GB763586A - Improvements in or relating to permanent magnet focussing arrangements for cathode ray tubes - Google Patents

Abstract

763,586. Cathode-ray tubes. MULLARD RADIO VALVE CO., Ltd. July 31, 1953 [Aug. 26, 1952], No. 21490/52. Class 39(1). A permanent magnet focussing mount for the cathode ray tube of a television receiver or like apparatus comprises a main support member 1 in the form of a metal plate provided with an aperture for the tube neck and having its edges bent over to increase the longitudinal rigidity and carrying guide rods 2 and 3 to which an apertured plate 4 is attached. An annular magnet 6 is secured to the plate 4 by means of three tabs 5 formed in the edge of the plate. A second annular magnet 7 is secured by tabs 8 to an apertured plate 9 secured to a tubular guide 10 which slides on the rod 2 and having two guide flanges 11 which lie on either side of the rod 3. Movement of the magnet 7 towards the magnet 6 is effected by a lever 13 fulcrummed on the plate 1 as by a pivot pin 14 and having a forked end bent to engage the back of the plate 9. The end 15 of the lever 13 is moved towards the plate 1 by any suitable means, for example a Bowden cable, so that the focus control can be brought to any convenient point of the receiver cabinet. The polarity of the magnets is such that they tend to repel one another, so no return spring is necessary. The assembly is correctly located with respect to the tube neck by a device, Fig. 3, of the " iris " type, comprising a circular plate 19 mounted on pins 16, 17 and 18 secured to the plate 1 and passing through three arcuate slots in the plate 19. Each pin forms a fulcrum for an arcuate lever 20 carrying a pin 21, which engages the wall of a hexagonal aperture in the plate 19, and a pad 22 of felt, rubber or the like. The plate 19 is urged in the direction to cause closure of the pads 22 on the tube neck by a spring 23 secured between the plate 19 and the plate 1. The magnets 6 and 7 are located in correct coaxial relationship during assembly of the apparatus by means of a mandrel having an end portion of the same diameter as the tube neck and a further portion of larger diameter on which the magnets are located. The mandrel is placed in position with one end gripped by the pads 22 and the other end located in the aperture in the plate 4, whereupon the tabs 8 and 9 are bent up to lock the magnets in position and the mandrel then removed.