Light spot suppression circuit for a cathode ray tube

In a cathode ray tube operable with a deflection voltage having a first forward sweep portion and a second flyback portion, a circuit for selectively inhibiting beam current in the tube when the first sweep portion is absent, the inhibiting means comprising, in combination, a first transistor connectable to a positive voltage source, a first serial path comprising a capacitor and a diode coupled to the first transistor, and means connecting the first serial path to the brightness control grid of the tube, the improvement in which the first transistor is normally maintained in a conductive state in the absence of the forward sweep portion of the deflection voltage;

• in which the first serial path is connected across the first transistor with the diode poled in the low impedance direction relative to the positive voltage of the source for normally establishing a high negative voltage at the first junction of the capacitor and the diode while the first transistor remains in its conductive condition;
• in which the connecting means comprises means for applying the voltage at the first junction to the brightness control grid of the tube whereby the high negative voltage normally at the first junction will inhibit beam current in the tube while the first transistor remains conductive;
• and in which the circuit further comprises means coupled to the base of the first transistor and responsive to the first sweep portion of the deflection voltage for positively switching the first transistor into a non-conductive state during the first sweep portion to impart to the first junction, and thereby to the brightness control grid of the tube, a positive step in voltage sufficient to permit the flow of beam current in the tube.