1,206,178. Converting. CKD PRAHA OBOROVY PODNIK. 20 Dec., 1967. [23 Dec., 1966], No.57877/67'. Heading H2F. A control circuit for a converter having controlled semi-conductor converting elements includes a synchronizing circuit 2 connected between a pulse source 1 and a blocking oscillator 5. Capacitor 29 discharges between pulses to produce a sawtooth signal which is fed via diode 27 to the oscillator 5. The oscillator may also be controlled by signals which are applied to transistor 61 in series with oscillator 5.