Abstract: This disclosure pertains to an apparatus utilized to detect the pitch in a string instrument, comparing it with a known standard, and providing an output error signal to an audio amplifier which drives a motor and gear train. The output shaft of the gear train is removably attached to the string tensioning pin. A timer activates the correcting motor in short bursts facilitating the manual striking of the string so as to produce a tone that may be measured. When the motor successfully adjusts the pitch produced by the string, the error signal is reduced sufficiently to prevent further motorized adjustments of the string tension. A meter is provided to indicate the relative pitch produced by the string by displaying the amplitude of the error signal. Locking means prevents the output shaft from being rotated by the motor, thereby enabling the user to manually adjust the pin when desired.