White-balance servicing circuit

Claim 1

In an apparatus comprising: a. a cathode drive network, having first and second input terminals, for supplying signals to a plurality of cathodes of a cathode ray tube, b. first amplifying means having a control electrode and first and second electrodes, said means being responsive to i. an operating potential, when applied to said second electrode, to cause flow therethrough of an electric current, and ii. variations in a signal, when applied between said control electrode and said first electrode, to vary said electric current flowing through said second electrode, means for applying a deflection input signal between said control electrode and said first electrode, and output means, responsive to current flow through said second electrode produced in response to said deflection input signal, to generate a deflection output signal, c. second amplifying means having a control electrode and first and second electrodes, said second amplifying means being responsive to i. variations in a signal, when applied between said control electrode and said first electrode, to vary an electric current flowing through said second electrode thereof when a bias voltage of first value concurrently is applied between said control electrode and said first electrode, and ii. a bias voltage of second value, when applied between said control element and said first electrode, to cut off flow of said electric current through said second electrode thereof, video input means for applying a video signal between said control electrode and said first electrode of said second amplifying means, means connecting said second electrode of said second amplifying means to said first input terminal of said cathode drive network, first direct-current-conductive impedance means having finite resistance to the flow therethrough of constant undirectional electric current, and means directly connecting said first direct-current-conductive impedance means between said first and second input terminals of said cathode drive network, the improvement comprising: d. first and second switching means, each having a first terminal, a second terminal, and means for permitting directcurrent-conduction from said first terminal thereof to said second terminal thereof when said switching means is closed and interrupting said conduction when said switching means is open, and means connecting said first terminal of each of said two switching means to a first source of operating potential, e. first biasing means, connected between said first source of operating potential and said second terminal of said first switching means, for producing a reference voltage at said second terminal of said first switching means when said first switching means is open, f. second direct-current-conductive means connecting said second terminal of said first switching means to said second input terminal of said cathode drive network, and g. second biasing means for (i) applying between said control electrode and said first electrode of said second amplifying means a biasing voltage having said first value when said second switching means is closed and having said second value when said second switching means is open, and (ii) preventing transmission of said video signal from said video input means to said second switching means, said output means comprising third direct-current-conductive means connecting said second terminal of said second switching means to said second electrode of said first amplifying means, for supplying an operating potential to said second electrode of said first amplifying means only when said second switching means is closed.