GB761663A - Improvements in or relating to focussing magnet systems for cathode ray tubes - Google Patents

Abstract

761,663. Electron lenses. GOULD, J. E. Aug. 18, 1954 [Sept. 21, 1953], No. 25935/53. Class 39(1). A magnetic focussing system for a cathode ray tube preferably a television receiver comprises three or more permanent magnet blocks 1, Figs. 2, 3 mounted radially around a ring 3 of mild steel or sintered iron which serves as one pole, the magnet blocks being magnetised in the radial direction, and the second pole is formed by a mild steel plate 4 axially spaced from ring 3 and having its periphery bent over at 5 to contact the outer ends of the magnet blocks 1. Ring 3 and plate 4 may be relatively adjustable to vary the axial gap between them. The magnet blocks of which there may be three (as shown) or six, are equally spaced circumferentially and mounted on flats 2 of ring 3; they may consist of " Alcomax III " (Registered Trade Mark) which is an alloy of Ni, Al, CO, Cu, Cb, Fe and " impurities " the proportions of which are given. The outer ends of the magnet blocks may be curved to match the curvature of flange 5. A triangular support 6 has flanges 7 contacting members 1, 3 and flaps 8 fitting over three flats 9 on ring 3. A washer 10 and three rivets 11 hold members 1, 3 against support 6. The structure so far described has a magnetic field distribution with unequal positive and negative peaks and with a large stray field on the right hand side, Fig. 3. By adding a second mild steel pole plate 12 the positive and negative peaks are made approximately equal, the field decreasing rapidly on each side of the assembly. By producing axial adjustment of at least one of plates 4, 12 with respect to ring 3 to reduce the axial gap both field peaks are reduced in amplitude by equal amounts. Image rotation is eliminated and the small leakage field on the left hand side, Fig. 3, does not interfere with the scanning coils or ion trap magnet. As shown, three slots 14 and screws 15, 16 enable 12 to be rotated (with an axial component of movement) relatively to ring 3. Screw 16 is larger than the others and can be used to lock members 4, 12 together after adjustment. Two holes 18 are tapped so that a handle can be fixed to 12 to assist in its adjustment. In a modification, Fig. 4 (not shown) both the inner and outer ends of the magnet blocks are flat and are mounted on flats on the members they contact. The assembly can be magnetized by means of two pancake coils, one on each side connected so as to produce initial repulsion and through which a heavy current is momentarily passed.