Reset Involving Non-adjustsble Truss Rod

[Andrew]

Hello again All. Been a while. Its nice to see that Frank is still here posting, among others of course.

I currently have on my bench an old Harmony archtop with a non adjustable reinforcement in the neck, which needs a neck reset. The relief in the neck measures .024" under tension (.020" no tension), so it is far from ideal, however the owner and i have decided that pulling the fingerboard to install new neck reinforcement would be overkill for what the guitar is, but he still wants it playable. Current action is about 4 mm (cant think of the imperial conversion, forgive me), and I cant just make the bridge shorter because the tongue rise from the 13th fret to 19th is about .054", so any attempt at normal action that way will fail. So it needs a reset to make it playable without a slide.

I came up with a little system (needs improvement) to overcome the tongue rise/ski ramp as I lay my straightedge on the frets to figure out where it aims at the bridge, positioned on the 1st and 14th frets. When it comes to compensating for the relief I figure I can just add that 20 thou or so to the height of where the straightedge should land at the bridge, and that should give me about the right amount of room to play with bridge height adjustments after the reset is done. But I feel like Im missing something.

Thoughts?

[onewent]

If the fb extension is ramping up it will need to be addressed.  On a guitar like that ..old Harmony, no truss, high relief, I'd likely pull the frets to level things out, and then reset and refret.  But that adds more cost to the project that perhaps the customer doesn't want to take on?  

One thing for sure, making a guess at the neck angle on a guitar with a squishy neck like an old Harmony w/o a truss rod is a real shot in the dark.  And after a reset, without the relief addressed, you're very likely to get buzzing from fret 7 through 13 or so.

But on the bright side, Harmony necks are easy to pull!  ..they often pop out dry .. check for a heel gap, sure sign the glue has given up the ghost.

Good luck with the project.  Tom

[Andrew]

The fb extension is ramped because of how it rests on the body at its existing angle, not due to an irregularity in the board itself. I will be levelling the frets or installing new ones as necessary, but the ramp needs to be addressed as a structural issue before that. Especially since Id like to avoid messing with the fretboard finish, so all levelling work will be done on the frets. Bit of a pain, but I can handle it.

I should point out that the guitar does have a truss rod, it's just not adjustable (checked with magnets). It actually stays very stable under tension - the relief only changes by 4 thou or so compared to slack. The top of the guitar is pretty good too - deflection (downward) measures about 8 thou. It by no means looks that stable lol - it's somewhat bubbly and misshapen, but everything is glued nice and tight, and there are no cracks anywhere. Ive had gauges all over this thing. Because it all appears pretty stable I figure I can get about 3/16ths or so of action without too much buzzing. The customer knows he's not gonna get low action from it and is totally fine with that. Plus he's a tractor mechanic, so a medium highish action should be no problem for his hands and what he wants to play on it.

[Roger Häggström]

I would heat bend the neck back to make the relief smaller first of all. Then measure the position of fretboard extension on the bridge with strings at tension. If the fretboard still has a slope up on the top I would put three shims with similar thickness to rise the straightedge above the bend. Two on the fretboard for the straightedge to rest on and one on the bridge. Then loosen the neck and shave off the heel as usual.

Before gluing the neck back it's important to test the neck without glue and with strings at tension. With a dovetail you can do that without a jig, but it is safer to use some kind of temporary jig using a clamp or two to make sure the neck stays on without glue! You probably have to repeat this a couple of times to get it right before the neck is glued down.

[Andrew]

I do not practise the heat straightening method on necks. Its doubly not an option on this fingerboard, as I would prefer to preserve the faux finish as much as possible.

That is essentially the method I used to overcome the ramp (using feeler gauges etc) and get my measurement at the bridge. I also measured the exact amount of rise in the extension (under tension and not) by shimming the two 'reader' frets until the straightedge was parallel to the highest point of the ramped frets. That reading was 60 thou and 54 thou.

Because the relief is 24 thou under tension, I suppose that the 1st fret would fall back about that much were I able to adjust the neck straight. Basically Im wondering if it would be better to read the frets/board as though that were the case, or if I should just read it as is, or perhaps split the difference. There is room at the bridge for adjustment after the fact, but I want to avoid changing that too much since it is pretty much correct as is ( much shorter and the guitar will get quiet, much taller and Im adding extra pressure to a funny looking old top that is currently fine as it sits).

I am familiar with the trial part of the procedure. I just want to get the right numbers so I dont have to try so hard, if you get my drift. Thats the idea anyway lol.

[Roger Häggström]

If you know the height you want to lower the string at the saddle, divide that measure with three and take away that amount on the tip of the heel. Not an exact measure, but a rule of thumb. I would take away a bit less and see what happens with the strings on at tension and do incremental shavings of the heel.

[Ned Knepp]

Be warned; I am an hobbyist, not a professional. I have played around with repair for many years but haven't worked on anything like the volume of instruments a Pro has for my advice to be taken too seriously. 

My experience with these cheaper archtops is that the neck joint is usually a mess and the steel rods in the neck usually need attention. Almost everyone of these, I've seen are loose at the base of the heel. What I've found is that the dovetail on the neck is often cut too short so it never seats firmly at the base of the slot. If the tail isn't firmly set at the bottom of the joint, it's just a matter of time before the sides of the joint develop a bit of slip and the joint gets loose, resulting in the almost universal  "screws in the bottom of the heel" , neck attachment technique.  

The reinforcing rods I've seen from these tend to be "not straight".  They often  appear to have been chopped to length rather than sawn leaving an (sharp) edge sticking up on the ends, something like a large version of the rolled edge on a scraper. This "spur" will usually project slightly above the edges of the slot ( or force the ends of the rod to do so if the spur is "down" in the slot, resulting in unwanted pressure on fingerboard at those points. I've seen this cause a loose fingerboard at the top of the neck and may contribute to the "ramp" effect you mentioned, by causing a small hump  around the 10th to 12th fret area. 

I understand that you don't want to do it but in the instances where I've separated  the fingerboard from the neck, I have been able to flatten the face of the rod, removing the pressure points and, when combined with flattening the neck surface under the FB as well, ended up with a much straighter/flatter neck to reset. I like flattening thing on this surface because it doesn't show up as much as  removing material from the fingerboard does, though it may mean that some adjustments to the width of the FB may be needed since removing material here slightly narrows the neck and, obviously, any unevenness in the FB, itself, is unaddressed. 

 I realize that you and the owner probably are not going to want to do all of this but I posted it anyway because it's been my experience that some of these old Harmony and Kay archtops make pretty good players, often better than their "value" might indicate, if you can get the neck right. I've got a Kay in my collection that came in pieces so I had not real idea how it would sound, that I kept because it ended up sounding and playing much better than I though it would, when I finished it.  

One more thought on relief;  My personal thought is that I don't really need or, particularly, want relief on my cantilevered necks. If the neck angle is good,  I feel that I get enough adjustment with the bridge to give me the playability I like. Where there is some "natural" relief, I tend to feel that it's sort of in the way to the setup I like. Just my opinion.   

[Andrew]

Thanks Ned, thats useful info about the ends of the rod. I considered that it may have taken a set, but not protrusions. Personally Id be all for taking the fingerboard off and doing whatever is needed inside, but Im trying to keep things reasonable for the guy. He spends more on tools than me lol. He knows what to expect and that if he decides down the road that he wants it to be slick enough for a full time player that its an option. And I will know what the neck joint looks like :). It is a pretty nice sounding guitar and everything is tight thus far. Im glad for any opinions you have on playability. I rarely play these days..