What is a set up? Well, it's like a tune up on your car. A few subtle adjustments can make a world of difference in the feel, response, and sound of your instrument.
Strings: They don't last as long as people think and could even effect your tuning, clarity, and intonation. (We'll get into intonation later) Perspiration gets into the windings of the strings, causing them to sound dead or unbalanced with the others. I almost always advise the strings be changed with a set up.
Fretboard maintenance: Another advantage to changing the strings is the access to the fingerboard and frets with the strings removed.
0000 steel wool will clean the grime off the fingerboard and frets. A scant amount of linseed or lemon oil will add some moisture to the unfinished rosewood or ebony.
Truss rod adjustment: Almost all stringed instruments have an adjustable truss rod to maintain the proper relief in the neck. The tension of the tuned strings, after a while can pull on the end of the neck, causing too much relief in the neck. What that means is the strings end up too high off the fingerboard, esp. around the mid-section of the fingerboard. If the strings were left off the instrument for a period of time, the neck can bow backwards, causing the strings to fret out or buzz on the frets. The neck should have a bit of relief or "bow" to play true, but too little and you end up with buzz; too much, and the strings are hard to press down on the fingerboard.
Saddle height at the bridge: On electrics and acoustics alike, if the saddle or bridge is too high, the strings are too far off the fingerboard, making it much harder to play. If the saddle or bridge is too low, the strings buzz on neighboring frets, much like the neck bowed too far in either direction. Electric guitars and basses have individual saddles to adjust the height and intonation for each string. Also the individual saddle adjustment makes it easier to set the saddles to the proper fingerboard radius. The fingerboard is rarely perfectly flat, but slightly curved, making it more comfortable to play across. The string saddles need to be set to the same relation to the fingerboard radius.
Intonation: Correct intonation allows the open picked string to play in proper pitch at the 12th fret and the harmonic note at the 12th fret. It is checked with a reliable electronic tuner or better yet, a strobe tuner. After tuning the open picked string, the string is picked at the 12th fret and should maintain the same 440 pitch. If it is flat on the 12th fret, the string length is too long, if it is sharp at the 12th fret, the string length is too short. Proper intonation should be set with new strings installed and stretched.
Tremolo adjustment: The tremolo unit has to be adjusted to maintain proper bridge height. Even breaking or removing a string, or changing string gauges, can throw the bridge out of whack if it has a tremolo system.
Electric instrument controls: All the volume, tone, switches, jacks, etc. should be checked and tighten if they have become loose. If they are allowed to become too loose, the entire control ends up roating when you turn the knob and eventually, it can lead to a wire breaking off the control. This is very common with the jack!
If the instrument has active electronics, the battery should be checked for it's available voltage. Musically speaking, a 9 volt battery is dead below 7 volts and should be changed.
Tuner adjustment: Most tuners on the headstock have an adjustment to keep them from slipping too far out of whack. Properly adjusted, the tuner should turn freely with just a bit of relief. If it's adjusted too tight, the tuner guts will wear out prematurely. The securing hardware should be tightened on the tuner also to prevent the tuner from rotating around in the headstock.
Cleaning: All of the instrument should be cleaned. A damp soft cloth should clean most grime off the finish. A good non-silicone polish will bring the luster up on the finish after it is nice and clean. Extremely grimy instruments are cleaned with a bit of naphtha solvent. This does not harm the finish. In fact, if the grime and grease is left on the finish too long, the finish will be compromised by trying to rub the grime off.
Most professionals know what it means to have a set up on a regular basis. Most new instruments probably can use a set up at the start. A change of string gauge is another reason for a set up. Beginning students will greatly benefit from a set up by having an instrument that they do not have to fight to learn how to play.
The set up is the start to a long lasting relationship with your ax!
