GB1452941A - Thyristor scanning tv circuits - Google Patents

Abstract

1452941 Converting STANDARD TELEPHONES & CABLES Ltd 16 May 1974 [22 May 1973] 24233/73 Heading H2F [Also in Division H3] In a television deflection circuit of the type comprising a trace thyristor TH3, a commutating thyristor TH2, a third thyristor TH1 in series with the supply to provide automatic picture width and EHT regulation and a rectified A.C. supply and smoothing circuit 4, the time constant of discharge of the smoothing circuit is so chosen that if the regulating and commutating thyristors are on together the circuit discharges below the holding current of the third thyristor between two positive half cycles of the supply. As described, if commutation continues to malfunction in this way over a number of cycles, the fuse will blow. The automatic width control voltage is derived from the negative peak of the wave at the line output transformer and is supplemented by EHT current feedback from a capacitor of the focusing voltage multiplier circuit to improve EHT stability. The control may have a time constant small enough to correct for mains ripple. The control signals for the thyristors are derived from two integrated circuits (Fig. 5). The righthand one, MIC, comprises two monostable circuits in cascade the first being driven from the output of circuit TBA fed with the video signal. The trace thyristor triggering pulse is derived by combining in diodes D2, D3 the outputs Q1 and Q2 of the two monostable circuits. The commutating thyristor is triggered from the first monostable circuit and the regulating thyristor is triggered by the second monostable circuit. The regulating signals from the line output circuit are compared with a reference provided by a Zener diode D1, the current through which controls the monostable circuit timing and thus its output pulse delay. Excessive delay is prevented by limiting the diodes D4, D5. A small mains transformer HT (Fig. 3) provides starting current for the drive and CR tube circuits and a diode circuit D2, D3 transfers the circuit to a supply derived from the line output circuit when it comes into operation. Fig. 8 (not shown) includes an arrangement avoiding the use of a mains transformer and including a Zener diode to stabilize the supply to the CRT heater circuit.