Scary loud bang when raising bridge

[HonketyHank]

I was raising the action on my Red Diamond oval hole A. Strings were at full tension so I used a small pliers I keep just for this job. I hadn't raised it up by much (didn't measure, but in terms of pitch, the G strings were about a quarter tone sharper) when I heard a sudden loud bang. I mean loud. My heart kinda did a back flip. On inspection, I can find nothing that might have caused that noise. No cracks, strings are intact. Now I am afraid to go any farther raising the bridge (or, really, doing anything) until I understand what made that noise.

Any ideas what might have happened?

Some background:
I had just finished with some minor work on the mandolin - I had had the James tailpiece off to tweak the break angle on it a little bit. Opened up a few nut slots slightly to eliminate the jumpies while tuning (A strings). I had fully loosened the truss rod nut while the strings were off so I could take a little bit off the OD of the washer (it was slightly proud of the headstock veneer). After reinstalling the TP and bringing the strings back up to tension I had retightened the truss rod tension to straighten out the neck - the best I could do still left a slight hump around the 12th fret (has always been that way since I have had it). That was when I decided to raise the action a bit to get rid of the annoying buzzy G string when picking it hard. I was just about to that point when I heard the noise.

[MikeEdgerton]

I'd be looking at the internal bracing. I'd probably loosen the strings while I was doing it.

[HonketyHank]

It has x bracing. I tapped on the top (strings still at full tension) but didn't detect any loose sound. I can get my mirror in there with a flashlight and see what I can see. Top does not look deformed.

[HonketyHank]

Can't see anything obviously bad inside. But I am not really a luthier (I did stay in a Holiday Inn Express once though).

No visible gaps between top and braces, no obvious cracks in the braces.

[Richard500]

Feeling lucky? How about a jumped thread on the bridge screw? Thin brass thumbwheel nut only has a few threads and at tension, you’ve got perhaps 15 lbs to raise. If that’s the issue, the loud bang might repeat, and this time you could see the saddle drop. Cure is a new thumbwheel, and maybe, screw.

[HonketyHank]

Hmmm. Interesting. I do remember that as I was turning the thumbscrew, it did not seem to be raising the bridge as much as I thought it should. Could be the thumbscrew threads. Will loosen strings and see what I find with the thumbscrew off. I do have a couple spares around here. Will report back.

[David Houchens]

I would suggest always backing off string pressure a bit before raising the saddle.

[Nevin]

I hope it is just the threads on the adjuster. I would de-tune and inspect the adjuster threads. I would expect visible damage. If not, I would keep the mandolin unstrung and get it to a luthier. You don't want to wait for the top to deform to find there is a cracked or loose brace.

[Simon DS]

No expert here but I’d consider the nut jumping the thread.

Also, a look at the reflection of light on the top, can you see reflected lines straight or symmetrical curves on both sides of the strings on the top?

[HonketyHank]

I don't see any damage to the threads inside the thumbwheel or on the post. It turns out I don't have any spare thumbwheels that fit the post threads or diameter. I doubt if it is relevant but one thing I did find was that the bridge post was not seated all the way into the bridge footer. It wasn't anywhere near coming out, but I could turn it with my fingers. It was only about a quarter turn from fully seated. It may have been that way from day one 30 or so years ago.

I may have to send it off to somebody. My local "luthier" is really mostly a guitar guy. I let him do refrets but I'm not confident about his mando-specific structural expertise. There is a good guy uptown in Portland but last I heard he was booked up for months.

Any other ideas that I could check or do would be appreciated.

[HonketyHank]

Simon, I did check reflections from the top when it was still strung up and everything looked smooth and regular.

[Larry Simonson]

How about a bridge bound string that suddenly released on raising the bridge?

[Dale Ludewig]

I would doubt that raising the action a little is going to cause a brace to come loose. The pressure change was minimal to the top. And it wasn't like you were hammering a wedge in there. I would think it has to be something else. Perhaps it's finally broken in.

[pops1]

I spend a little extra time when I make a saddle so it fits well to the adjusting wheel. I can turn the bass side up under full tension with my fingers. I don't think I am capable or strong enough to strip threads with my fingers so I don't worry about it. The treble side is too close to the finger rest to get a good grip so I can only do that once in a while. I use a tool between the bridge foot and saddle to take some pressure off and the wheels turn easily then. Been doing this for decades and have not had a problem with screw threads on either the post or wheel. I don't think you did the damage by adjusting your bridge up. Not sure what it was that made the noise, would be curious as to what it sounded like. Could you have had the truss rod in a different position and the neck moved to the lesser truss rod tension?

[mrmando]

Maybe the bluegrass police were banging on your door to remind you to loosen the strings before raising the action?

[HonketyHank]

pops1, Ithink you might be onto something. first of all, the sound was very sharp and percussive. more like two things hitting together than a piece of wood breaking. I had the truss rod nut and washer out earlier. I am now wondering if the truss rod or that washer might have suddenly reseated itself.

[Cameronium]

I had a similar thing happen to mandolin no.1, I heard a very uncomfortable crack and then found the saddle in two pieces......parted like the red sea.....except with a crack. I had made the saddle part a little too thin thinking I could get a hair more adjustment out if it. The string pressure disagreed.....

[tvezza]

The metal screw part of the mechanism screws into the wooden bridge base with a tiny hex driver. The wooden base of the bridge is tapped to receive the metal screw. It seems more likely that this tapped wooden thread may have given way than the metal on metal slip you are describing here. Also sometimes this metal screw is turning with the metal wheel and may have come unscrewed under load. This later does’t match what you describe as when the wheel and screw turn together the bridge will seems to operate properly and go up and down, but by moving in the wooden threads.

I saw a mandolin that had this problem of the wheel and screw being slight rusted and when the bridge was lowered the bottom of the screw had started to go out of the bottom of the bridge into the mandolin top!

I once tried to make bridge wheels out of old silver mercury dimes thinking it would look cool. It turned out the silver was too soft and quickly stripped.

[HonketyHank]

Thanks for all the replies. They have helped. After the above observations and then thinking more about the issue, I concluded that the most likely noisemaker was the truss rod washer having been formerly "seated", me removing and then replacing it, but not getting it exactly into its former seat, and then reseating as I moderately increased the string tension by raising the bridge. So I reinstalled the bridge, brought the strings back up to tension with no issues. Then I brought up the bridge height to eliminate buzzes with no issues. Have been playing it for about an hour with no issues.

Will report back if everything goes to perdition in a wheelbarrow all of a sudden, but for now I think I am back in business.

Thanks again for the help.

[CES]

Duuuuude! I’m really glad everything seems to be OK. I have a Brekke bridge on my Weber OM but I usually detune at least a little if I adjust it up (down I don’t worry about).

[Timbofood]

I’m in the “something slipped” camp.
I had an interesting chat and exercise with Bill Halsey when he left me his prototype to test. I wanted to raise the action a bit and he showed me the way to raise it under tension safely. Well, he instructed me so I would not be scared to do it when he wasn’t standing right next to me. I will admit I was a little shaken about it but, his technique was to slip a wide blade screwdriver (or similar lever) and raise the saddle just enough to take the pressure off the post and then raise the thumbscrew.
I’ve done it any number of times, it still makes me sweat just a little but, I’ve never had a mishap.

[HonketyHank]

I have often wished they still had those big oversize thumbwheels. And I have often wondered if their weight really affected the tone or volume all that much. Any objective data on that?

[Pardon me if I have hijacked my own thread.]

[William Smith]

This has happened to me a few times as I've adjusted my bridges before at full tension and in my times, I've done this it was the saddle that made the pop bang zoo because it got caught up in a thread on the post and then at tension the saddle will pop down. For me it was when lowering action not raising it though.

[HonketyHank]

Hmmm. I was raising the bass-side. If the saddle was on a bit crooked so that the treble side was caught and not sitting on the thumbwheel, raising the bass side could free the treble side and let it slam down. I'm liking that scenario even more than the trussrod.

[Richard500]

While we’re sort of on this subject, if one does much bridge fussing, as apparently some do seasonally, a “string raiser” can be useful. A homemade version can be nothing more than an L-shaped plywood scrap, or more elaborately, two screws and two plates. Either type can allow not only taking the load off the dinky thumbwheels, but allow re-positioning or swapping the bridge without scraping the top, especially for intonation when tension might be maintained.
For my rescues of the distressed mandolins, I also need to change the height of (non-adjustable) bridges, and have to keep tension on to do it.