GB1311184A - Convergence correction circuits - Google Patents

Abstract

1311184 Cathode-ray tube circuits PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 4 June 1970 [7 June 1969 7 March 1970] 27012/70 Heading H4T An arrangement for obtaining dynamic convergence in a three-gun CRT for colour television comprises two cores wound on a common core, the first carrying a periodic current and the second a steady current derived from the amplitude of the periodic current so that the net field is stabilized at one of its extreme values. As shown in Fig. 1 parabolic and square-wave signals at scan-frequency are supplied to inputs 1, 2 where the former is differentiated before being combined with the latter. The combined signal (variable in amplitude by 7) is supplied to two identical circuits 11-19, 11<SP>1</SP>-19<SP>1</SP> which control the convergence of red and green beams respectively, potentiometer 9 permitting differential control between the beams. Each arrangement comprises a complementary pair of transistors (11, 12) operating in class B conditions, the output current through coil 17 being the integral of the input (the coil inductance being relatively large) and thus the desired combination of linear (sawtooth) and parabolic waveforms. The current through one of the transistors is smoothed by 15, and by proper choice of the ratio of the turns of coils 13 and 17 can be made to ensure that the resultant total field has a minimum of zero (this occurs at zero deflection when static convergence only will be effective). For a flyback ratio of 20% the turns ratio of coils 13 and 17 should be approximately 2À65 : 1. The operation of the Fig. 1 embodiment is stated to be satisfactory where the parabolic component is large compared to the sawtooth component; where this condition will not necessarily hold modifications to this basic circuit may be made to adjust the current through 13 by providing either an adjustable current supply to transistor 11 not passing through 13 (Figs. 6A, 6B, not shown), by effectively varying resistance 14 (Fig. 8, not shown) or by the use of a third coil (Figs. 7A, 7B, not shown).