A servo system is described whereby a rotating drum has its angular speed and angular position controlled to fine limits by using a crystal oscillator as a stable reference. The device was developed for use as a “magnetic storage system” forming part of a digital computing machine; it has, however, other possible applications.The various components and their functions are described individually and an overall analysis developed. The theoretical performance is compared with that obtained experimentally, and a discussion of the factors affecting the final accuracy is included.