MINERVA COLOR MF1615 CHASSIS GSC100 29301-374.01 (08) CRT TUBE TOSHIBA 420ALB22

The CRT TUBE TOSHIBA 420ALB22 is the first toshiba In-Line tube for the 16 Inches screen format. Note the convergence assy.  

 

 

 

 

 

 MINERVA  COLOR  MF1615   CHASSIS GSC100  29301-374.01 (08) CRT TUBE  TOSHIBA 420ALB22  IN-LINE TYPE TRIPLE ELECTRON GUN ASSEMBLY:

An in-line type triple electron gun assembly which comprises three electron gun members arranged in the same plane with the axis of the side gun members inclined at a predetermined angle to that of the central gun member; and support members fitted to the gun members for their integral assembly, said support member comprising two longitudinal elements each disposed between two adjacent gun members and at least one bridge element connecting said two longitudinal elements.


An in-line type triple electron gun assembly comprising: 2. An electron gun assembly according to claim 1 wherein said at least one lateral component of at least one of the support members is fused to the cathode electrodes and plane grid electrodes of electron gun members. 3. An electron gun assembly according to claim 1 wherein said at least one lateral component of at least one of the support members is fused to the cathode electrodes and plane grid electrodes of all of the electron gun members. 4. An electron gun assembly according to claim 1 wherein each of said support members is generally H-shaped and comprises two longitudinal components and a lateral component bridging said two longitudinal components. 5. An electron gun assembly according to claim 4 wherein said lateral component of at least one of the H-shaped support members is fused to the cathode electrode and plane grid electrodes of electron gun members. 6. An electron gun assembly according to claim 4 wherein said lateral component of at least one of the H-shaped support members is fused to the cathode electrode and plane grid electrodes of all of the electron gun members. 7. An electron gun assembly according to claim 4 wherein said generally H-shaped support members each include a metal support member bridging the ends of the longitudinal components thereof. 8. An electron gun assembly according to claim 5 wherein said generally H-shaped support members each include a metal support member bridging the ends of the longitudinal components thereof. 9. An electron gun assembly according to claim 1 wherein each of said support members comprises one longitudinal component and a plurality of lateral components integrally connected to said longitudinal component. 10. An electron gun assembly according to claim 9 wherein at least one of said lateral components is fused to the cathode electrodes and plane grid electrodes of electron gun members.

Description:

This invention relates to an in-line type triple electron gun assembly.

The prior art in-line type triple electron gun assembly is fabricated by fitting glass beads to the prescribed positions between two adjacent ones of three electron gun members arranged in the same plane by the aid of brackets fitted to grid electrodes constituting each electron gun member so as to fix together the three electron gun members.

With the prior art in-line type triple electron gun assembly, the glass beads are provided separately from each other, making the relative position of the gun members unstable and in consequence giving rise to their mutual displacement. Particularly, the brackets fitted to the grid electrodes constituting each gun member are bent intricately and subject to a certain degree of mechanical stress during fabrication, so that application of heat required for the assembly of gun members most likely deforms the brackets. Though very slight for the individual brackets, this deformation assumes a prominent proportion for all the brackets, resulting in the noticeable displacement of the electron gun assembly as a whole. This displacement of the constituent gun members occurs particularly in their axial direction, leads to the irregular arrangement of its grid electrodes.

It is accordingly the object of this invention to provide an electron gun assembly whose constituent members are accurately arranged to keep the assembly as a whole free from any deformation.

SUMMARY OF THE INVENTION

According to this invention, triple electron gun members each having a cathode electrode, plane grid electrode and cylindrical electrodes coaxially arranged in succession are placed side by side in the same plane with a prescribed convergence angle defined therebetween. To both sides of the electron gun assembly are fused two support members to hold the respective gun members securely in place. Each support member comprises at least one lateral or bridging component fused in common to at least the cathode electrodes and plane grid electrodes of the respective electron gun members and at least one longitudinal component intersecting said at least one lateral component at right angles and fitted in common to said electron gun members through the later described means which are fused to said electron gun members.

The present invention can be more fully understood from the following detailed description when taken in connection with the accompanying drawings, in which:

FIG. 1 is a side view of an in-line type triple electron gun assembly according to the first embodiment of this invention;

FIG. 2 is a sectional view on line 2--2 of the electron gun assembly of FIG. 1;

FIG. 3 is a perspective view of a plane grid electrode of a side electron gun member;

FIG. 4 is a perspective view of a plane grid electrode of a central electron gun member;

FIG. 5 is a side view of a triple electron gun assembly according to the second embodiment of the invention; and

FIG. 6 is a sectional view on line 6--6 of the triple electron gun assembly of FIG. 5.

As seen from FIG. 1, each electron gun member 12 of an in-line type triple electron gun assembly 11 comprises a cathode electrode 13, a first plane grid electrode 14 and second to fourth cylindrical grid electrodes 15 to 17, all coaxially arranged in succession. Said electron gun assembly 11 comprises triple electron gun members 12 each having the above-mentioned arrangement, and a magnetic convergence assembly 18 disposed ahead of the furthest end of the fourth cylindrical grid electrodes of the electron gun members 12. The juxtaposed triple electron gun members 12 are securely held between two generally H-shaped support members 19 made of, for example, glass. The cathode electrode 13 of the electron gun member 12 comprises a heater received in a generally funnel-shaped support member 20. The first plane grid electrode 14s of each side electron gun member 12 comprises, as shown in FIGS. 2 and 3, a semicircular effective plate electrode 22 bored with a hole 21s for allowing the passage of a beam of electrons and an electrode holder 23 erected on said plate electrode 22, thus presenting an L-shaped cross section as a whole. Both side grid electrodes 14 each constructed as described above are so disposed as to cause the electrode holders 23 to face each other. As shown in FIG. 4 first plane grid electrode 14c of the central electron gun member 12 comprises a channel-shaped effective grid electrode 24 bored with a hole 21c allowing the passage of a beam of electrons and electrode holders 25 projecting outward from both ends of said effective electrode 24.

The H-shaped support member 19 is so designed that when it is fused to the electron gun assembly 11, the lateral component 19b bridging the two longitudinal components 19a is so positioned as to face the first grid electrode and cathode electrode of each electron gun member 12. As shown in FIG. 1. lateral component 19b is integral with (i.e., integrally connected to) components 19a and is made of the same material as components 19a. Said H-shaped support member 19 is fitted to the electron gun assembly 11 through the support member 20 of the cathode electrodes 13, the holders 23 and 25 of the first plane grid electrodes 14s and 14c, and brackets 26 fused to the cylindrical grid electrodes 15 to 17. A metal support element 19C bridges the ends of the H-shaped support member 19.

According to the in-line type triple electron gun assembly of this invention of the aforementioned arrangement, the three electron gun members 12 are fixed in place by the H-shaped support members 19, so that the respective electron gun members 12 and the electrodes included therein can be located accurately with the support members 19 used as the base. Since the electron gun members 12 are substantially free from any deformation or distortion, a beam of electrons emitted from the electron gun members 12 can display excellent characteristics of emission and focusing simply by slightly adjusting an external magnetic field. Further, the electrodes of the electron gun members 12 are independently supported by the H-shaped support members 19, so that the brackets 26 are required to hold only the grid electrodes to which they are fused, namely, are not subject to any extra load. Therefore, the brackets 26 can be made of thin light material having a relatively small mechanical strength. This offers various advantages that the brackets 26 can be easily fabricated; work stress occurring in the brackets 26 is reduced; when the thin brackets 26 are fused to the electrodes the roundness of said electrodes is little affected; and the electron lens constituted by the electron gun members is substantially free from aberration.

The lateral component 19b of the H-shaped support member 19 is fused to the first plane grid electrode and cathode electrode of each electron gun member, thereby minimizing the deformation of the mechanically weak first plane gride electrode which would occur when the triple electron gun members are assembled and the harmful effect of displacement caused by said distortion between the axis of the first grid electrode and those of the other grid electrodes. Further, the above-mentioned lateral component 19b plays the part of elevating the overall mechanical strength of the electron gun members constituting the in-line type electron gun assembly which is mechanically weaker than the delta-shaped type.

There will now be described by reference to FIG. 5 the second embodiment of this invention. The triple electron gun members arranged in the same manner as in the first embodiment have the electrodes operated with the same potential or three juxtaposed electrodes. The three electron gun members 12 are securely held between two insulating support members 30 each formed of lateral components 31, 32, 33 and 34 fused to the respective crosswise groups 13, 14, 15, 16 and 17, each group consisting of said three juxtaposed electrodes and a longitudinal component 35 intersecting said lateral components at right angles. Lateral components 31-34 are integral with and are made of the same material as the longitudinal component 35.

The electron gun assembly of the second embodiment shown in FIG. 5 attains not only the same effect as the first embodiment but also displays the following advantage that since the lateral components support the crosswise groups each consisting of three juxtaposed grid electrodes operated with the same potential, the electrodes of the lengthwise groups operated with different potentials can be spaced relatively far from each other, thereby elevating the degree of insulation therebetween.

 MAGNETIC CONVERGENCE DEVICE FOR USE IN AN IN-LINE TOSHIBA CRT TYPE COLOR CATHODE RAY TUBE: Explanation of the Convergence Unit on Toshiba CRT Tube; A pair of E-shaped cores are mounted radially in opposite directions on the outer surface of a neck portion of an inline type color cathode ray tube. Each of the E-shaped cores has a center leg and two side legs about which dynamic convergence coils are wound in radial arrangement relative to the neck portion so as to converge electron beams from the cathode ray tubes along the longitudinal direction of the center legs of the respective cores. First and second disc-shaped permanent magnets are rotatably mounted on a cross-piece connecting the one side ends of the respective legs, and provide adjustable static fluxes cooperating with the dynamic convergence flux by travelling through the center and side legs and across the open ends thereof. 1. A magnetic convergence device for use in an in-line type color cathode ray tube comprising: 2. A magnetic convergence device as claimed in claim 1 wherein each of said E-shaped cores has a cross-piece separated into first, second and third divisions respectively connected to said center and side legs with first and second magnetic gaps provided between the adjacent ones of said divisions; and said permanent magnets are so positioned as to bridge the magnetic gaps. 3. A magnetic convergence device as claimed in claim 1 wherein said means for rotatably supporting the permanent magnets has two supporting members, each of which is pivotally mounted on said frame and has an adjusting wheel, a rod member with a rectangular head, each of said magnets having a rectangular bore for slidably receiving said rectangular head, and a spring member for resiliently pressing said magnet against said cross-piece. 4. A magnetic convergence device as claimed in claim 3 wherein the first and second adjusting wheels included in said two supporting members are positioned at different distances from the end wall of the frame. 5. A magnetic converence device as claimed in claim 4 wherein the peripheral portions of the first and second adjusting wheels overlap each other as viewed in the lengthwise direction of the convergence device. 6. A magnetic convergence device as claimed in claim 4 wherein said first and second adjusting wheels are so disposed as to prevent their peripheral portions from overlapping each other as viewed in the lengthwise direction of the convergence device. 7. A magnetic convergence device as claimed in claim 1 wherein said means for rotatably supporting the permanent magnets include supporting rods secured to said frame, said magnets having bores for rotatably receiving the free end of said supporting rods and spring members for resiliently pressing said magnets against said cross-piece. 8. A magnetic convergence device as claimed in claim 1 wherein said means for rotatably supporting the permanent magnets include spring members stretched between said frame and permanent magnets for resiliently pressing said magnets against said cross-piece. 9. A magnetic convergence device as claimed in claim 8 wherein said spring member is a compression coil spring. 10. A magnetic convergence device as claimed in claim 8 wherein said spring member is a hook-shaped leaf spring, the curved portion thereof engaging the surface of said magnets and the straight portion being secured to said frame. 11. A magnetic convergence device as claimed in claim 1 comprising four disc-shaped permanent magnets, and wherein said means for rotatably mounting said magnets includes means for rotatably mounting two of said permanent magnets on respective cross-pieces of each of said E-shaped cores.

Description:

This invention relates to a magnetic convergence device for converging the multiple electron beams of an in-line type color cathode ray tube and more particularly to a magnetic convergence device having an improved static magnetic convergence means cooperating with a dynamic convergence means. An in-line type color cathode ray tube has recently come into use as a color cathode ray tube. The in-line type tube generally has three electron guns arranged in linear relationship, and the electron beams emitted from the guns are directed to a fluorescent screen through the neck portion keeping the in-line arrangement of said electron guns. Among these beams, the center beam is usually for a green color and the two side beams are respectively for red and blue colors. To provide a clear and proper color picture over the entire area of the fluorescent screen, it is necessary for the three electron beams convergently to impinge on a given small area of the fluorescent screen. For this purpose, a dynamic convergence means and a static convergence means cooperating therewith are generally provided on the outer surface of the neck portion in connectqon with the red and blue color electron beams. Further, the red and blue color electron beams should be deflected all over the fluorescent screen so as to obtain a good beam convergence, but any of the conventional convergence devices has failed to effect proper deflection. Accordingly, it is an object of this invention to provide a magnetic convergence device for use in an in-line type color cathode ray tube capable of adjusting the convergence of multiple electron beams accurately and uniformly. SUMMARY OF THE INVENTION In accordance with this invention, the above-mentioned object can be achieved by providing a magnetic convergence device for use in an in-line tupe color cathode ray tube comprising a pair of E-shaped cores, each of said cores having a center leg, two side legs and a cross-piece connecting the ends of said center and said legs; a nonmagnetic frame having a tubular member for mounting said E-shaped cores radially in facing relationship on the outer surface of the neck portion of said cathode ray tube; dynamic convergence coils wound about said legs for generating a dynamic convergence flux therethrough; two disc-shaped permanent magnets for creating an adjustable static flux cooperating with said dynamic convergence flux by travelling through said legs and across the open ends thereof; and means for rotatably mounting said permanent magnets on said cross-piece of at least one of said E-shaped cores. The present invention can be more fully understood from the following detailed description when taken in connection with the accompanying drawings, in which: FIG. 1 is a front view of a magnetic convergence device according to an embodiment of this invention; FIG. 2 is a side view, partly in section, along line 2--2 of FIG. 1; FIG. 3 is a schematic front view of the magnetic convergence device of FIG. 1, presenting the operation thereof; FIG. 4 shows a modification of the convergence adjusting mechanism of the device of FIG. 1; FIGS. 5 to 7 show other modifications of the static convergence adjusting mechanism of the device of FIG. 1; and FIG. 8 is a fractional schematic front view of another embodiment of the invention. Referring to FIGS. 1 and 2 of the accompanying drawings, a mounting frame 10 is made of a nonmagnetic material, preferably a plastic material. The mounting frame 10 has a tubular member 11 fitted around the outer surface of the neck portion 12 of an in-line type color cathode ray tube. The three electron beams 13, 14 and 15 representing red, green and blue colors of the color cathode ray tube are aligned in line with one another and separated by magnetic shield plates 16 and 17 so as to prevent interaction between the magnetic fields applied to the respective electron 13 to 15. The beams, in particular the beams 13 and 15, are therefore independently adjustable and their convergence is controlled by magnetic assemblies 18 and 19. The assemblies 18 and 19 are positioned external to the neck portion 12 of the cathode ray tube and adjacent to the internal magnetic shield plates 16 and 17. The assemblies 18 and 19 create a flux for deflecting the respective beams 13 and 15. Each assembly, for example, assembly 18 comprises a dynamic electromagnet 20 and two disc-shaped permanent magnets 21 and 22. Since each assembly is identical in structure, description of the parts of the assembly 18 will suffice and is applicable to those of the other assembly 19 which are designated by the same numerals having a letter a suffixed thereto. The dynamic electromaget 20 has an E-shaped iron core prepared by powder metallurgy and consisting of a center leg 23, two side legs 24 and 25 and a cross-piece, the cross-piece being divided into a portion 26 connected to the center leg 23 and two other portions 27 and 28 connected to the side legs 24 and 25. Further, said cross--piece has magnetic gaps formed by nonmagnetic adhesive spacers 29 and 30 disposed in the boundaries of the adjacent ones of the aforesaid three portions 26, 27 and 28. Coils 31 and 32 are wound about the side legs 24 and 25 to create an alternting flux by alternating current passing therethrough in addition to the static fluxes of permanent magnets 21 and 22. Each dynamic electromagnet 20 is secured between side shoulders 33 longitudinally of the subject magnetic convergence device with the innermost ends of the side legs 24 and 25 tightly fitted to the inside of the top shoulders 34 so as to prevent the electromagnet 20 from unduly approaching the neck portion, that is, to allow a presecribed space therebetween. The aforementioned shoulders 33 and 34 are formed on the frame 10 so as to support the side legs 24 and 25 and also fit the innermost ends of the legs 23 to 25 around the outer surface of the neck portion 12. Between the dynamic electromagnet 20 and the end wall 36 of the frame 10 are provided permanent magnets 21 and 22 respectively supported by rotatable supporting members comprised of rods 39 and 40 and adjusting wheels 41 and 42. The rods 39 and 40 have projections 37 fitted into perforations 38 provided in the wall 36 of the frame 10. Between the adjusting wheels 41 and 42 and permanent magnets 21 and 22, there are wound compression coil springs 43 and 44 around the rods 39 and 40 to press the permanent magnets 21 and 22 against the crosspiece of the dynamic electromagnet 20. The permanent magnets 21 and 22 have bores 45 rectangular in cross section to allow the passage therethrough of the top portions of the rods 39 and 40 having a rectangular cross section similar to that of the bores 45. Thus the permanent magnets 21 and 22 are normally pressed against the dynamic electromagnet 20, and, when rotated by manually turning the adjusting wheels 41 and 22, can adjust the direction in which there is created a static flux therefrom. The permanent magnets 21 and 22 are respectively so positioned as to bridge the boundaries defined by the central portion 26 of the cross-piece 28 with the adjacent portions 27 and 28. The height of the side portions of the frame 10 gradually decreases toward the end wall 36 to facilitate the manual rotation of adjusting wheels 41 and 42. There will now be described by reference to FIG. 3 the operation of the magnetic convergence device shown in FIGS. 1 and 2. When the permanent magnet 21 generates a flux in the same direction, that is, the same polarity arrangement NS as the permanent magnet 22 as shown in the left side of FIG. 3, then the resultant compound flux F mainly passes through the paired side legs 24 and 25 by travelling across a space defined between their mutually facing open ends, causing the red color electron beam 13 to be deflected in the direction indicated by the arrow I or II. The deflecting direction I or II of the red color electron beam 13 is determined by the directions in which the compound flux F and said beam 13 are travelling. For example, when the compound flux F takes a course shown by the arrow and the red color electron beam 13 is assumed to pass from the under to the upper surface of the drawing sheet, then said beam 13 will be deflected in the direction of the arrow I. And if the beam 13 travels conversely from the upper to the under surface thereof, then the beam 13 will be deflected in the direction of the arrow II. On the other hand, when the permanent magnets 21 and 22 create fluxes in opposite polarity arrangements, for example, of SN-NS as shown in the right side of FIG. 3, then the resultant compound flux Fa passes through the center leg 23a and is thereafter divided into two portions flowing from the open end of the center leg 23a to the open ends of the side legs 24a and 25a. Further when the permanent magnets 21 and 22 create fluxes in different opposite polarity arrangements from the previous case, that is, NS-SN, then the compound flux Fa travels conversely from the open ends of the side legs 24a and 25a to the open end of the center leg 23a. Accordingly, the blue color electron beam 15 is deflected by the compound flux Fa in the direction shown by the arrow III or IV. The deflecting direction of said beam 15 is determined similarly in accordance with the directions in which the compound flux Fa and the blue color electron beam 15 are travelling. The peripheral portions of the adjusting wheels 41 and 42 of FIG. 1 are separated as viewed in the crosswise direction of the magnetic convergence device but overlap each other as viewed in the lengthwise direction of said device. However as illustrated in FIG. 4, to facilitate the manual rotation of the wheels 41 and 42, they may be so disposed as to have the peripheral portions thereof separated as viewed in the crosswise direction of the magnetic convergence device but prevented from overlapping each other as viewed in the lengthwise dirction thereof as in the previous case. FIGS. 5 to 7 show the modifications of the static convergence adjusting mechanism of the magnetic convergence device of FIG. 1. The static convergence adjusting mechanism of FIG. 5 comprises a supporting rod 50 fixed to the end wall 36, the free end of the rod 50 rotatably supporting the permanent magnet 21b by being received in a bore 51 provided therein, and a compression coil spring 52 for resiliently pressing the magnet 21b against the cross-piece of the electromagnet 20. The static convergence adjusting mechanism of FIG. 6 comprises a compression coil spring 53 stretched between the end wall 36 and permanent magnet 21 for resiliently pressing the magnet 21 against the cross-piece of the electromagnet 20 so as to permit the rotation of said magnet 21. The static convergence adjusting mechanism of FIG. 7 comprises a hook-shaped leaf spring 54, the curved portion thereof engaging the surface of the magnet 21 so as to permit its rotation and the straight portion thereof being secured to the end wall 36. In another embodiment of the invention of FIG. 8, an integrally formed E-shaped core 55 has side legs 56 and 57, a center leg 58 and a cross-piece 59 connecting the ends of the legs 56 to 58. Dynamic coils 60, 61 and 62 are wound about the respective legs 56 to 58. The other elements of the embodiment of FIG. 8 are operated in the same manner as those of FIG. 1 and description thereof is omitted. The foreggoing description relates to the case where the three legs had such lengths as permitted their close abutment against the periphery of the neck portion of the color cathode ray tube, with their end faces varied accordingly. However, this invention is also applicable even where the center and paired side legs constituting the E-shaped core have substantially the same length and cross section.

TOSHIBA MAGNETIC CONVERGENCE DEVICE FOR USE IN AN IN-LINE TYPE COLOR CATHODE RAY TUBE A mounting plate having first and second arcuate slits is mounted on the neck portion of an in-line type color cathode ray tube. On both surfaces of the mounting plate are slidably disposed first and second arcuate racks and first and second magnetic convergence units each having an E-shaped convergence core, by means of screws passing through the arcuate slits. First and second pinions geared to the arcuate racks are fixed to one end of first and second shafts rotatably fitted to the mounting plate, the other end of the shafts being fixed to the first and second adjusting wheels. 1. A magnetic convergence device for use in an in-line type color cathode ray tube comprising a nonmagnetic mounting plate having first and second arcuate slits and a plurality of split pieces being clamped by a clamping means so as to fix said mounting plate on the outer surface of the neck portion of said cathode ray tube; first and second magnetic convergence units positioned on one surface of said mounting plate; and means for adjustably fitting said magnetic convergence units to permit their displacement along said arcuate slits, each of said magnetic convergence units having a nonmagnetic frame, a dynamic convergence electromagnet including an E-shaped core and two disc-shaped permanent magnets associated with said electromagnet. 2. A magnetic convergence device as claimed in claim 1 wherein said means for adjustably fitting said magnetic convergence units comprise first and second arcuate racks positioned on the other surface of said mounting plate; connecting means for connecting a pair of arcuate racks to a pair of magnetic convergence units through said arcuate slits; first and second pinions geared to said arcuate racks and fixed to one end of first and second shafts rotatably disposed on said mounting plate; and first and second adjusting wheels fixed to the other end of said shafts. 3. A magnetic convergence device as claimed in claim 1 wherein said means for adjustably fitting said magnetic convergence units comprises a plurality of screws for fixing said magnetic convergence units to said mounting plate through said arcuate slits, the diameter of the head of the screw being larger than the width of the arcuate slit. 4. A magnetic convergence device as claimed in claim 2 wherein said adjusting wheel is graduated on the periphery to indicate the rotating position of said magnetic convergence units on said mounting plate.

Description:

This invention relates to a magnetic convergence device for use in an in-line type color cathode ray tube and more particularly to a magnetic convergence device having first and second individually and mechanically adjustable magnetic convergence units. An in-line type color cathode ray tube has recently come into use as a color cathode ray tube. The in-line type tube generally has three electron guns arranged in linear relationship, and the electron beams emitted from the guns are directed to a fluorescent screen through the neck portion of said tube maintaining the in-line arrangement of said electron guns. Among these beams, the central beam is usually for a green color and the two side beams are for red and blue colors. To present a clear and proper color picture over the entire area of the screen, it is necessary for the three electron beams convergently to impinge on a given small area of the screen. For this purpose, a dynamic convergence means and a static convergence means cooperating therewith are generally provided on the outer surface of the neck portion in connection with the red and blue color electron beams so as to electrically and magnetically adjust their directions. Further, the red and blue color electron beams should be deflected individually all over the fluorescent screen so as to obtain a good beam convergence, but none of the conventional convergence devices has succeeded in effecting proper deflection. Accordingly, it is an object of this invention to provide a magnetic convergence device for use in an in-line type color cathode ray tube capable of adjusting the convergence of multiple electron beams accurately and uniformly. In accordance with this invention, the above-mentioned object can be achieved by providing a magnetic convergence device for use in an in-line type color cathode ray tube comprising a nonmagnetic mounting plate having first and second arcuate slits and a plurality of split pieces being clamped by a clamping means so as to fix said mounting plate on the outer surface of the neck portion of said cathode ray tube; first and second magnetic convergence units positioned on one surface of said mounting plate; means for adjustably fitting said magnetic convergence units to permit their displacement along said arcuate slits, each of said magnetic convergence units having a nonmagnetic frame, a dynamic convergence electromagnet including an E-shaped core and two disc-shaped permanent magnets associated with the electromagnet. The present invention can be more fully understood from the following detailed description when taken in connection with the accompanying drawings, in which: FIG. 1 is a side view of a magnetic convergence device mounted on the outer surface of the neck portion of an in-line type color cathode ray tube according to an embodiment of this invention; FIG. 2 is a plan view taken along line II--II of FIG. 1; FIG. 3 is a plan view taken along line III--III of FIG. 1; and FIG. 4 is a plan view of another embodiment of the invention. Referring to FIGS. 1 to 3 of the accompanying drawings, there is disposed a disc-shaped nonmagnetic mounting plate 3 on the outer surface of a neck portion 1 of an in-line type color cathode ray tube 2. The mounting plate 3 has a bore 4 provided at the center into which the neck portion 1 is inserted, and a plurality of, for example, six split pieces 5 formed integrally with the mounting plate 3 and extending from the inner surface of the bore 4. There is provided a clamping means consisting of a clamping band 6 and a bolt 7 for clamping the split pieces 5 to the neck portion 1. First and second arcuate slits 8 are formed in the mounting plate 3 in concentric relationship with the bore 4. On one surface of the mounting plate 3 facing an electron gun assembly (not shown) disposed at one end part of the neck portion 1 are positioned first and second arcuate racks 9, which have a curvature corresponding to that of the racks, 8 and a width slightly larger than that of the slits 9. On the other surface of the mounting plate 3 facing a fluorescent screen (not shown) of the cathode ray tube 2 are positioned first and second magnetic convergence units 10, which are connected to the arcuate racks 9 by means of screws 11 passing through the arcuate slits 8. Thus, the arcuate racks 9 and magnetic convergence units 10 are so fitted to the mounting plate 3 as to slide through arcuate slits 8. Each of the magnetic convergence units 10 comprises a nonmagnetic rectangular frame 12 having a bottom plate 13 connected to the arcuate rack 9, a dynamic convergence electromagnet including an E-shaped core 14 and dynamic convergence coils 15 wound thereon, and two disc-shaped permanent magnets 16 positioned between the cross-piece of the E-shaped core 14 and frame 12. The magnets 16 are pressed against the cross-piece of the core 14 by compression coil springs 17. The electron gun assembly of the in-line type color cathode ray tube 2 has three linearly arranged electron guns and consequently the emitted three electron beams 18, 19 and 20 respectively corresponding to red, green and blue colors are also linearly disposed and further separated by magnetic shield plates 21 and 22 so as to prevent interaction between the magnetic fields applied to the electron beams 18 to 20. The beams, in particular the beams 18 and 20, are therefore independently adjustable and their convergence can be controlled by the magnetic convergence units 10, which create dynamic and static fluxes from the free ends of the E-shaped cores 14 for deflecting the beams 18 and 20. First and second pinion 23 are geared to the arcuate racks 9. Each of the pinions 23 is fixed to one end of a shaft 24 rotatably mounted on a holder 25 formed on the surface of the mounting plate 3. At the other end of the shaft 24 is provided an adjusting wheel 26 for rotating the pinion 23. The peripheral portion of one surface of the adjusting wheels 26 is provided with a scale 27 for indicating the rotating position of the magnetic convergence units 10 on the mounting plate 3. In FIGS. 1 to 3, three electron beams 18 to 20 emitted from the linearly arranged three electron guns travel through the neck portion 1 and are deflected in the deflection device 28 mounted on the outer surface of the funnel portion 29 of the color cathode ray tube 2 so as to convergently impinge on the fluorescent screen. In the neck portion 1 the red and blue beams 18 and 20 are magnetically subjected to deflection by the magnetic convergence units. The magnetic convergence is achieved using a dynamic convergence flux obtained by applying an alternating current to the coils 15 and a static convergence flux obtained by adjusting the arrangement of the magnetic poles of the two disc-shaped permanent magnets 16. According to this embodiment, the magnetic convergence is further achieved by mechanically displacing the magnetic convergence units 10 along the arcuate slits 8 through rotation of the adjusting wheels 26. In the magnetic convergence device of FIG. 4, the magnetic convergence units 10 are fitted to the mounting plate 3 by screws 30 passing through the slits 8. The diameter of the screw head is larger than the width of the slit 8 and the diameter of the threaded portion of the screw 30 passing through the slit 8 is slightly smaller than the width of the slit 8. Therefore, when all of the screws 30 are loosened, the magnetic convergence units 10 can be easily displaced along the arcuate slits 8 so as to precisely deflect the electron beams 18 and 20. The other elements of the embodiment of FIG. 4 are operated in the same manner as those of FIG. 1 and description thereof is omitted.