How To Fix A Guitar

The following four adjustments can be made to all kinds of guitars to fix them. These adjustments will fix all guitars and make them playable. The adjustments are the amount of relief in the neck using an adjustable truss rod, the string height at the saddle, the string height at the nut and the intonation.
These adjustments should be made at least once on every guitar. Most manufacturers do not take time to properly do these adjustments. A manufacturer only does these adjustments for the average player, but not for the individual player.
How do you adjust the amount of the neck bow? It is supposed to be simple. Every adjustable truss rod shares the same principles of operation. Every truss rod has a threaded nut tightened on a threaded metal rod. The tension in the rod changes the curvature of the neck in which the the rod is embedded. To adjust the rod, you need to tighten or loosen the nut on the rod. When tighten the nut, the tension increases the tension in the rod as well the amount which the rod counteracts the pull of the strings. This should reduce the bow in the neck.
As far as adjusting the saddle height, you can do this either before or after adjusting the string height at the nut. You should start by measuring the distance from the top of the twelfth fret to the bottom of the sixth string. You should do this when the guitar's strings are at full strength. You should measure laying a 6-inch ruler, on edge, adjacent to and parallel to the string.
The ruler is supported at one end of the twelfth fret and along its length by adjacent frets, eleven, ten, nine, etc. You can use other methods to measure from the top of the twelfth to the bottom of the string.
Another thing that you might have to fix is the string height. You can start by using elementary geometry. You will find that the change in the string height at the twelfth fret needs to be about twice the of change at the saddle. If a string height at the twelfth fret is 4/32" and the desired measurement is 3/32", the change in height will have to be lowered by 1/32" at the twelfth fret is about 2/32".
After taking measurements, you should calculate the amount that each string needs to be lowered at the saddle. You should make adjustments the saddle must project at least 1/16" from the top of the bridge. This should make sure that the strings exert a sufficient downward force on the saddle to stop the strings from vibrating side-to-side on the top surface of the saddle. If you cannot maintain this 1/16" projection, it will be necessary to reset or shave the bridge. This should be done by a professional repairer or a skilled amateur. The last adjustment that you should make is to the string height at the nut.
The required tools are a short straight edge (ruler), a standard set of feeler gages, a set of calibrated nut files, an X-acto saw and a tear-drop needle file. You should start by measuring the height of the first fret. You measure the first fret by placing a straight edge on the top of the first two frets so it straddles on the first and second frets. The slide feeler gages should be place between the fingerboard and the straight edge until the gages fill in the space between the fingerboard and the straight edge. When it doesn't fit the required measurement for a string height at the nut, you should adjust it.