US2926272A - Combined shielding and centering means for cathode ray tubes - Google Patents

Description

F/ELD .STAENGTH Feb. 23, 1960 v, D! PAOLO Er AL 2,926,272

COMBINED SHIELDING AND CENTERING MEANS FOR CATHODE RAY TUBES Filed June 24, 1958 F/@ v 4 a & CONT/POL 62/0 72 INVENTORS mom/95 K 0/ mow 015mm:- mo/v f/VD PAN/E BY /f fi i V/IJ/LEVJKIJ a; DEF! Ecr/a/v 60/45 QM 4w F/GJ.

arm/mgr ilnited States Patent COMBINED SHIELDING AND CENTERING MEANS Y FOR CATHODE RAY TUBES Application June 24, 195s, Serial No. 744,121 7 9 Claims. 01. 313-75) The present invention rel-ates to television type display systems and more particularly to improvements in beam shielding and centering devices for such systems. I

Thepresent'trend in television systems is to reduce the over-all length of the television picture tube, first by increasing the maximum deflection angle of the beam, thereby to reduce the length of the bulb for a given screen size, and secondly by reducing the length of the neck of the cathode-ray tube as much as possible. I In reducing the length of the neck it becomes necessary to move the cathode and first and second grids of the electron gun closer to the deflection yoke. This has introduced the need for means for shielding these elements of the electron gun from the elfects of the fringing field of the deflection yoke.

The reduction in the length of the neck of the cathoderay tube has also made it impossible tolocate magnetic centering devices-on the neck of the cathode-ray tube betweenthe electron gun and the deflection yoke. The removal oflthe centering devices from the neck of the cathode-ray tube has made it necessary to find other means for positioning the point of impingement of the undeflected cathode-ray beam at the desired spot on the picture tube screen. Attempts have been made to in: crease the precision of manufacture of cathode-ray tubes so that no centering device is required. This greatly increases the unit cost of acceptable picture tubes. Other attempts have been made to rely on electrical corrective circuits associated with the yoke to provide centering. Another alternative which has been tried is to provide some form of magnetic centering devices within the yoke itself. Each ofthese last two alternatives has its disadvantages and generally adds to the cost of the picture tube and deflection yoke assembly.

The copending application of Thomas V. Di Paolo, Serial No. 696,840, filed November 15, 1957, discloses a novel laminated shield member forreducing the fringingfiux at the end of the deflection yoke. This novel laminated shielding member requires very little space on the neck of the cathode-ray tube and effectively reduces the fringing flux but does not provide any centering action.

-It is an object-of the present invention to provide an improved laminated shielding member which requires very little spaceon the neck of the picture tubeand which performs the dual functions of magnetic shielding and beam centering. i

'It is-a further object 'of the present invention to provide a novel on-the-neck beam centering means which controls the position-of the beam within the electrostatic focusing region. L Still another object of the present invention is to provide a novel beamcentering arrangement external of the deflection'yoke which does not require any substantial amount of additional space on the neck of the cathoderay tube.

These and other objects of the present invention are achieved by providing a laminated magnetic shielding member comprising a highly conductive non-magnetic 2 sheet-like member and a sheet-like member of high permeabili ty secured thereto. The high permeability merriber is formed with a gap therein dividing said high per meability member into two magnetically separate portions. An adjustable magnet is secured to said laminated magnetic shielding member in a position to bridge the gap between said two separate portions. Control of beam centering is accomplished by positioning the magnet to vary the strength of the magnetic field impressed between the two magnetically separate portions of high permeability. The entire shield assembly may be rotated to provide additional control of beam centering if desired.

For a better understanding of the present invention together with other and further objects thereof reference should now be made to the following detailed description which is to be read in conjunction with the accompanying drawings in which:

Fig. 1 is a view partially in section of a cathode-ray tube and deflection yoke assembly incorporating the novel centering device of the invention;

Fig. 2A is a side view of the combined shielding member and centering device of the present invention;

Fig. 2B is a plan view of the shielding member of Fig. 2A:

Fig. 3 is a graph showing the reduction in fringing flux which may be accomplished through the use of the novel shielding and centering device of the present invention;

Fig. 4 is a graph showing the change in centering which may be accomplished through the rotation of the centering magnet; and

Fig. 5 is a fragmentary view showing one possible modification of the structure shownin Fig. 1.

Turning now to Fig. 1, member 10 is the neck portion of a cathode-ray tube on which the deflection yoke assembly incorporating the present invention is mounted. The neck flare of the cathode-ray tube is shown at 12. The end turns of one of the horizontal deflection coils are shown at 14 and 14a. One vertical deflection coil is shown at 16. An insulating coil form 18 separates the the horizontal and vertical deflection coils 14 and 16 and serves to maintain these coils in their proper position on the neck 10 of the cathode-ray tube. Member 20 represents the magnetic yoke core which serves as the external magnetic path for flux set up by horizontal and vertical deflection coils.

The three electrodes identified by the reference numeral 24 form the electrostatic focusing means for the cathode-ray tube shown in Fig. 1. The beam forming assembly, including cathode, control grid and screen grid, are shown generally at 26.

The laminated shielding and beam centering assembly which comprises the present invention is shown is crosssection at 28. Assembly 28 is apertured to receive the neck end of the cathode-ray tube and is held in place adjacent the end turns 14a of the deflection coils by the insulating housing 39 which mechanically covers the terminals of the deflection coils and associated components. An insulating member 31 may be included between the end turns 14a and the assembly 28 to prevent accidental contact between circuit leads and the conductive portion of assembly 28. Member 31 performs the additional function of spacing in the shield and centering assembly 28 from the end turns 14:: by a fraction of an inch. This reduces the shunting eifect of this assembly on the deflection fields. The centering magnet which forms a portion of assembly 28 is shown at 32. Magnet 32 is held in place on assembly 28 by a rivet 33 which forms a pivot about which magnet 32 may be rotated.

Turning now to Figs. 2A and 2B for a more complete description of the novel shielding and centering assembly members 42 and 44. may be controlled by properly selecting the size of gap 3 28, it will be seen that assembly 28 comprises a disc 40 of conductive material such as copper or aluminum. Disc 40 is formed with aperture 41 to receive the neck portion 19 of the cathode-ray tube. Secured to disc 49 are two half discs 42 and 44. Half disccs 42 and 44 are formed of a material having a high magnetic permeability such as silicon steel. Half discs 42 and 44 may be secured to disc 4% by means of rivets 46 which pass through both members. Other forms of securing means may be substituted if desired. The edge portions 48 and 50 of the half disc at are separated from the confronting edge portions 52 and 54 of half disc 44- to form a gap 56. The purpose of this gap is to add reluctance in the path of the low frequency vertical deflection field, a portion of which is shunted by shielding assembly 23. The curved edge surfaces 57 and 58 of half discs 44 and 42, respec tively, define an aperture which has substantially the same diameter as aperture 41. Y

The centering magnet 32 shown in Figs. 2A and 2B bridges the gap between'the two half discs 42 and 44. Centering magnet 32 is magnetized along a diameter as indicated by the letters N and S in Fig. 2B which designate the north and south poles of the magnet 32. Magnet 32 may be rotated about its support 33 thereby to vary the angle between the north-south axis of the magnet and the gap 56.

Turning once again to Fig. l, it will be seen that the combined shielding and centering assembly28, shown at Figs. 2A and 2B, is disposed with the conductive member 4- adjacent the end turns 14a of the horizontal deflection coils. The gap 56 is disposed transverse to the direction of the low frequency vertical field across the neck of the cathode-ray tube. As shown in Fig. 1, housing 30 is provided with an opening 60 in the rear wall thereof so that magnet 32 may be rotated without removing housing 30.

The manner in which the laminated shielding member 28 of the present invention prevents fringing of the horizontal and Vertical deflection fields is described in detail Briefly in the above-mentioned copending application. the shielding member operates in the following manner. The eddy currents set up in the highly conductive member 40 by the approximately kilocycle horizontal scanning field tend to confine the flux from the end regions of the coils to the region of neck 16 to the left of the assembly 28 as shown in Fig. 1. Therefore the novel laminated shield of the present invention shunts much less of the high frequency horizontal deflection field around the neck of the tube than would be shunted by a single layer shield of high permeability material at the same location. Disc 4% also aids in reducing hysteresis losses in half discs 42 and 44 by reducing the concentration of high frequency magnetic flux in high permeability portions 42 and 44. The reduction in the shunting effect and the reduction of the hysteresis loss both 7 tend to minimize the loss of horizontal deflection power in the assembly 28.

The eddy currents induced in conductive disc 46 as a result of the relatively low frequency vertical scanning field are usually not suflicient to confine the vertical field to the region to the left of disc 49. Therefore some of the fringing flux passes through conductive disc 49' to the low reluctance members 42 and 44. This leakage flux is conducted around the neckof the tube through The amount of this shunted flux 56 in the otherwise low reluctance path afforded by members 42 and 44. In general, the size of this gap will represent a compromise between the amount of defocusing which can be tolerated at the edge of the picture as a result of residual fringing flux and the permissible loss of vertical deflection power resulting from this shunting effect of the shielding means. 'In one typical embodiment of the invention gap 56 has a width of W of an to receive magnet 32. This opening 60 is large enough inch. As mentioned in the copending application'it has been determined experimentally that the shunting effect of the laminated shield member 28 with low frequency deflection field can be changed measurably by including gap 56. This is so even though there are other relatively large air gaps in the path of the vertical deflection flux.

Fig. 3 is a plot of the field strength of -the fringing deflection flux along the axis of tube 10 as a'function of the distance from the end plane of the deflection yoke. The position of the control grid of the system of Fig. l

is shown as a convenient reference point in evaluating the curves. Curve represents the field strength without assembly 28 in place. Curve 72 represents the field strength with assembly 28 inplace. It should be noted that the amount of fringing flux at the control grid region has been reduced by a factor of approximately 3. At the same time the field strength at the end plane of the deflection coils is reduced only slightly. I i

Fig. 4 is a plot of the change in static beam position which may be achieved by rotating magnet 32. The zero or reference direction is taken withthe north-south axis of magnet 32 aligned with the edges 50 and 54.,

Rotating themagnet from this reference position places the north-south axis across the gap 56. As a result there is a steady flux which flows through half discs 42 and 44 and bridges the gap between edges 52 and 48. Some of this steady flux also fringes between arcuate edges 57 and 58--that is, across the region occupied by'the neck portion 10 of the cathode-ray tube. The flux which fringes'across the region occupied'by the neck portion 10 of the cathode-ray tube causes a deflection of the cathode ray beam in the vertical direction. Theamount of deflection in the vertical direction will increase as the angle between the north-south axis of magnet 32 and the gap 56 increases. The maximum deflection as represented by peak 74 in Fig. 4 is reached when, the north-south axis is perpendicular to the reference axis, that is perpendicular to :edge 54 or 50. Further rotation of the magnet 32 will reduce the amount of magnetic flux induced in half discs 42 and 44. This'reduction in flux will reduce the vertical deflection of the beam. The deflectionof the beam from its normal rest position will again be a minimum when the north-south-axis again moves into 7 alignment with edges 50 and 54.

into contact with the half disc d2. Misalignment of the beam-spot on the screen of the cathode-ray tube is usually caused by misalignments between the gun assembly and the neck of the ca'thoderay tube rather than-by misalignments in the gun assembly'itself. Thereforecentering of the spot on the screen of the cathode-ray tube may require an actual misalignment of the beam with the axis of the gun. For this reason it is desirable to locate the region in which centering occurs as far forward on the electron gun. assembly as possible. This location of the centering region minimizes the distance that the beam is displaced fromthe axis of the focusingassembly by the necessary misalignment of the beam axis. and. the gun axis introduced by thecentering means. Too great a displacement of the beam may place it in a regionof poor J focus and may cause the beam to strike elementsof the gun or the neck of the cathode tube and. thuscause shadowing of the picture on the screen. The novel shielding and centering device of .the presentinvention piaces the centering region much further forward than is possible with conventional on-the-neck centering devices.

lf the assembly 28 is fixed in position with slot 56 vertical, rotation of magnet 32 will control the position of the spot only in a vertical plane. Since the tolerance on beam position in the vertical plane maybe relaxed if the present centering device is employed,-it'-will usually be possible to achieve sufficient precision in the manufacture of the cathode-ray tube so that no horizontal centering is required. Slight misalignments of the beam in the horizontal direction may be corrected by appropriate adjustments of the horizontal linearity and width controls. If additional centering in the horizontal direction is required the entire assembly 28 may be made rotatable about the neck portion of the cathode-ray tube. If assembly 28 is made rotatable the spacing between assembly 28 and the end turns 14a of the deflection coils should be increased somewhat to compensate for the additional shunting effect which'willoccur when gap 56 is moved from its normal positidn at right angles to the low frequency deflection flux. Alternatively, any of the well known forms of within-the-yoke centering devices may be employed in conjunction with the vertical centering device of this invention to center the beam in the horizontal direction. A preferred alternative is to provide a second set of half discs (not shown) in a plane parallel to but spaced from shielding and centering assembly 28. For example, the second set of half discs may be placed on the outside of support member 30. The gap between this second set of half discs is preferably located at right angles to gap 56. This gap in the second set of discs may be bridged with a second adjustable magnet which may be moved to control the position of the beam in a .horizontal direction. For a more detailed description of a preferred form of centering assembly of this type which will control the position of the beam in two :mutually perpendicular directions reference should be :made to the copending application of Henry S. Vasilevskis, Serial No. 755,270, filed August 15, 195 8.

While the discs 40 in assembly 28 have been shown :as havinga generally circular shape, it is to be understood ithat they :may have any other convenient shape such as :square, Ehexagonal or the like. Similarly the two half discs 42 and 44 need not define exactly the area defined by the conductive disc 40. Disc 40 is preferably but :not necessarily larger than the combined area of half discs 42 and 44 so that these half discs of highly permeiable material are completely shielded from the high frequency horizontal field. The laminated assembly 28 is :shown as held together by rivets. Obviously any other :suitable fastening means may be substituted therefor. Similarly, magnet 32 may have a shape other than circular and may be supported by means other than the one shown in the drawing. For example it may comprise a simple bar magnet held in place by its own magnetism. Means such as non-magnetic shims placed between the magnet and the high permeability half discs may be employed if desired to control the strength of the magnetic field induced in the half discs. The magnet may be held in place by cement or other convenient fastening means once the desired adjustment has been achieved.

Fig. 5 illustrates one possible modification of the structure of Fig. 1, which in some instances may permit easier adjustment of the position of magnet 32. Half discs 42 and 44 are formed with ear portions 80 and 82. Magnet 32 is pivoted near the edge of half discs 40 and 42 so that it projects through an opening 84 in the cylindrical portion of housing 30. Magnet 32 may be rotated by engaging the edge thereof with a thumb or finger. The shielding and centering characteristics of the embodiment shown in Fig. 5 are similar to the corresponding characteristics of the embodiment of Fig. 1.

While the invention has been described with reference to certain preferred embodiments thereof, it will be apparent that various modifications and other embodiments thereof will occur to those skilled in the art within the scope of the invention. Accordingly we desired the scope of our invention to be limited only by the appended claims.

What is claimed is:

1. A laminated magnetic shielding and beam centering assembly for a cathode-ray tube comprising a highly conductive nonmagnetic sheet-like member and a sheet-likemember of high permeability secured thereto in a parallel, closely adjacent relationship, said high permeability mem-. ber being formed with a gap therein dividing said high permeability member into two magnetically separate portions, said laminated member being formed with an aperture therein centered on said gap and having a dimension substantially larger than the narrow dimension of said .gap in a plane of said sheet-like member, and a permanent magnet bridging said gap in a position-to establish a magnetic field between said two magnetically separate portions of said high permeability member and across said aperture.

2. A laminated magnetic shielding and beam centering assembly as recited in claim 1 wherein said magnet is circular in cross-section and magnetized along a diameter, said magnet being pivotally mounted about an axis passing substantially through the center of said crosssection and in proximity to said gap.

3. A laminated magnetic shielding and beam centering assembly as recited in claim 1, wherein the north-south axis of said magnet lies parallel to the plane of said sheet-like members, said assembly further comprising means pivotally mounting said magnet about an axis transverse to said north-south axis and transverse to said sheet-like members, said axis intersecting said sheet-like members in the region of said gap.

4. In combination with a cathode-ray tube system having magnet deflection means for creating a relatively high frequency magnetic deflection field in a first plane and a relatively low frequency magnetic field in a second plane, a laminated magnetic shielding and beam centering assembly comprising a highly conductive sheet-like member apertured to receive the neck of said cathoderay tube and disposed in adjacency with said deflection means, a high permeability sheet-like member secured to said highly conductive sheet-like member in a parallel, closely adjacent relationship on the side remote from said deflection means, said high permeability member being apertured to receive the neck of said cathode-ray tube and being further formed With a gap extending transverse to said low frequency field and intersecting said aperture, said gap and said aperture dividing said high permeability member into two magnetically separate portions, and an adjustable permanent magnet member bridging said gap, said magnet being positioned so as to establish a magnetic field between said two portions of said high permeability member and across said neck of said cathode-ray tube.

5. The combination as recited in claim 4 wherein said magnet is disposed with the north-south axis thereof parallel to the plane of said sheet-like members, said combination further comprising means pivotally mounting said magnet for rotation in the plane parallel to the plane of said sheet-like members, the axis of rotation of said magnet passing substantially through said gap.

6. In a television display system, in combination with a cathode-ray tube, a set of horizontal deflection coils energized at a relatively high frequency and providing a field in a first direction transverse to the beam axis of said cathode-ray tube and a set of vertical deflection coils energized at a relatively low frequency and providing a field in a second direction transverse to the beam axis of said cathode-ray tube, a low-loss magnetic shield and beam centering assembly comprising a highly conductive non-magnetic sheet member disposed transversely to said axis in proximity to one end of said two sets of deflection coils, said sheet member being apertured to receive the neck of said cathode-ray tube, and a pair of coplanar plates of high permeability material disposed parallel to and closely adjacent said sheet member on the side of said sheet member remote from said coils, se-

z a lectedportions of an edge ofone of said plates being disposed in spaced juxtaposition to corresponding portions of said other plate thereby to define a gap extending substantially transversely to said axis and transverse to the field set up by said vertical deflection coils, and a permanent magnet bridging said gap, said magnet being so oriented as to establish a magnetic field between said pair of coplanar plates and across the neck of said cathoderay tube. a

The combination recited in claim 6 wherein said magnet is disposed with the north-south axis thereof substantially parallel to the plane of said sheet member and means pivotally mounting said magnet about an axis transverse to said north-south axis, the pivotal axis of said magnet passing substantially through said gap.

. 8. The combination as recited in claim 7 wherein said magnet has the shape of ashort, circular cylinder, said magnet being magnetized along a diameter and pivotally mounted about the cylindrical axis.

9. The combination as recited in claim 6 in which said coplanar plates are semicircular in shape, and in which said two plates together defining a circular area with said gap disposed along a diameter thereof.

References Cited in the file of this patent UNITED STATES PATENTS 2,634,381 Kafka Apr. 7, 1953 2,717,323 Clay Sept. 6, 1955 2,761,989 Barkow Sept. 4, 1956 2,813,212 Grundmann Nov. 12, 1957 2,817,782 Over et a1. Dec. 24, 1957

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