GB886412A - Improvements in or relating to deflection and focusing apparatus for cathode ray tubes - Google Patents

Abstract

886,412. Cathode-ray tubes. MULLARD Ltd. March 4, 1958 [March 13, 1957; Dec. 23, 1957], Nos. 8280/57 and 6915/58. Class 39(1) A cathode-ray tube scan magnification and beam focusing system includes, in one arrangement, an orthogonal pair of quadripole focusing lenses L1, L2, Figs. 14A and 14 B, and a quadripole magnifying lens L3 which has divergent properties in the line scan or X plane, Fig. 14A, so as to increase directly the angle of scan in that plane, but overconvergent properties in the frame or Y plane, Fig. 14B, so as to bring the beam to a focus within the lens field. In this plane the angle of scan is increased in the reverse sense. The effective deflection centre of the deflection means, which may be magnetic or electrostatic, is located on the gun side of the effective centre of the magnifying lens. In another arrangement the quadripole focusing lenses are replaced by a single electromagnetic spherical lens which may be operated as an immersion lens with one or more internal foci. The magnifying lens may have two or more internal foci. The quadripole lenses may be electrostatic, as in Fig. 13, or electro-magnetic, as in Fig. 11, or may be composed of permanent bar magnets as in Fig. 21, which depicts one form of magnifying lens L3, and in Figs. 23 and 25, which depict two forms of focusing lens L1 or L2. In Fig. 21 four pairs of bar magnets NS are located by non-magnetic spacing blocks 32 and are shunted by mild steel elements 30. A further pair of bar magnets mounted in vertically adjustable non-magnetic housings 31 are provided for correcting the shape of the raster. In Fig. 23 the arrangement includes soft iron polepieces 33 and four magnets 34. The field strength is varied by rotation of a ring 35 which contains alternate brass (36) and steel (37) segments. In Fig. 25 each magnet 39 is secured in the holder 47 by end pressure applied by a screw 40. The field strength is varied by axial movement of a brass threaded ring 27 which has a steel insert 28. The structure is held on the tube neck by a clamp 41, Fig. 26, which tightens on to a rubber washer 42 when drawn by screws 43 towards a semicircular abutment 44 on the plate 45. The structure may be adjusted radially, large clearances being provided for this purpose by the fixing holes 46. Scanning may be effected by applying the scanning currents to the windings of the focusing structure of Fig. 11, where this structure forms lens L2 of Figs. 14A and 14B; or the frame currents may be applied to lens L1 and the line currents to lens L2; or separate deflection coils for both scans may be wound round both the yokes of lenses L1 and L2. In another arrangement a separate scanning yoke is provided between lenses L2 and L3. This yoke 48, Fig. 20, is made up of a pair of annular 12-toothed ferroxcube yokes. The frame coils 49 are toroidal while the line coils 50, 51 are saddle shaped. Auxiliary coils 25 employing frame scan current convect for deflection defocusing. The auxiliary coils 25 may be placed at or near the magnifying lens L3 instead of being incorporated in the scanning coil system of Fig. 20. A circular magnetic shunt may be placed between the scanning system and the lens L3 to act as a weaker lens of opposite polarity to the magnifying lens L3 in order to reduce spot magnification. A deflection system may be used in which the beam is deflected first in one direction and then in the opposite direction so as to cross the optical axis. The principles determining the positioning of the various elements of the complete system particularly with reference to the choice of quadripole or spherical focusing lenses and the correction of various defects such as deflection defocusing, changes in deflection sensitivity and reduction of spot magnification are discussed in detail. The invention is described with reference to raster scanning applications e.g. in television oscilloscope or storage tubes but reference is also made to uni-dimensional line scanning applications. Specifications 827,851 and 846,544 are referred to.