GB614418A - Improvements in beam-deflecting circuits for cathode ray tubes - Google Patents

Abstract

614,418. Valve generating circuits. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Jan. 14, 1946, No. 1184. Convention date, Jan. 13, 1945. [Class 40 (vi)] In a beam deflecting circuit for a cathode-ray tube, a grid-controlled damping tube with its cathode earthed for alternating currents, is connected across the deflecting coils, which are coupled through a transformer to an electric discharge tube. The characteristics of the damping tube are modified to improve linearity. In Fig. 1, the power valve 10 supplies current via transformer 14 to deflection coils 12, across which is connected damping valve 50. A sawtooth waveform which is applied to valve 10 is generated by a circuit comprising condenser 28 which is periodically discharged by gridcontrolled valve 38. A portion of the output from transformer 14 is fed to the grid of valve 10 via resistor 46 to include negative peaks in the waveform 36 applied to the grid of valve 10, to ensure that this valve is cut off during flyback. Resistor 24, which is adjustable, controls the output of valve 10 by varying the screen potential, and also controls the amplitude of the input waveform due to the connection of charging resistor 30. Potentiometer 58 enables the direct current through the deflection coils 12 to be controlled for centring purposes. The cathodes of valve 50 and the lower end of the deflection coils are effectively at earth potential for alternating currents. During the flyback period, high-frequency oscillations are developed in the secondary circuit, but the connections of the transformer are such that after one half cycle of oscillation the anodes and grids of valve 50 are driven positive so that this valve conducts and damps out the oscillations. Due to the effect of condenser 66, the potential of the grids of valve 50 falls exponentially, at first quickly due to the effect of grid current, and then more slowly depending mainly on the value of resistors 68 and 74. This arrangement assists in damping out the initial parasitic oscillations at the beginning of scansion. Resistor 74 is made adjustable to provide a linearity control. The anode current of valve 50 then slowly decreases, the total effect of this current and that due to valve 10 resulting in current through the deflector coils of improved linearity. Bias for valve 50 is obtained by grid rectification and the use of elements 70 and 72. Specifications 542,032 and 566,835 are referred to.