Sigma soundboard belly advise sought

[Jerry Ryan]

I'm a novice in these repairs (still don't charge for work, but someday...). Learning from books recommended here, Frets.com, StewMac Tips and from posting a few times, always received good advise & input. Every project requires more tools, new supplies!!

Question: Where to start? A friend brought me an nice old Sigma dreadnaught guitar with a bad soundboard belly issue. Typical from what I have seen, dips ahead of the bridge visably, about a 1/4"+ rise behind the bridge.No cracks, finish is still good. Bridge is lifting from the back, the outside edges are mostly still attached. I used my IO camera to inspect the inside, bracing is all solid, bridgeplate look good, but thin apx 1/8".

I have used the JLD brace for similar issues, on 'run of the mill' guitars, but not as dramatic a belly as this one. However, I know there are better ways and someday I'll use my skills on 'Good' instruments. My reading says to start by removing the bridge and replacing or re-inforcing the bridge plate?

[R.J. Fulton]

Learn good base techniques, and always remember that every repair is different and must be approached as being potentially unique. Every repair has it's own personality, and usually they're all just a different kind of asshole.
There's never one right way in repairs in my experience. However the fundamentals and alot of foresight and patience will usually see you through a well thought out process no matter the nuances.
R.J.

[R.J. Fulton]

I prefer to remove the bridge plate first with the bridge attached then remove the bridge. Make new bridgeplate out of something solid like maple and give a slight radius to match the top's original dome. Probably somewhere between 25 and 30 radius depending on the situation and severity of the belly. I also like to oversize the new bridgeplate in a repair like this to give the top's belly more surface area to grab to and to help level it out when reglued. I make sure the new bridgeplate is nice and tight for fit and clamp it in with as even pressure over the whole plate as possible. I made a plexiglass type caul. In really extreme case I've sometimes used wingnuts through bridgeplate and top attached through a piece of cork bottomed plywood screwed down tight to level it and insure contact with the top and bridgeplate when reglued and used soundhole clamps to augment the pressure on the outside edges of the bridgeplate which always want to curl away a bit with this technique. Hope this makes sense and hope it helps. Good luck and all my best. I agree with Greg in his last comment about heroic measures and also the grain orientation. However practice makes perfect and the perfect practice is on lesser instuments and pawn shop specials. It will really make you appreciate working on a Martin or other well made instrument down the road. They always seem to go so much smoother as far as major repairs.
R.J.

[Jerry Ryan]

RJ,

Thanks for the input. I bought some fine grained hard maple to make a new bridge plate, now I'm making the hook shaped scraper tool to remove the plate and shaping an aluminum heating block for my deap throat c-clamp for heating the glue on the bridge plate. I took the bridge off already. As stated all repairs have their ouwn personality, the bridge has been off before and there is a 'splice' of wood under the bridge that the grain runs lengthway with the bridge. Tight clean job done on that repair. However with the bridge off its obvious the belly begins to be really obvious at the back edge of the bridge.

How far beyond the bridge can a bridge plate extend to help this repair? Stock bridges are 2 1/2" wide, I am thinking adding a 1/2"?

Jerry

[griff]

Forget using the camera to inspect the bracing, get in there with a thin palette knife, it sounds like the top has let go of its bracing (.25" is a pretty severe difference between front and back of the bridge). The fact that the bridge is lifting from the back is another indicator of structural failure, and possibly that the soundboard is damaged under the bridge so use extra care removing it. I personally wouldn't attempt to radius the new bridge plate (provided it even needs one...check those braces carefully first) it's very difficult to ensure a perfect fit, and you want to flatten the top as much as possible in that area.

The hobby store birch ply would not be hard enough to use as a bridge plate.

[Jerry Ryan]

Griff;

Thanks for the input! The dental camera I use has a 10x magnification with a lighted lens. Really shows flaws, etc. I will be sure there is no seperation by checking with a palate knife. I bought some fine grain hard maple for this job. How thick can I go with a bridge plate. I read 1/8" is the std.?

Jerry