GB1432254A - Television receivers - Google Patents

Abstract

1432254 Cathode-ray tube circuits BANG & OLUFSEN AS 12 June 1973 [15 June 1972] 27849/73 Heading H4T In an arrangement for automatically adjusting the cut-off point of the beam in a cathoderay display tube a measure of the cathode current is periodically produced and the bias potential between the cathode and a grid electrode is automatically adjusted to reduce the cathode current to substantially zero. As shown, during the scanning of the first or last few lines of each frame of a television picture, such lines being produced outside the visible screen area, a control circuit 30 responsive to an output from the vertical deflection generator 28 operates electronic switches 20 and 22 (only one shown) to replace the video (colour) input from video amplifier 18 to the control grids 12 by a constant voltage Vg and to interrupt the connection of the cathode(s) 10 to earth and to insert in those paths respective currant measuring devices 32 which produce an output voltage proportional to the cathode currents. The voltage produced by each device 32 is compared in a comparator 34 with the voltage Vg to produce respective output voltages which are employed via respective voltage dividers 16. to so adjust the voltages supplied to the screen electrodes associated with the respective electron guns such that the cathode current is reduced to a value close to zero (i.e. the cut-off point) the comparator or the voltage dividers including a time constant circuit which "holds" the adjusted voltages until the next frame occurs. In an alternative (and preferred) embodiment the voltages supplied to the screen electrodes are maintained constant and during the adjustment period, the cathode currents are employed to charge respective capacitors the voltages on which increase the cathode-voltages until the differences between the latter and the control grids becomes sufficient to suppress the cathodecurrent, the capacitor maintaining this condition until the next adjustment period. Instead of adjusting the cut-off points of all the guns simultaneously they may be adjusted sequentially and thus some of the circuitry may be common to all guns.