US2749486A - Focussing magnet systems for cathode ray tubes - Google Patents

Focussing magnet systems for cathode ray tubes Download PDF

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Publication number
US2749486A
US2749486A US454985A US45498554A US2749486A US 2749486 A US2749486 A US 2749486A US 454985 A US454985 A US 454985A US 45498554 A US45498554 A US 45498554A US 2749486 A US2749486 A US 2749486A
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pole
ring
focussing
cathode ray
axial
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Expired - Lifetime
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US454985A
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English (en)
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Gould John Edward
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J29/00—Details of cathode-ray tubes or of electron-beam tubes of the types covered by group H01J31/00
    • H01J29/46—Arrangements of electrodes and associated parts for generating or controlling the ray or beam, e.g. electron-optical arrangement
    • H01J29/58—Arrangements for focusing or reflecting ray or beam
    • H01J29/64—Magnetic lenses
    • H01J29/68—Magnetic lenses using permanent magnets only

Definitions

  • Cathode ray tubes for television receivers usually carry on the neck of the tube between the electron gun and the cone an ion trap system, a focussing system, and a scanning system, all three serving to produce fixed or variable magnetic fields on the axis of the tube. It is desirable to keep the neck as short as possible and these fieldproducing systems must therefore be limited in their axial dimensions and must be located close to each other. Stray field from the focussing system extending along the axis is likely to interfere with the desired field distribution in either or both of the adjacent systems.
  • a focussing system comprising an electromagnet shrouded with permeable material such as iron has, in its characteristic distribution of magnetic field along the tube axis, a peak value at the axial midpoint, reducing rapidly to low Values with increasing axial distance from the midpoint.
  • the use of a permanent magnet focussing system is advantageous, e. g. by reducing the rectified power requirements in the set, but the usual arrangement of a permanent magnet ring or blocks magnetised in the direction of the axis of the tube and carrying at each end pole pieces with circular holes gives a peak value in the characteristic field distribution reducing rapidly through zero field to negative peaks (of about 20% of the strength of the main peak) and thereafter changing relatively slowly with increasing axial distance (see Figure 8 of Permanent magnet lenses for television tubes, D.
  • a focussing system has a number of permanent magnet blocks mounted radially round a central pole consisting of a ring of permeable material, such as mild steel or sintered iron, with their polarisation in the radial direction, and a further pole consisting of a mild steel plate parallel to the plane of the ring, to form with the ring an axial gap, with a central hole coaxial with the ring and its periphery bent over to contact the outer ends of the radial magnet blocks.
  • the ring and the plate pole may be adjustable to vary the axial gap between them.
  • the axial field distribution curve of such an assembly spacing has a positive peak that falls to a low value very quickly on the mild steel pole side of the assembly, and a lower negative peak that rises more slowly from its peak value on the magnet and ring side of the assembly.
  • the extension of the stray field on the opposite side of the assembly is relatively unimportant.
  • At least one of the mild steel pole plates may be axially adjustable with respect to the ring.
  • the axial field distribution curve for an assembly with two symmetrical axial gaps shows equal but opposite peak fields under the two gaps, the field decreasing rapidly on each side of the assembly. If one pole plate is adjusted to give a reduced axial gap, both field peaks are reduced in value but remain approximately equal.
  • a satisfactory range of focussing power can be achieved without appreciably changing the symmetrical distribution of the axial field or the rapid fall in the leakage field. It is an advantage of such symmetrical positive and negative field systems that image rotation is eliminated and also the small leakage field gives little interference with either scanning coils or the ion trap magnet system.
  • Three or more magnet blocks may be used to give substantially uniform flux distribution round the cylindrical pole face of the ring.
  • Figure 1 is a plan of a focussing system
  • Figure 2 is a section on the line 2-2 of Figure 1;
  • Figure 3 is a section on the line 33 of Figure 2.
  • Figure 4 is a fragmentary view corresponding to Figure 2 showing an alternative arrangement for mounting the magnets.
  • the outer shell 12 is adjustable axially with respect to the ring 3, axial movement being derived from rotation of the outer shell 12 with respect to the inner shell 4 by means of three slots 14 in the wall 13 of the outer shell 12, which slots 14 are inclined at a slight angle to the plane of the ring 3 and magnets 1.
  • Two of the slots 14 embrace the heads of small cheese-headed screws 15 screwed firmly into the wall 5 of the inner shell 4 and the third slot embraces the shank of a larger screw 16, which may be used to lock the two shells 4, 12 together after adjustment.
  • the gap between the ring 3 and the shell 12 may be varied from 0.6 cm. to 1.1 cms.
  • Three tapped holes 17 in the inner shell 4 are used to attach the system to an upright of the chassis of a television receiver.
  • the neck of the cathode ray tube passes through a hole in the upright and the central hole of the focussing system and is usually supported by resilient packing, e. g. rubber,
  • the outer ends of the magnet blocks 1 must be finished to the same curvature as the cylindrical wall of the inner shell 4 in order to get a close fit.
  • the magnet blocks 19 (only one shown) are mounted on flats 20 round a ring 21 and the cylindrical wall 22 of the inner shell is formed with fiat portions 23 fitting closely over the square ends of the blocks 19.
  • the cylindrical wall 24 of the outer shell 25 is similar to that in the previous construction and all other constructional details are the same.
  • the focussing power of a magnetic electron lens is proportional to the quantity:
  • Magnetisation of the assembly is readily elfected by the use of pancake type magnetising coils, one on each side of the assembly, and connected so as to produce mutual repulsion, through which a heavy current is momentarily passed.
  • the invention thus enables adequate focussing strength to be maintained with modest axial dimensions of the assembly, and efficient utilisation of permanent magnet material.
  • a focussing magnet system for cathode ray tubes comprising a central ring pole of magnetically permeable material, a number of permanent magnet blocks mounted on the central ring pole each with its direction of polarisation extending radially outwardly from the outer periphery of the ring pole, and a further pole consisting of a magnetically permeable plate parallel to the plane of the ring pole and spaced from the ring pole to form a gap, with a central hole coaxial with the ring pole, and with its periphery bent over to contact the radially outer ends of the magnet blocks, like poles of the magnets respectively contacting the ring pole and the further pole.
  • a focussing magnet system for cathode ray tubes comprising a central ring pole of magnetically permeable material, a number of permanent magnet blocks mounted on the central ring pole each with its direction of polarisation extending radially outwardly from the outer periphery of the ring pole, a further pole consisting of a magnetically permeable plate parallel to the plane of the ring pole and spaced from the ring pole to form a gap, with a central hole coaxial with the ring pole, and with its periphery bent over to contact the radially outer ends of the magnet blocks, like poles of the magnets respectively contacting the ring pole and the further pole, and a second magnetically permeable plate pole with coaxial hole and bent-over periphery disposed in reverse position with respect to the first plate pole to form a second axial gap.
  • a focussing magnet system as in claim 3, comprising flats on the outer periphery of the ring pole to receive the inner ends of the magnet blocks, further intermediate flats on the ring pole, and a non-magnetic support interposed between the ring pole and one plate pole, said support being formed with flanges for engaging the adjacent end surface of the ring pole to determine the extent of the gap between the ring pole and the plate pole and flaps for engaging the intermediate flats.

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  • Microwave Tubes (AREA)
  • Image-Pickup Tubes, Image-Amplification Tubes, And Storage Tubes (AREA)
US454985A 1953-09-21 1954-09-09 Focussing magnet systems for cathode ray tubes Expired - Lifetime US2749486A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB25935/53A GB761663A (en) 1953-09-21 1953-09-21 Improvements in or relating to focussing magnet systems for cathode ray tubes

Publications (1)

Publication Number Publication Date
US2749486A true US2749486A (en) 1956-06-05

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US454985A Expired - Lifetime US2749486A (en) 1953-09-21 1954-09-09 Focussing magnet systems for cathode ray tubes

Country Status (5)

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US (1) US2749486A (en, 2012)
BE (1) BE531679A (en, 2012)
CH (1) CH320563A (en, 2012)
FR (1) FR1106914A (en, 2012)
GB (1) GB761663A (en, 2012)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2864021A (en) * 1955-09-01 1958-12-09 Rca Corp Color kinescope adjunct
US3059156A (en) * 1957-06-19 1962-10-16 Moriya Saburo Miyata Means for controlling magnetic fields

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2416687A (en) * 1944-03-30 1947-03-04 Bell Telephone Labor Inc Magnetic focussing device
US2418487A (en) * 1944-03-31 1947-04-08 Bell Telephone Labor Inc Cathode ray magnetic focusing device
US2591820A (en) * 1948-02-07 1952-04-08 Pye Ltd Suspension mounting for the focussing device of cathode-ray tubes
US2640868A (en) * 1950-11-04 1953-06-02 All Star Products Inc Magnetic focusing device for cathode-ray tubes

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2416687A (en) * 1944-03-30 1947-03-04 Bell Telephone Labor Inc Magnetic focussing device
US2418487A (en) * 1944-03-31 1947-04-08 Bell Telephone Labor Inc Cathode ray magnetic focusing device
US2591820A (en) * 1948-02-07 1952-04-08 Pye Ltd Suspension mounting for the focussing device of cathode-ray tubes
US2640868A (en) * 1950-11-04 1953-06-02 All Star Products Inc Magnetic focusing device for cathode-ray tubes

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2864021A (en) * 1955-09-01 1958-12-09 Rca Corp Color kinescope adjunct
US3059156A (en) * 1957-06-19 1962-10-16 Moriya Saburo Miyata Means for controlling magnetic fields

Also Published As

Publication number Publication date
FR1106914A (fr) 1955-12-27
CH320563A (fr) 1957-03-31
GB761663A (en) 1956-11-21
BE531679A (en, 2012)

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