Cheap repair on bendy neck?

[Tavy]

Folks, I have a strum-stick in the 'shop with an unreasonably bendy neck (I can make it bend quite easily with my fingers), it only has to manage 4 lightly strung dulcimer strings but can't cope even with that. It's a cheap ($150) instrument that I've worked on before because it was both unplayable and untunable straight from the shop, a setup and some new planetary tuners fixed that, but over time the neck has developed quite a bow.

I figured there couldn't be any stiffener inside given how easily it moves, so as a last-ditch repair (the owner really likes the sound), I offered to route in through the back of the neck and fill it full of carbon fibre, and much to my surprise I found carbon in there already:



I am most definitely NOT going to route out the existing fibre!

Can't even put this one on the fire with carbon in there

Any thoughts? I wondered about fixing "something" that's good under tension above the fibre and then capping off: but I only have about 5mm of depth to play with. A compression rod would be great, but there's literally nowhere for the nut to go (it would come out in the middle of one of the tuning pegs).

It is a last ditch effort, so chucking it in the bin is an option, but I hate being beat !

[rcc56]

Suggestion #1: To quote King Arthur and his knights from Monty Python and the Holy Grail, "Run away!!"

Suggestion #2: If you're not ready to run away, fill the channel with the stiffest piece of hardwood you can find, sand to 220, spray with something tough from the hardware store, and then run away.

Addendum to suggestion #2: If the owner brings it back later, run away.

[Richard500]

Provided that the existing carbon rod really is both oriented fiber in epoxy, and not set in 90 degrees wrong; that is, the larger dimension parallel to the fretboard, then it’s possible that it is not adequately glued to the wood along its length. Unless it’s anchored, it isn’t going to add stiffness. So look into this (by flexing the neck) and maybe flood the thing with say, thin CA, then add a well-glued cap on the back of grain-oriented hardwood.
A possible tension element, needing a little fabrication, could be a hefty steel wire anchored on a transverse rod in the headstock and laid close to the back of the neck, terminating inside the instrument with threaded hardware, if this thing has an oval hole for access.
Or - burn it. Should be safe in a woodstove or furnace, but not an open fire.

[mandocello8]

tavy, this does not add up, "has a carbon fiber insert" and "i can flex it with my fingers". are you sure that stuff is actually carbon fiber and not black coat hanger plastic material? (I guess you try to scratch it?). if it's really carbon fiber and it's uncompressible/unsctretchable, then when you flex the neck, something in there will have to move. if richard500's guess is right, this insert is no longer glued to the neck wood, when you flex the neck the two of them move relative to each other. (no idea how to fix that, perhaps fretboard has to come off to re-glue "the dark side" of the insert).

also looking at the photograph, it looks like you had dug deep enough to have space for one more carbon fiber insert to add on top of the existing one (in case it's too thin, not stiff enough). there may be no space left to add a wood veneer to hide it, but ask the owner, "you want stiff or pretty?".

"the owner really likes the sound", i was just now in a similar situation with the japanese made mandocello. an adjustable truss rod retrofit, wedge fretboard, cost of work about equal to ebay value of instrument. but I really like the sound, the woodwork and mandocellos are damn hard to come by, so it was worth it. (in my case, it is now both stiff *and* pretty).

good luck with this repair, i hope you can make it work by hook or crook.

[Tavy]

Tis a strange one: definitely carbon fibre, yes the fibres run along the length, yes seems to be reasonably well fixed in (I can't see it move when flexing the neck, I was hoping it might have pulled right out!), but not on the centre line, and not very deep or wide. I could laminate more strip in there and hope for the best I guess, but as this is for a friend, and at mates rates already, I'm wondering if I couldn't just build him a better one... the instruments not much more than a neck anyway...