GB883692A - Multi-beam convergence system - Google Patents

Abstract

883,692. Cathode-ray tube operating circuits. RADIO CORPORATION OF AMERICA. March 27, 1958 [April 24, 1957], No. 9914/58. Class 40(7). In a colour television receiver utilizing a cathode-ray tube having three electron guns and three electromagnets 12G, 12R, 12B for converging the beams to a common point on the tube screen and wherein parabolic current waves are derived from the horizontal and vertical scanning generators 10 and 14 and applied to the electromagnets so as to maintain the convergence during scanning, circuit means are provided so as to bring the trough portions of the parabolic waves, which correspond to the middle of the scan, to a reference potential. In the case of the horizontal waves, the circuit means comprise diodes 40 which function to clamp the troughs of the waves to earth potential. In the case of the vertical waves, the circuit means comprises an inductor 60 whose resistance is selected in relation to that of potentiometer 54 such that the correct proportion of the direct current through valve 48 is shunted off to bring the current to zero at the trough of the superimposed parabolic current. By maintaining the troughs of the parabolic waves at a reference level, the arrangement according to the invention simplifies the adjustment of the circuit to maintain accurate convergence over the whole of the scanned raster. The circuit for producing the horizontal parabolic current waves are supplied from the horizontal generator at terminals 20 and comprise combinations of inductors, capacitors and resistors as shown. The red and green circuits are coupled in a balanced or differential form, whilst the blue circuit remains separate. Switches 35 permit additional waves of appropriate polarity derived from terminals 20<SP>1</SP> to be applied to the electromagnets. The vertical parabolic current waves are derived directly by applying sawtooth voltage waves from potentiometers 54 to the electromagnet. Controlled amounts of sawtooth wave may also be applied from potentiometers 53.