GB676629A - Improvements in or relating to focussing devices for cathode ray tubes - Google Patents

Abstract

676,629. Cathode-ray tubes. GOODMANS INDUSTRIES, Ltd. March 31, 1950 [April 19, 1949], No. 10383/49. Class 39 (i). [Also in Group XXXV] A focusing device for a television receiver comprises an annular magnet with separate polepieces 2, 4, Fig. 1, enclosed in a non-ferrous casing 5, to define a main flux path a of fixed reluctance, and means such as a movable ferrous sleeve 11 are provided for varying the reluctance of an auxiliary flux path b for focusing adjustment, the casing 5 also enclosing centring means 7, 10 for centring the device on the tube neck. In the form shown, the permanent magnet assembly 1, 2, 4 is centred by means of a non-magnetic ring 7 engaging the tube neck by means of a rubber annulus 10, and which is adjustable transversely in two mutually perpendicular directions by two threaded studs 8 against the tension of springs 9. The sleeve 11 has a pair of helical slots 12, Fig. 4, through which the studs 8 pass so that it may be moved by rotating it upon the casing 5. Studs 14 co-operating with a further pair of helical slots 12a allow the sleeve 11 to be moved helically throughout the entire range of the path b. The pole-piece 4 is of L-shaped cross-section. The focusing device as a whole may be carried by a universal mounting for tilting as required and may be attached thereto by bolts 15. The focusing plane may be altered by reversing the focusing device on its support. In a modification, the magnet is an annular assembly of studs or arcuate strips between disc polepieces, and the adjusting means is one or more ferrous screwed studs extending transversely through one polepiece, either near the outer periphery of polepieces which extend beyond the outer face of the magnet, or between the component parts of the magnet. In another modification a steel ring is arranged near the outer edges of the polepieces. The ring is split and the ends connected by a screwed bolt so that the ring can be contracted or expanded. In all forms the magnetic structure may be split diametrically into two or more sections to facilitate assembly, and the sections may be fitted together symmetrically or asymmetrically, for example to provide a specific shape of spot or to correct existing distortion. If the subsidiary flux path is of greater reluctance than the main flux path it is preferably in parallel therewith, or if of similar or smaller reluctance it is preferably in series therewith. The Provisional Specification states in addition that the main flux path is of low reluctance. Both polepieces may have inwardly directed flanges, or one polepiece may be flat and the other of curved cross-section. As an alternative to using a split steel ring, a solid or split soft iron ring may be so arranged as to be adjustable in a transverse or inclined plane. The ratio of the fluxes across the main and subsidiary paths may be 5: 1.