Guitar and Bass Repair: Perfect the Tuning of Your Instrument - Intonation Troubleshooting Part 1

Have you ever found yourself in this situation? You’ve set the intonation on your guitar or bass perfectly. Your tuner reads exactly the same pitch for the harmonic at the twelfth fret and the fretted note at the twelfth, but some frets still produce notes that are out of tune. The previous Xylem article discussed intonation and how to adjust it properly for guitars and basses. This two-part article will address various intonation problems and their solutions. After you have completed any of the adjustments below, you should re-intonate your instrument. While reading, keep in mind that instruments that intonate perfectly for every fret are very rare and you should expect slight variations from perfect pitch. However, you should be able to fix variations greater than about 6 cents sharp or flat in most cases.

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Bad Tuners, Low Batteries and Old Strings

When checking the intonation of an instrument, make sure to use a decent quality chromatic tuner with fresh batteries. The intonation may seem to be off simply because a cheap tuner, or a good one with low batteries, produces inaccurate readings. Another potential cheap fix for bad intonation is a string replacement. As guitar and bass strings get old, they become corroded, develop pits and dings, and the cores stretch more easily, all of which can affect intonation.

Excessive Relief

Ruling out a faulty tuner or old strings, the next thing to check is the relief of the neck. If your instrument’s intonation is properly set at the 12th fret, but is still incorrect at certain other frets (especially those in the middle of the fretboard), it may have too much relief. Excessive relief requires that the strings travel further to contact the frets, which causes the strings to stretch further and results in a slightly sharp pitch. To correct this problem, optimize the relief of your instrument (refer to the Xylem truss rod articles, parts one through three for detailed procedures for optimizing relief).

Nut and Saddle Slots

On a well-made nut, the slots are angled down towards the headstock and, as a result, the string contacts the nut at a single point in the slot nearest the fretboard (Figures 1 & 2). If the nut slots have more than one contact point, or the contact point is further away from the fretboard, intonation may be thrown off. If the nut slots of your instrument are not cut at such an angle, and/or the highest points of the slots are not right at the fretboard face of the nut, the nut slots may need to be filed. You will need a set of gauged nut-slotting files that match your instrument’s string gauges to file the slots properly (these can be purchased from luthier supply companies, try to get files that are about .003" larger than your string gauges). File away material in small increments, making sure that the depths of the slots do not exceed ½ to ¾ of the string’s diameters. If the slots are already too deep to allow further filing, you may need to install a new nut. Once the fretboard faces of the slots are higher than the rest, you will have fixed the problem. If you are not comfortable filing the nut yourself, most repair techs and luthiers (including myself) charge between $20-$50 for nut modifications.

A similar procedure can be done to the slotted saddles on some instruments. Extra caution should be used when filing these saddles, as they may not be as easily replaced as the nut. Saddles that consist of an edge only (such as those found on most acoustic guitars) should not be filed in this manner. Also, the strings should be pressed down into the saddle slots with a fingernail to ensure that the strings are contacting the leading edge of the slot.

The second part of this article will discuss how to modify a nut that is too high for proper intonation and a few simple, cheap ways to improve the intonation of an instrument with fixed saddles or moveable saddles that are adjusted all the way back or forward. Thanks for reading, see you next month!

Copyright © Anthony Olinger, Xylem Handmade Basses and Guitars 2008.