GB458746A - Improvements in and relating to cathode ray tubes - Google Patents

Abstract

458,746. Cathode-ray tubes; television. KEYSTON, J. E., Holly Cottage, Wood End Green, Hayes, and NICOLL, F. H., 17, The Drive, Ickenham, both in Middlesex, and KLEMPERER, O., 70, Syke Ings, Iver, Buckinghamshire. March 19, 1935, Nos. 8604 and 13495. [Classes 39 (i), 40 (iii), and 40 (v)] Relates to arrangements for keeping the electron beam of a cathode-ray tube or imagedissector accurately focused during its deflection. Compensating for shape of screen. For a television transmitting tube with an inclined, photo-electric, mosaic screen 1, Fig. 3, or with other screen not having its centre of curvature at the point of deflection 4, the focal length of the electron lens is fluctuated with the deflection of the ray 3 in one or both directions and is increased when the ray is directed towards more remote parts of the screen. A fluctuating potential derived at 36, 37 from the deflecting oscillations is fed to a compensating electrode 30 surrounding the gap between focusing anodes 22, 24. The electrode 30 is connected to the cathode 20 through a leak 38 and may be biassed relatively to the cathode. In a modification, Fig. 4 (not shown), the anode 24 is a coating on the wall of the tube and the electrode 30 is a short cylinder or apertured diaphragm of internal diameter intermediate between the diameters of the anodes 22, 24. The compensating electrode need not be combined with the final electron lens ; in a cathoderay tube for television reception, part of the modulating cylinder is insulated from the remainder and used for fluctuating the focal length. The circuit shown in Fig. 3 compensates for deflections vertically up and down the screen 1 by the plates 4, 5 or by coils ; to compensate for deflections across the screen, potentials of symmetrical zigzag waveform are applied to the compensating electrode and are derived at terminal 66, Fig. 7, from the sawtooth deflecting oscillations applied at terminals 51, 52. The valve 53 is biassed so that during part of each oscillation, it is non-conducting and a condenser 59 is charged, the condenser then discharging through the valve 53. The resistance-condenser circuit 55, 59 may be replaced by a resistance-inductance circuit. The phase of the oscillations is reversed by a valve 63. A circuit modifying the waveform may be inserted between terminal 66 and the compensating electrode. In another arrangement, the potentials applied to the compensating electrode are obtained from the saw-tooth oscillations by integrating the latter, a condenser being charged in series with a resistance. Compensating for distorting effect of deflecting plates. Fluctuating potentials derived from the deflecting oscillations are fed to the compensating electrode or to a main lens electrode 22, Fig. 9, to compensate for the distorting effect of electrostatic deflecting plates on the beam and so to maintain at minimum size the spot on the fluorescent screen 71 shaped as part of a sphere with its centre near the deflecting plates. A blocking oscillator 81 generates exponentially curved saw - tooth oscillations at a frequency controlled by synchronizing impulses applied at 92, 93, the condenser 94 being repeatedly charged through the primary winding 82 of a transformer 77. These oscillations are applied to the anode 22 through the secondary 80 and also to the deflecting plates 4, 5 through a filter circuit 96 ... 98 converting the waveform to straight-line sawteeth. To prevent changes in brightness of the spot on the screen 71 with changes in the potential of the anode 22, corresponding potential changes are also applied in phase opposition through a circuit 100, 101 to the cathode 20 or cathode-shield 21 or to a grid between the cathode and first anode. In a modification, the condenser 94 is large and the filter circuit 96 ... 98 is omitted so that straight-line saw-tooth oscillations are applied to the anode 22 and to the deflecting plates. Image dissectors. In a tube with an electron image deflected magnetically over a stationary scanning aperture, accurate focusing during deflection is maintained by supplying oscillations of symmetrical zigzag waveform and of the frequencies of the deflecting oscillations to the magnetic focusing coil or by superimposing such oscillations on the potential difference between the photo-electric cathode and the anode. Specifications 445,094 and 458,744 are referred to.