A combined manual and motorized adjustment drive for adjusting the string tension of a stringed instrument having a tuning peg, a drive motor, a force-transmitting member for transmitting a drive force to the tuning peg and a manually operable adjustment element which couples with the force-transmitting member for manual adjustability of the rotary position of the tuning peg wherein said adjustment drive is improved in that in allows a compact design in addition to reliable self-retention and good adjustability of the manual drive. The force-transmitting member is an at least three-stage reduction gear having a first gear stage on which the drive motor acts with an output shaft and a last gear stage which acts on the tuning peg in order to rotate the latter, and the adjustment element acts on a gear stage of the reduction gear between the first and the last gear stages to introduce the force.