Setting the intonation on your Epiphone SG requires a fresh set of strings and an electronic guitar tuner. A guitar that is properly intonated requires that the string's vibrating distance between the nut, 12th fret and the bridge saddle be equal. If the intonation is off, the guitar will play in tune in the lower note registers but will be out of tune in the higher note registers. Since each guitar string differs in gauge diameter and vibration frequency, intonation can change when a lighter or heavier gauge string is installed or when other repairs or adjustments are made.
Plug the guitar into a guitar tuner with a guitar cable and tune to standard pitch.
Play the low "E" (6th) string open and adjust the tuning if necessary.
Play the low "E" string on the 12th fret (octave "E" note), and compare the tuner readout to that of the open "E" string. If the note readings are the same, no intonation adjustment is needed. If the note is high or low (flat or sharp), proceed to the next step.
Turn the screw on the front of the bridge to adjust the string's corresponding saddle with a flathead screwdriver. If the 12th fret note is too high (sharp or "#"), turn the screw clockwise to move the saddle forward. If the 12th fret note is too low (flat or "b"), turn the screw counter-clockwise to move the saddle backward.
Re-tune the open string with the tuner, then recheck the note on the 12th fret. Adjust the saddle to set the intonation again if necessary. Adjusting the intonation may require several attempts on each string saddle before it is correct.
Repeat steps 2 through 4 for the remaining A5, D4, G3, B2 and high E1 strings.
When making saddle adjustments, loosen the string slightly to facilitate easier screwdriver access and screw-turning. Turn the saddle adjustment screw about 1/4 of a turn at a time to avoid overcompensating. Performing intonations with new strings installed on your guitar will give you truer readings on the tuner.
Do not force the adjustment screw if it is difficult to turn, as the screw threads may strip, making adjustment impossible. Apply a drop of household lubricating oil to the screw threads and allow to sit for an hour before trying again. If the adjustment screw will not turn after oil application, bring the instrument to a professional repair shop for evaluation. Make neck and bridge height adjustments before moving saddles. These adjustments will alter the intonation and will require you to perform the intonation procedure again.