CAUTION: THE FOLLOWING PROCEDURES ARE FOR EXPERIENCED USERS. DAMAGE CAN OCCUR IF TRUSS RODS ARE INCORRECTLY ADJUSTED. THIS TYPE OF DAMAGE IS NOT COVERED BY YAMAHA GUITAR WARRANTY.
1) Check approximate string height of strings in order to evaluate the action before starting adjustments. All measurements to be done at the 12th fret of the guitar between the bottom of the string and the top of the fret. These measurements are approximations not specific dimensions.
Laminate Tops, F Series, APX/CPX 500
1st String: 2.0mm / .07874 inches (5/64th)
6th String: 2.8mm / .11024 inches (7/64th)
Solid Tops, FG, L series, APX/CPX700/900
1st String: 1.8mm / .07087 inches (4/64th)
6th String: 2.6mm / .10236 inches (6/64th)
2) Check string height at the nut by using a feeler gauge between the bottom of the string and the top of the first fret, it should be .022 +/- .002 for acoustic guitars for as a starting point. This is on the high side.
3) Check curvature of neck to determine if truss rod needs adjustment. The trussrod is NOT used to specifically lower the action but to adjust the necks straightness.
a) With all strings tuned tuned to pitch , a slight curvature of the neck is necessary and normal to avoid string buzzing. To check this place a capo on the first fret and while pressing down on the string at the 15th fret, check for appx .010 clearance at the 8th fret .
b) If the clearance is too great, loosen the strings and then tighten the truss rod by turning clockwise appx 1/8 of a turn. Retune and recheck,tightening more if necessary.
c) If the clearance is too little or string is touching the 8th fret then loosen the trussrod by turning counterclockwise. Recheck and readjust as necessary.
d) If the adjustments are having no effect, then cease this procedure and take guitar to a qualified repairman.
CAUTION: if the trussrod is extremely tight and will not move in either direction stop adjustment procedure and consult a professional luthier. If enough torque is applied to a frozen rod the truss rod can break . If the truss rod breaks it is a major repair and expensive to fix.
TIP: If the trussrod is extremely tight or seems frozen loosen the trussrod by first turning in the loosening or counterclockwise direction until it ” frees” up , then tighten as necessary.
TIP: if the neck is not response to truss adjustments, tune to pitch and let it sit overnight in a warm suitably humid environment to see if the neck will move over time.