GB802982A - Improvements relating to apparatus for protecting the phosphor screen of a cathode-ray tube - Google Patents

Abstract

802,982. Cathode-ray tube operating circuits. FERRANTI, Ltd. July 31, 1956 [May 5, 1955], No. 13012/55. Class 40 (7). The embodiment of a cathode-ray tube screen protection circuit includes a capacitor 28 between control grid of a projection-type television tube 30 and earth, the tube beam being cut off unless the capacitor carries a charge above a critical value. Positive-going line flyback pulses are applied at 12 and positive-going frame flyback pulses at 20. Assuming that valve 22 is biased to cut-off, the amplified line pulses at screen grid 15 positively charge capacitor 28 via capacitor 24 and brightness control 26, negative pulse peaks being removed by diode 25 (which may be a metal rectifier) and the long time-constant of components 26, 27, 28 affording integration of the charging pulses prevents unwanted modulation of the beam current. When the line time-base is running normally, capacitor 28 has a sustained positive charge sufficient to permit tube beam current to flow. At switching on, capacitor 28, until it is charged to above the critical value, will maintain the beam cut-off. The connection of capacitor 28 to the anode of valve 10 serves to rapidly discharge the capacitor when the line time-base fails and the receiver is in operation; the negative signal at suppressor grid 17 ensures that the anode of valve 10 is cut off from space current when the line time-base is operating normally. With no charging pulses to capacitor 28 the viewer cannot bring the beam on again by manipulation of brightness control 26. The effect of frame pulses is to produce intermittent conduction in valve 22, thus causing a momentary reduction of voltage across resistor 26 but the long-time constant of circuit 26 to 28 prohibits unwanted tube beam modulation. If there are no frame pulses at switching on, valve 22 conducts continuously and the resulting low potential of screen grid 15 prevents charging of capacitor 28 and the tube beam remains cut off; and similarly when the frame time-base fails in operation, in which case beam cut-off is less rapid than when line pulses fail since capacitor 28 takes longer to discharge through resistors 26, 27 than through valve 10. The circuit has certain features of inherent selfprotection and it is preferable for valve 22 to share the cathode of valve 10. The lower end of capacitor 28 may alternatively be connected to H.T. positive; the line and frame pulses may be derived by other than flyback means; and the capacitor may be charged directly by line pulses. Where the cathode-ray tube uses electrostatic deflection an extra stage may be provided to give suitable line pulses from the line time-base.