Patent GB-767437-A (GB)
Title: Improvements in or relating to focusing magnet systems for cathode ray tubes
Inventor: KNIGHT FRED
IPC: H01J29/68
Filed: 1954-02-05
Abstract: 767,437. Electron beam focussing apparatus. MARRISON & CATHERALL, Ltd. Dec. 20, 1954 [Feb. 5, 1954], No. 3404/54. Class 39(1). A magnetic focussing system for a cathode ray receiving tube comprises a plurality of permanent magnet blocks 1, Figs. 1, 2, arranged around the axis, the blocks each having axially and radially extending portions as by making them L-shaped, Fig. 1, or U-shaped, Fig. 5, the magnet being polarised along the radial and axial portions thereof, a ring pole piece 5 in contact with the inner ends of the radial magnet portions and a second ring pole piece 6 contacting the ends of the axial limbs of the magnets the magnet blocks and pole pieces being so relatively disposed axially that the magnetic pull on at least one side of the field plate does not extend substantially beyond the end of the magnet system. There may be only 2 magnet blocks but there are preferably 3, 4 or even more and they are made of an alloy of 13 per cent Ni, 8 per cent Al, 23 per cent Co, 3 per cent Cu, the balance being iron except perhaps for 0À8 per cent or 2À5 per cent niobium. They are cast in multiple stick or ring form and then after heat treatment and grinding are broken into individual magnets. The casing 2, which may be of thermosetting, thermoplastic or cold setting synthetic resin, has a flange 10 with projections 9 formed with recesses 8 to receive the magnet blocks which are held in position by a retaining ring 13. Casing 2 and ring 13 can be integral. Mild steel or sintered iron pole piece 5 has flats to contact the magnet blocks and pole piece 6 of mild steel is triangular and has a notch 15 to receive rib 16 bent from magnetic ring 17. A handle 24 enables ring 17 to be adjusted pivotally in its own plane and to slide with respect to notch 15 so that the axis of the magnetic field can be tilted to control the position of the pictures. A retaining ring 18 is secured by eyelets 19 to member 10, eyelets 19 also serving to secure the assembly to a support. Magnetic shunt sleeve 20 has three slots 22 engaged by three screws 21 and is rotated by handle 23 the resulting axial movement thereof effectively varying the axial gap between the pole pieces. In another embodiment, Fig. 5, the magnets 25 are U-shaped and have two ring polepieces 28, 29 and an axially movable magnetic shunt ring 31 is again provided. The space inside the U is filled with non-magnetic material 30. In a modification the shunt ring is replaced by axially adjustable steel shunt screws, Fig. 6 (not shown) or sliding plungers. In another embodiment, Figs. 1, 8 (not shown), using L- shaped magnets, a magnetic end plate has tapered axial wings extending outside the magnets so that by rotating the plate relative to a fixed magnetic pole piece the shunting effect can be varied. In another form, Fig. 9 (not shown), also using L-shaped magnets, a biassing coil is provided between the pole pieces and the current through the coil can be varied to adjust the magnetic field. The embodiments of Figs. 5-8 (not shown), may have means similar to Fig. 4 (not shown), for picture shift adjustment. In further forms the screw and slot 21, 22 of Fig. 1 can be replaced by pin and slot or other quick start thread equivalent. The device of Fig. 1 shifts the flux plate axially due to movement of sleeve 20 ; to avoid this a plurality of axial steel screws equal in number to the number of magnet blocks and in threaded connection with one pole piece extend towards the other pole piece and are adjustable axially to vary the effective gap between the pole pieces. The pole piece can have wings in which case there is no magnetic plate adjacent thereto. The focussing device, together with scanning coils and ion trap magnets, surround the - tube neck.