Spot killer circuit

Claim 2

A spot killer circuit wherein a transistor is interconnected in series between the first grid of a picture tube and a low voltage supply for supplying a voltage to said first grid; the input point of said transistor is connected to a high voltage supply through a time-constant circuit which may cut off said transistor for a time interval required for discharging the charge on the anode of said picture tube when a power switch is opened, the high voltage supply supplying a higher voltage than said low voltage supply; and the output point of said transistor is connected to said high voltage supply, whereby when the power switch is opened, said transistor is cut off and consequently the potential of the first grid of said picture tube rises.

Claim 3

A spot killer circuit as set forth in claim 1 wherein said transistor is interconnected between the first grid of said picture tube and said low voltage supply, a first time-constant circuit is connected between said low voltage supply and the ground; said input point of said transistor is connected to said high voltage supply through a second time-constant circuit; and a time constant of said first time-constant circuit is made shorter than a time-constant of said second time-constant circuit.

Claim 4

A spot killer circuit as set forth in claim 1 wherein the reference point of said transistor is connected to said low voltage supply which supplies a voltage to the first grid of said picture tube; the input point of said transistor is connected through a capacitor to said high voltage supply; the output point of said transistor is connected to a low voltage side of a circuit for controlling the first grid voltage of said picture tube; and the output of said transistor is connected to the first grid of said picture tube through a capacitor, whereby in the normal operation the voltage at the output point of said transistor may be equal to the voltage at the reference point thereof but when the power switch is opened, said transistor may be cut-off in response to the variation in the voltage at the input point of said transistor or to the variation in the voltage at the reference point thereof or to the variations in both the voltages between the output and reference points and consequently the voltage at the output point may be abruptly increased.

Claim 5

A spot killer circuit as set forth in claim 1 wherein the reference point of said transistor is connected to said low voltage supply which supplies a voltage to said first grid of said picture tube; the input of said transistor is connected through a capacitor to said high voltage supply; the output point of said transistor is connected only through a capacitor to the first grid of said picture tube; and the low voltage side of a circuit for controlling the voltage at the first grid of said picture tube is connected to said low voltage supply.

Claim 6

A spot killer circuit as set forth in claim 1 wherein the reference point of said transistor is connected to said low voltage supply which supplies a voltage to the first grid of said picture tube; the input point of said transistor is connected through a capacitor to said high voltage supply; and the output point of said transistor is connected to the first grid of said picture tube only through a conductive path, whereby when the power switch is turned off, the voltage at the first grid of said picture tube may be raised to a voltage substantially equal to or higher than the cathode voltage of said picture tube.