Abstract: A yarn feed gearbox control system for controlling the let-off speed of a yarn feed gearbox. The control system is adapted for use with a warp knitting machine having a main shaft and a gearbox operative to control the feed rate of yarn delivery from a yarn beam, the gearbox including an adjustment spindle extending therefrom. The control system includes a yarn feed rate detector which measures the rate of yarn delivery from the beam, a computer which receives yarn feed rate signals from the yarn feed rate detector and generates control signals corresponding thereto, and a control device which controls the speed of rotation of the spindle in accordance with the control signals. The control system may be further provided with a main shaft detector to measure the speed of rotation of the main shaft of the knitting machine. The control system may also be provided with a beam revolution detector to generate a revolution signal corresponding to each revolution of the yarn beam.