Bridge Repair Help on Acoustic Guitar

[envirodat]

I have been learning how to repair guitars and a teacher friend brought this to me to fix for a student. My goal on repairs is find guitars, fix them up and give them to people who may not be able to afford one. Obviously, I am not working on Martins but perhaps one day I will be brave enough to do so.

This guitar is something I have not run into. Although I have done several bridge repairs, I have not seen it where the bridge looks like it bent the top of the sound board. The guitar is a mahogany (laminate) Tanglewood Crossroads TWCRP that the student uses. The people tried to glue it in and when it pulled out again, I was given the chance to fix it. I forgot to take some pictures of what it looked like before. There were chunks missing.

Below are two pictures of where I am at currently. In each photo the gaps are easy to see. The bridge seems to be sitting on a bulge that makes me think it has been deformed. I would love to fix this because it is something different. My initial thought is to sand down the top but I worry about weakening the structure. When I look at the underside of the bridge plate, it appears fine, no gaps.

I am not experienced enough to convince myself that if I glue it on (after proper cleanup) like I have done in the past that this will hold right and there won’t be any gaps. If someone in the group has run into something like this and has any insight they could share, it would be greatly appreciated. I have read through threads by Freeman, Guitarbuilder and 1bad914 but nothing like this.

Thanks in advance if you can point me to a path of getting this fixed.

 

[Freeman Keller]

Remember that the top of flat topped guitars is not flat. There is some intentional dome built it, and over the years it will deform under string tension. I don't normally see that much front to back, most of the time its across the top but certainly that is common.

When I reglue a bridge my goal is to get every bit of the old glue off, if there is any finish on the top I get rid of that and I try to make the bridge fit the contour of the top whatever that might be. Frequently I'll put a piece of sandpaper on the top and make small sanding movements with the bridge. In particular I want the wings of the bridge to be tight against the top.



I think your glue joint will be strong enough if it is wood to wood and the contours match. Use several deep clamps and I always try to put clamps on the tips of the wings (the inside will be just outside the X.

 

[guitarbuilder]

I would take a sharp chisel and hold it perpendicular to scrape off any old glue and dirt. You want new cells to glue to. Then You want at least one deep clamp and a caul to spread the pressure. You'll want a caul underneath to keep the braces from getting crushed. The top caul should be distributing pressure down on all parts of the bridge but be small enough to get glue off when it oozes out. At the same time, you want to remove all the junk from the underside of the bridge. IF it isn't flat, but looks like it was sanded to a dome like shape, leave it. If not, sand it flat and the string pull should put a bend in it.

On my first guitars I actually scraped a bit to accentuate the arc produced by string pull, but the last 10 or so, I just go flat. Hot hide glue would be best here but in reality yellow glue...plain old titebond will do the job. You want to resist creep. You can do that with a good bond.


Do a couple dry runs, tape your cauls in place, adjust your clamps so you don't have to fool around when you actually do the job for real.


(link removed)

FRETS.COM

FRETS.COM

StewMac Acoustic Bridge Tools Instructions - StewMac

These tools make difficult bridge repairs easier and more profitable.

www.stewmac.com

HF has deep c clamps, but make sure you can get it into the guitar and out.

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[crazydave911]

∆∆∆ This

 

[envirodat]

Thank you Freeman for the advice as usual. I do know that the guitar is not flat and there should be a dome. I learned that in your post about set ups and your help in the past on my first bridge repair (which is still holding strong all these years). However, it is always good to revisit information again.

Thank you Guitarbuilder for the information. I think you are on to something about with the strings pulling it up. They had some heavy strings on it (no idea why you would do that for a kid learning but it may have been that's what they had). This dang thing has a speed bump that makes me think the soundboard was dented. I will have to peel the tape off and I think it will be a little more obvious. I think some measurements can also help show it so I'll get those.

I guess my question is if the soundboard was deformed because of the bridge pushing down into it, could it be fixed or should I sand the bridge to the shape the best I can?

 

[guitarbuilder]

You should get an inspection mirror inside to see how the bridge plate and braces look before you proceed with gluing the bridge on.

 

[envirodat]

Thanks. I wondered about that too and took a quick picture. I think I am going to have to feel about with a feeler gauge to see if there are gaps.

Here is the plate

This is the top. The line where the arrows are is where it looks like the sound board got bent.

 

[KokoTele]

That bridge plate looks like it might be a bit undersized for the guitar, contributing to the problem. Look carefully at the braces and make sure that they're still glued solidly.

It's hard to tell from the pics, but it looks like the bridge plate needs to be reglued or replaced. I got a hardware store chisel and heated it with a propane torch so I could bend the end into a J shape to hook under the back of a bridge plate for repairs like this. When replacing the plate you can use cauls to clamp the top and bridge plate back into shape.

TJ Thompson invented a set of matching curved cauls for a repair like this. The idea is that you heat the glue and wood to make it pliable and force it back into shape. I've had reasonably good success with this method, but the cauls are pricey for a guy in your situation.

 

[Freeman Keller]

That looks like a mahogany top, it might be laminated in which case it might be separating interally. Its not uncommon for bridges to take some of the top wood along with it. In theory a well done glue joint should be stronger than the wood. The other thing that I think I see is finish in the bridge foot print - your arrows are pointing to where they have scored when removing finish but I'm not sure they got it all. Unfortunately some builders don't do a very good job of masking or removing finishing before putting the bridge on - wood glues do not adhere well to finish.

I'll also caution about clamping cauls. You want to apply clamping pressure between the bridge and the bridge plate, be very very careful of braces - particularly where the wings sit over the arms of the X. I made some special caul out of UHMW (cutting board material) that fits between the X braces and indexes on the pin holes



UHMW is great for cauls - it machines well and glue does not stick to it. If you decide to make wood cauls wrap them in waxed paper. It is embarrassing to glue your caul to the inside of the guitar. You will have glue squeeze out into the pin holes, drill and ream them after the bridge is back on.

I'm not too worried about the bridge plate, I make them a lot smaller

 

[envirodat]

Thanks KokoTele on the pointer to the Thompson Belly reducer. In the video from StewMac it sure looked like the issue. I have a feeling that is what I will need to do. Yeah, they are expensive, but I hope to keep on repairing guitars and having another tool would be good.

 

[Gsweng]

I have been learning how to repair guitars and a teacher friend brought this to me to fix for a student. My goal on repairs is find guitars, fix them up and give them to people who may not be able to afford one. Obviously, I am not working on Martins but perhaps one day I will be brave enough to do so.

This guitar is something I have not run into. Although I have done several bridge repairs, I have not seen it where the bridge looks like it bent the top of the sound board. The guitar is a mahogany (laminate) Tanglewood Crossroads TWCRP that the student uses. The people tried to glue it in and when it pulled out again, I was given the chance to fix it. I forgot to take some pictures of what it looked like before. There were chunks missing.

Below are two pictures of where I am at currently. In each photo the gaps are easy to see. The bridge seems to be sitting on a bulge that makes me think it has been deformed. I would love to fix this because it is something different. My initial thought is to sand down the top but I worry about weakening the structure. When I look at the underside of the bridge plate, it appears fine, no gaps.

I am not experienced enough to convince myself that if I glue it on (after proper cleanup) like I have done in the past that this will hold right and there won’t be any gaps. If someone in the group has run into something like this and has any insight they could share, it would be greatly appreciated. I have read through threads by Freeman, Guitarbuilder and 1bad914 but nothing like this.

Thanks in advance if you can point me to a path of getting this fixed.

View attachment 1173682
View attachment 1173683

Bill, this is what the bridge doctor does, it eliminates the bulge below the bridge and in so doing, it pushes up the area on the lower part of the soundhole, above the bridge. Also, the variation of bridge Doctor I have used requires a hole be drilled in the bridge to accept a screw that will tighten the lower part of the bridge to the soundboard. First, get the bridge glued down and clamped to the soundboard. There are some good YouTube’s on the bridge Doctor and you can see how it works. This could be the ticket.

 

[Gsweng]

In your pictures, the bridge does appear to be sitting on a dome. Is the actual bridge level to the soundboard or is it canted downward into the soundhole? If so, the BD can swivel the bridge upward.

 

[envirodat]

It got canted down into the sound hole. It looks like the very thin top started to buckle under the bridge towards the sound hole. If you look at the one picture with the arrows, you can almost see a crease in it. I should be getting my bridge clamps tomorrow and I hope to get the process working to see what difference it makes. In the video from StewMac, you can see how it worked. However, my cheap guitar is not an old Martin. So I am hoping the heat helps the laminate.

 

[KokoTele]

Bill, this is what the bridge doctor does, it eliminates the bulge below the bridge and in so doing, it pushes up the area on the lower part of the soundhole, above the bridge. Also, the variation of bridge Doctor I have used requires a hole be drilled in the bridge to accept a screw that will tighten the lower part of the bridge to the soundboard. First, get the bridge glued down and clamped to the soundboard. There are some good YouTube’s on the bridge Doctor and you can see how it works. This could be the ticket.

The Bridge Doctor is a good solution a lot of times, though I'm not sure it's right for this one. They make two versions, one that uses a screw through the bridge and one that uses a brass pin through the bridge pin hole. That pin is cross drilled so you can thread the string through it.

It comes with a perloid dot to hide the screw head, though I usually fill the hole with sawdust and glue. Done well, it's almost invisible.

 

[Boreas]

Thanks KokoTele on the pointer to the Thompson Belly reducer. In the video from StewMac it sure looked like the issue. I have a feeling that is what I will need to do. Yeah, they are expensive, but I hope to keep on repairing guitars and having another tool would be good.

I dunno - it looks to me like much of the top has collapsed - similar to what you may see in an overly-dry situation. I am assuming this is not the case since the OP is in PA - not known for its deserts. I suspect the braces near the bridge have become unglued or warped. Some glues can soften and slip with heat. This may be a good opportunity to learn how to remove a top or back to access the bracing.

 

[Gsweng]

The Bridge Doctor is a good solution a lot of times, though I'm not sure it's right for this one. They make two versions, one that uses a screw through the bridge and one that uses a brass pin through the bridge pin hole. That pin is cross drilled so you can thread the string through it.

It comes with a perloid dot to hide the screw head, though I usually fill the hole with sawdust and glue. Done well, it's almost invisible.

True, it’s hard to say. I agree with Boreas, check all the bracing first.

 

[envirodat]

Thank you both! More stuff to learn.

 

[Freeman Keller]

If you are seriously thinking about the bridge doctor stop and think about how it works. It might reduce the bulge behind the bridge but probably won't do much of anything on the one in the pictures. And since it keeps the bridge from rotating around its axis it kills the way a pinned bridge works to make sounds.

We can talk more about this if you really want.

 

[Boreas]

True, it’s hard to say. I agree with Boreas, check all the bracing first.

You only want to re-glue the bridge IF the top and bracing is sound. Sometimes tapping will reveal bad joints, but best to get under the hood. Often, if you are patient, your smart phone can pick up gaps and other issues.

I forgot to ask - is it a solid or laminated top?

 

[Gsweng]

If you are seriously thinking about the bridge doctor stop and think about how it works. It might reduce the bulge behind the bridge but probably won't do much of anything on the one in the pictures. And since it keeps the bridge from rotating around its axis it kills the way a pinned bridge works to make sounds.

We can talk more about this if you really want.

That is true, the bridge does need to rotate. The bridge Doctor operation can be complicated. My experience (only based on four uses or so) is that you can eliminate a cant in the bridge and position the bridge in a more level position. Then the bridge can rotate about the new position. If the BD is tightened too much, it can actually prevent the bridge from rotating at all, with serious consequences to volume and tone. But again, this is based on my limited uses of the BD.