GB1074700A - Improvements in or relating to circuit arrangements for use in television receivers - Google Patents

Abstract

1,074,700. Television. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd- April 7, 1966 [April 10, 1965], No. 15605/66. Heading H4F. A television receiver circuit arrangement for maintaining a constant black level brightness of the displayed picture when variations in the supply voltage V b occur comprises a potentio. meter resistor 14, Fig. 1, across only part of which is D.C. coupled a voltage dependent resistor (VDR) 20, the variable tap 13 of the potentiometer being D.C. coupled to either the control grid of the display tube I as shown or to the cathode, Fig. 3, not shown, the cathode or control grid which is not so coupled being D.C. coupled to the video output stage end of a load resistor 9 for the receiver video output stage 1. Terminal 16 of the potentiometer 14 may be coupled via a fixed resistor 19 to the positive supply terminal in order to prevent variable tapping 13 being moved up to the positive terminal of the supply voltage, the other terminal 15 of the potentiometer' being connected to earth. The fixed terminal 17 of the VDR on the potentiometer 14 is such that it is normally best for the user to adjust the tapping 13 to a position opposite it and thus it' may be desirable to provide the brightness knob with a mark indicating when tapping 13 is opposite terminal 17. The Specification includes' a detailed' discussion of the circuit operation. If the resistance of the portion between the terminals 15 and 17 of potentiometer 14.does not have a correct value a resistor 31, shown dotted, may be connected in parallel therewith, so that the resulting parallel combination has such a value that the desired voltage for 'the tube control grid is obtained at terminal 17. Resistor 31 may be made' variable so as to compensate for spreads in the' constants of various VDR's. To prevent tapping 13 attaining earth potential a further fixed resistor. may be connected between terminal 15 and earth. If the resistance of the portion between terminals 16 and 17 of the potentiometer is not correct resistor 19 may be altered. Specification 898,913 is referred to.