GB667695A - Improvements in or relating to television receivers - Google Patents

Abstract

667,695. Television receivers. COLE, Ltd., E. K., and TYRER, A. Sept. 7, 1950 [Sept. 8, 1949], No. 23241/49. Class'40 (iii). In a television receiver employing a cathoderay tube means are provided, for preventing damage to the screen of the tube should either the line or frame scanning potentials fail, comprising triode valves A and B, which may be in the form of a double triode, the frame-scanning potential being applied to valve A and the linescanning potential being applied to valve B, and valve A including in its cathode circuit a relay coil L which controls a switch X in the circuit of valve B which circuit also contains the brilliance control resistor P of the receiver. Should either scanning potential fail the current flow through the corresponding valve decreases, so that, either directly or through the action of relay units L, X the current through brilliance control resistor P falls and the potential at the tapping T1 which is taken to the control grid of the cathode-ray tube falls and thus biases the beam of the cathode-ray tube off. In addition, if the frame-scanning potential fails, switch Y opens and disables the E.H.T. generator. The line-scanning and frame-scanning may alternatively be applied to valves A and B respectively, the component values being modified suitably. In Fig. 2 (not shown), is shown an arrangement wherein the brilliance control resistor is in the anode current of valve B for use when the brilliance-control potential is applied to the cathode of the cathode-ray tube. It is stated that the invention is particularly suitable for use in receivers of the projection type wherein small cathode-ray tubes employing high energy electron beams are used.