Patent GB-1391852-A (GB)
Title: Vertical convergence circuit
IPC: H04N9/28
Filed: 1972-03-27
Abstract: 1391852 Cathode-ray tube circuits NIPPON VICTOR KK 27 March 1972 [29 March 1971 3 June 1971 8 July 1971 (2) 19 July 1971] 14230/72 Heading H4T In a dynamic vertical convergence circuit for a three-gun shadow-mask colour picture tube all of the waveforms required are derived from a sawtooth voltage of vertical deflection frequency. Fig. 1 shows one embodiment, in which the vertical frequency sawtooth wave a derived from the vertical output transformer 1 causes a current of parabolic waveform amplitude adjusted by resistor 18 to flow in the series connected Green and Red convergence windings 20, 21 respectively, the tilt of this waveform being adjustable by means of potentiometer 25 which produces a sawtooth current in the windings 20, 21 by applying the waveform b (essentially a pulse type waveform), derived by differentiation of waveform a via capacitor 30 and the resistor 25, Additionally, a low value capacitor 15 and a potentiometer 16 differentiate waveform a to produce waveform d which causes a parabolic waveform voltage e to be produced across a capacitor 17 and by adjustment of potentiometer 16 a differential tilt voltage is supplied to a differential amplitude control potentiometer 22. For convergence of the blue beam a Blue convergence winding 27 is supplied at one end with a sawtooth a via potentiometer 29 to cause a current of parabolic waveform to flow therein and the tilt of this waveform is adjusted by means of potentiometer 26 which is supplied with the waveform b and connected to the other end of winding 27 to cause a sawtooth current to flow therein. In the embodiment described above, the waveform b because of its nature, is only effective by adjustment of potentiometer 25 to cause an increase in the correction current during scanning of the upper part of the picture and when it is desired to orientate the tube so that the Blue gun is below the Red and Green (the opposite of the conventional manner), so that, when viewed obliquely downwardly the curvature of the screen corrects pincushion distortion of the Red and Green raster, the pincushion distortion of the less conspicuous blue raster being neglected, a second embodiment provides that the range of tilt correction is increased by providing a parabolic voltage waveform b (Fig. 6), produced across the capacitor 41 to allow of tilt correction of the lower part of the screen. The range of the tilt correction for the Blue convergence is similarly increased by the parabolic voltage waveform produced across capacitor 43. In the second embodiment (Fig. 6), the presence of the diode 42 and resistor 44 shunting the relatively large value capacitors 41, 43 (to provide a D.C. return path), result in loss of dynamic convergence in the vicinity of the screen centre and to overcome this in a third embodiment (Fig. 9), the potentiometers 25 and 26 are provided with centre-taps connected to earth. The principle of operation of this arrange. ment is discussed with reference to Figs. 10A to 10E and Figs. 11A to 11C (none shown), and results in that, when the sliding contacts of the potentiometers are positioned above the earth taps, the waveform b is effective for tilt control whereas when the contacts are positioned below the earth tap the presence of the resistance in series with the windings 20, 21 and 27 is effectively added to the resistance of the windings and adjustably varies the integration time constant of the latter. In a fourth embodiment, the arrangement of Fig. 9 is modified by the omission of the earth taps, earthing the lower ends of the potentiometers 25 and 26 and providing respective switches between the upper ends of the potentiometers and the capacitor 30 so that by selective operation of the switches various tilt configurations may be produced (Fig. 12, not shown). Fig. 13 shows a fifth embodiment, in which the potentiometers 25 and 26 are supplied with a parabolic waveform f derived from the waveform a by an integrating circuit comprising a resistor 60 and capacitor 61, the arrangement being otherwise similar to the embodiments of Figs. 1 and 6.