Abstract: A high voltage shutdown circuit that tracks with brightness changes in the CRT. An ABL (automatic brightness level) signal is applied to adjust the conduction point of a latch transistor that is connected in a trigger arrangement with a zener diode and driven by a sensing winding on a high voltage transformer. The ABL voltage is applied to an inverting transistor and an opposite polarity potential is developed across a capacitor connected in series with the latch transistor. Because of the loading on the high voltage generating means, the voltage across the sensing winding does not accurately reflect the actual high voltage at the CRT and the circuit prevents premature high voltage shutdown under high beam current conditions.

Abstract: A projection cathode ray tube burn protection circuit includes a buffer transistor supplied with a 60 Hz square wave signal from an optical coupler. A PNP and NPN combination of transistors full wave rectifiers the buffer output. The rectified signal is divided down to reduce the magnitude of the alternating current components therein and applied to the base of a switch transistor that is normally maintained in saturation for swamping out the alternating current components. The switch transistor drives a high voltage shut down transistor and a blank transistor for shutting down the high voltage generating system and for blanking the cathode ray tube, respectively, in the event the power input to the optical coupler is disrupted.

Abstract: A shutdown circuit for a projection type CRT comprises a transistor switch for operating a relay, responsive to a turn-off signal, to immediately break the high B+ operating voltage connection to a horizontal deflection system. A switched mode power supply is supplied with the turn-off signal, after a delay caused by a resistance-capacitance circuit, for shutting down the low voltage drive voltage to the horizontal deflection system after occurrence of the turn-off signal.

Abstract: A system for preventing phosphor burn in a cathode ray tube (CRT) without a shadow mask such as used in a projection television receiver is disclosed. Horizontal drive circuit failure or the intermittent operation thereof is detected by means of a fast falling, horizontally derived, high voltage sweep transformer output which results in the application of a stable positive voltage to raster blanking circuitry for the duration of horizontal drive circuit failure or intermittent operation. Short time constant coupling circuitry permits the near simultaneous shutdown of the CRT upon detection of a horizontal drive circuit malfunction in avoiding the burning of a vertical line on the unprotected phosphor-coated faceplate of the CRT.

Abstract: A television receiver protection system providing X-ray protection to the viewer responds to rectified horizontal flyback pulses supplied to the gate electrode of a silicon controlled rectifier through a zener diode to trigger the silicon controlled rectifier into conduction when the high voltage level from the flyback transformer of the receiver becomes abnormally high. The silicon controlled rectifier is connected across the power supply for the horizontal oscillator, so that the horizontal oscillator is rendered inoperative when the silicon controlled rectifier becomes conductive.