JPS6432766A - Spot killer circuit for cathode ray tube - Google Patents

Abstract

PURPOSE:To properly and simply prevent the spot of electronic beams by providing a switching circuit, which changes the partial pressure ratio of a partial pressure circuit in accordance with the impression of a signal to indicate the cutoff of an alternating voltage source, so that the electronic beams may not be emitted from the cathode of the titled cathode ray tube at the time of the cutoff of the alternating voltage source. CONSTITUTION:Since a capacitor C1 is charged during the action of a CTR, for example, when the power source of the CRT is turned off, the capacitor is discharged at a required time constant. For that reason, a voltage V1 can be held at a negative voltage for the required time, and a transistor TR2 goes to a turn-on condition. As this result, the collector current of the TR2 increases, a base voltage V3 of a TR1 for feedback increases in a ground direction. Thus, a first grid G1 can be held at a cutoff voltage or below for the required time, the electronic beams to move due to the residual capacity of an anode are cut off, and the generation of the spot can be prevented.