tone. I am still more interested in a repair than in a replacement.

I am wondering if i could warm up the plate under compressive tension in the hope that the glue between the plies would be reactivated, i.e, try to trigger what happened accidentally with the bass bar. Or is that nonsense?

In any case - meanwhile i expect the repair the bottom plate to be more work than the top.
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Re: Repairing a old Gibson archtop

Postby David King » Wed Mar 22, 2017 4:08 pm

What about soaking the inside of the plate with a penetrating epoxy to fill all the cracks? Reactivating the old glue with steam might work but without the original mold and press I don't see how you could get very far.
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Re: Repairing a old Gibson archtop

Postby Beate Ritzert » Wed Mar 22, 2017 6:41 pm

Epoxy would change the acoustic properties, wouldn't it?

And why epoxy and not, for example, thinned fish glue?
I am already using fish glue to repair the breakouts, and i have noticed that thinned fish glue creeps into cracks pretty well.

And what about the 2nd glueing boundary between the central ply and the outer one?

Anyway, thanks for the suggestion of using glue with a low viscosity.
I think, i'll do both, try to reactivate the glue between the plies and use thin glue on the inner surface.
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Re: Repairing a old Gibson archtop

Postby David King » Wed Mar 22, 2017 7:08 pm

Beate,
I think I'm an "epoxy-and-CA-glue-first" kind of guy rather than a fish glue first. Yes thinned fish glue ought to accomplish exactly the same thing but it won't fare as well the next time the guitar gets stranded under water. <g>
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Re: Repairing a old Gibson archtop

Postby Bob Francis » Wed Mar 22, 2017 8:37 pm

Thanks all this is a very interesting thread.
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Re: Repairing a old Gibson archtop

Postby Beate Ritzert » Wed Mar 22, 2017 8:52 pm

David King wrote:... well the next time the guitar gets stranded under water. <g>


... well, the the bottom plate will swim away alone by itself ...
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Re: Repairing a old Gibson archtop

Postby Barry Daniels » Thu Mar 23, 2017 12:15 pm

Don't worry about the acoustics of a delaminated plywood plate as there really isn't any. ;-)
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Re: Repairing a old Gibson archtop

Postby Beate Ritzert » Thu Mar 23, 2017 1:04 pm

Indeed. It makes "plopp" if You tap it.
All my speculations aim toward improving this again.
Just for stability it should be sufficient to cure the breakouts at the edge of the plate and with that inevitably also cure the delamination around the edge. Well, my efforts do actually lead to a slight improvement; although the job is not finished, something like a tone starts to develop instead of the "plopp".
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Re: Repairing a old Gibson archtop

Postby Steven Smith » Thu Mar 23, 2017 3:48 pm

Beate Ritzert wrote: ... something like a tone starts to develop instead of the "plopp".


Progress!!
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Re: Repairing a old Gibson archtop

Postby Beate Ritzert » Thu Mar 23, 2017 4:21 pm

;-)
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Re: Repairing a old Gibson archtop

Postby Beate Ritzert » Wed Mar 29, 2017 2:49 pm

Well, the forum has massive problems accepting new contributions. Just a short version of my former post. In order to avoid data loss in tiny edits.

Meanwhile i have applied heat, tension and humidity to the plate as an attempt to re-activate the glue joints between the laminates. The improvement is very convincing.

Here the procedure:

a) dry run:

Image

Image

Make the plate wet from underneath and apply moderate heat - just a bit more than hand warm. 3 hours under the lamps and 11 hours clamped while cooling. As expected I needed to repair many of the breakouts, but that was of course straightforward to deal with. Finally the plate has been smoothed a bit with grit 80. I am still considering to try the approach with the warm and thinned fish glue in addition - just out of curiosity.

Image
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Re: Repairing a old Gibson archtop

Postby Beate Ritzert » Fri Mar 31, 2017 3:12 pm

Today i prepared the bottom plate. Closed the last obvios breakouts and cleared the inner surface with a scraper. Now it looks a lot better than from the outside... With the exception of the attempt to soak glue into the the tiny cracks of the plate i consider it done. Actually there are a few tiny breakouts left over at the edge. If this was an expensive instrument, i would have closed them, too, of course. But i do not think these will affect the function of the plate in any way, so my decision is for pragmatism.

But i noticed a new problem, unfortunately a really unpleasant one: tapping the top often showed a rattle - not always. Today i found the reason: on the treble side - the one with the intact bar - the top is loose. Surprisingly in a way not visible from the inside: the rim appears to be attached to the lining as bad or good as everywhere else and the lining appears to be safely glued to the top. But the proble is visible from the outside if You search for it; the rim stands out a bit and it can be pressed inward.

Mhmm, i am a bit curious what to expect when i remove the binding. In addition to the expected delamination at this spot there must be something else.


Now the following problem arises: there is a race confict between that repair step and my planned attempt to reform the top, simply because i need the rim fixed for the latter but the repair of the loose rim / delamination of the top will probaply open up again if i apply humidity and heat to the top.
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Re: Repairing a old Gibson archtop

Postby Beate Ritzert » Fri Mar 31, 2017 4:22 pm

Well, that's the problem:

Image

I decided to to this with PVA glue which should withstand my attempt to raise the top better than HHG or fish glue.
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Re: Repairing a old Gibson archtop

Postby Barry Daniels » Fri Mar 31, 2017 4:32 pm

I would work some epoxy in there as far as possible then clamp it.
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Re: Repairing a old Gibson archtop

Postby Beate Ritzert » Sun Apr 02, 2017 6:19 pm

At that time i had already applied the PVA glue - with the exception of the neck joint the guitar has been built with PVA glue anyway. There have been a few more such spots, and i hope i found them all.

Today i bult a stamp and try to press the top a bit outward:

Image

BTW. the rims seem to be massive. In the picture You see the traces of the saw - apparently a large table saw - used to cut the rims. They were a lot more pronounced; i smoothed them a bit with a scraper.

I am still a bit unsure how to stabilize the braces. Maybe give them some kind of lining and compensate that by scalloppinng them. And then decide wether to add a few more braces in some places.
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Re: Repairing a old Gibson archtop

Postby Beate Ritzert » Tue Apr 04, 2017 5:59 pm

So, apparently the top came out almost symmetric.

Of course the brace became loose again. But i admit a was not unhappy at all about that; the reglue occured accidentally and the fit was everything but good.

Through my loupes i saw that it was indeed the laminate which tore off over more than half the length of the brace, except of abount the first inch. The surface of the glue layer could be clearly seen through the gap, almost everything seemed to have remained mostly intact on the inner laminate layer. And hence unreachable.

I decided to reglue the brace with fish glue, thinned just as much that it could be injected through a 1.2 mm needle. Please note the four holes i drilled into the gap in order to be able to inject the glue even there.

Image

Admittedly i do not trust the stability of this joint - although i expect fish glue to perfom better on the rests of the older glues that for example CA. So i'll add some kind of lining, maybe cut from the rests of the bars of the thinline.
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