Abstract:
Means for aligning the electron beam of an electron beam in a cathode ray tube so as to adjust the alignment and which includes a rear bobbin portion surrounding the cathode ray tube and which is mounted at least one cylindrical shaped magnet that is magnetized in the direction of a diameter of the cylinder and which can be rotated so as to correct for misalignment in the tube. In one embodiment, two rotatable magnets mounted at right angles relative to each other are utilized and in the second embodiment a single magnet can be utilized and in which the supporting bobbin in which the magnet is mounted can be rotated by 180° is provided.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to alignment apparatus for an electron beam in a cathode ray tube for externally adjusting the alignment of the electron beam. 
     2. Description of the Prior Art 
     Conventional alignment apparatus for electron beams of cathode ray tubes utilize two rotatable permanent magnet rings which surround the outer portion of the tube where the correction is desired. By adjusting the angular position of these rotatable magnetic rings, the magnitude and direction of the magnetic field for the electron beam in a cathode ray tube can be controlled and the alignment of the electron beam can be made. In the prior art devices, a magnetic shield plate does not generally surround the outside of the portion where the adjustment of the electron beam occurs. Thus, the electron beam is subjected to the influence of an external or environmental magnetic field. So as to avoid the influence of such external magnetic field, a magnetic shield plate may cover the outside of the two magnetic rings. However, in this structure, the means for rotating the magnetic rings relative to each other becomes very complicated. 
     A second conventional alignment apparatus for electron beam of the prior art comprises an alignment correction coil which surrounds the outside of the electron gun where the beam alignment correction is required. Orthogonal electromagnetic fields generated by the alignment correction coil correct the alignment of the electron beam. 
     A magnetic shield plate extends over the alignment correction coil to prevent the influence of external and environmental magnetic shields. In this structure, however, there must be applied a constant supply of current for generating the orthogonal electromagnetic field and the structure becomes complicated and expensive and high power is consumed on a continuous basis. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an alignment apparatus for the electron beam in a cathode ray tube which avoids the difficulties of the prior art. Another object of the invention is to provide an alignment apparatus for an electron beam which is simple in construction. 
     Another object is to provide an alignment apparatus for an electron beam which is subject to undesirable external and environmental magnetic fields. 
     A feature of the invention comprises mounting one or two cylindrical magnets in the magnetic deflection region of a cathode ray tube and wherein the magnets can be rotated so as to compensate the electron beam of the cathode ray tube. 
     Other objects, features and advantages of the invention will be readily apparent from the following description of certain preferred embodiments thereof taken in conjunction with the accompanying drawings although variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure, and in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a prior art alignment adjustment apparatus for an electron beam tube; 
     FIG. 2 is a cross-sectional view of another prior art alignment adjustment apparatus for an electron beam tube; 
     FIG. 3 is a cross-sectional view of an embodiment of an alignment apparatus for an electron beam tube according to the invention; 
     FIG. 4 is a rear plan view of the embodiment illustrated in FIG. 3; 
     FIG. 5A is a isometric view of the compensating magnet according to the invention; 
     FIG. 5B is an end view of the compensating magnet according to the invention; 
     FIG. 6 is a cross-sectional view of the magnetic holder and magnet according to the invention; 
     FIG. 7 is a plot of the relationship between the magnet and the magnetic field plate for explaining the invention; and 
     FIG. 8 is a graph illustrating the magnetic field magnitude and direction which varies as a function of the rotation of the magnet. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a prior art compensating device for the electron beam of a pickup tube 1, which has a cathode 2 which generates an electron beam which passes through grids 3 and 4 for accelerating the electron beam. A bobbin 5 surrounds the pickup tube 1 and a deflection coil 6 and a focusing coil 7 are mounted on the bobbin. A magnetic shield plate of generally cylindrical shape 8 which may be made of permalloy surrounds the deflection focusing coils 6 and 7 as shown. Alignment adjustment apparatus for the electron beam in the pickup tube 1 comprises two permanent magnetic rings 9a and 9b which are mounted adjacent each other and which are rotatably mounted relative to the pickup tube and relative to each other and surround the outside portion of the pickup tube 1 where the beam alignment correction is required. This corresponds to the portion of the tube between the grid 3 and the grid 4. By rotating the magnetic rings 9a and 9b the magnitude and direction of the magnetic field for controlling the path of the electron beam in the pickup tube 1 can be controlled and the beam alignment can be adjusted. 
     In this structure, since the two magnetic rings 9a and 9b are attached to the bobbin 5 and are mounted to be rotatable relative to the tube, the magnetic shield plate 8 must generally terminate before the rings 9a and 9b and thus the shield does not cover or surround the portion of the pickup tube where the alignment of the electron beam is accomplished. For this reason, the portion where the electron beam is not shielded by the shield 8 is subjected to the influences of magnetic fields which can comprise external and environmental magnetic fields caused by leakage magnetic flux or terrestrial magnetization. The electron beam in a pickup tube 1 is always subject to the influence of such external magnetic fields. So as to avoid the influence of the external magnetic fields, a magnetic shield plate 8 can be extended to surround or cover the two magnetic rings 9a and 9b. However, this results in the structure for rotating the magnetic rings 9a and 9b relative to each other being very complicated. 
     FIG. 2 illustrates another prior art device wherein instead of the magnetic rings 9a and 9b an alignment correction coil 10 which surrounds the outer portion where the beam correction alignment is provided at the portion between the grids 3 and 4. The alignment correction coil 10 must be supplied with a suitable current to generate orthogonal electromagnetic field to adjust the alignment of the electron beam in the pickup tube 1 and in this case the magnetic shield 8 extends so that it covers the alignment correction coil 10 so as to eliminate the influence of external magnetic fields. However, in the prior art device of FIG. 2, constant supply of current for alignment correction must be supplied to the coil 10 to generate the orthogonal electromagnetic field. This makes the structure complicated, expensive and requires the consumption of high power on a continuous basis. 
     FIGS. 3 and 4 illustrate one embodiment of an alignment adjustment apparatus for electron beam according to the invention. 
     The cathode 102 produces an electron beam which passes through the grid electrodes 103 and 104 of the pickup tube 101. A main bobbin 105 surrounds the pickup tube 1 and carries deflection and focusing coils 106 and 107 and a cylindrical shield 108 covers the bobbin 105. 
     Adjacent the portion which surrounds the electrodes 103 and 104, a relatively thick rear bobbin 105a is attached to the main bobbin 105. The rear bobbin 105a surrounds the outside of the pickup tube 101 where the adjustment correction is required, that is the portion between the grids 103 and 104 in the pickup tube 101. The magnetic shield plate 108 made of permalloy extends over the deflection coil 106, the focusing coil 105 and that portion of the rear bobbin 5a which surrounds the adjustment region of the pickup tube 101. 
     A pair of cylindrical holes 111a and 111b are formed in the rear bobbin 105a and their axis extend parallel to the axis of the pickup tube 101 from the rear end surface of the bobbin 105a to the region where beam alignment correction is required, that is the region which is radially offset from the space between the grids 103 and 104. 
     As illustrated in FIG. 4, the openings 111a and 111b are formed to lie on axes which are at right angles to each other relative to the center of the pickup tube and assuming that the center of the pickup tube is the origin of the X-Y coordinate system thus formed, the holes 111a and 111b are formed on the X and Y axes, respectively. A pair of cylindrical shape magnets 112 are magnetized diametrically as illustrated in FIGS. 5A and 5B. The magnets 112 may be made of ferrite and are inserted into the holes 111a and 111b so that they may be rotated about their axes. 
     FIGS. 5A and 5B illustrate the perspective and rear view of the cylindrical shape magnets 112 according to the invention. Each of the magnets 112 are mounted in cylindrical shaped holders 113 which are formed with a cavity for receiving the magnets 112 as illustrated in FIG. 6 and are formed with a threaded external portion 113a at their outer end and a slot 113b so as to allow the holder 113 to be rotated with a screw driver so as to rotate the magnet 112. The outer ends of the openings 111a and 111b are also threaded so as to mate with the threads 113a of the holders 113 and the holders 113 and the magnets 112 are threadedly received in the openings 111a and 111b. 
     In use, the beam from the cathode 102 is aligned by inserting a screw driver into the slots 113b to rotate the holder 113 and the magnet 112 until the beam is properly aligned. 
     FIG. 7 explains the principle of the alignment adjustment for the electron beam. The cylindrical magnet 112 is mounted within the inside of the tubular magnetic shield plate 108 of permalloy and FIG. 7 illustrates the magnet 112 mounted relative to the origin of the X-Y axis which coincides with the center axial line of the pickup tube 101 and in FIG. 7 the magnet 112 has its center A mounted on the Y axis of the X-Y coordinate system. As the cylindrical shaped magnet 112 is rotated around its axis with a screw driver which is inserted into the slot 113b the direction and magnitude of the magnetic field at the origin can be changed which will compensate for the extraneous magnetic fields effecting the beam of the pickup tube. In a particular embodiment, the radius of the tubular magnetic shield plate 108 was 16 mm, the distance between the origin O and the axis A of the cylindrical magnet 112 was 13.4 mm, the length of the cylindrical magnet 112 was 3 mm and the diameter of the cylindrical magnet 112 was 3 mm. With this structure, the magnitude of the field of the cylindrical magnet 112 was 2 gauss at a point 13 mm perpendicular to the polarized direction of the magnet. As the cylindrical magnet 112 is rotated about its axis, the direction and magnitude of the magnetic field at the origin O will vary as illustrated in FIG. 8 where the field strength in gauss is plotted against the angle θ which is the angle of rotation of the magnet 112 as illustrated in FIG. 7. 
     As shown in FIG. 8, the magnitude of the magnetic field changes substantially on the Y axis as a function of the rotating angle θ of the magnet 112. Thus, when two separate magnets 112 are respectively placed on the X and Y axis as illustrated in FIG. 4, the direction and magnitude of the magnetic field at the origin O can be changed in an optimum fashion by rotating each of the magnets 112 around its axis. Thus, an adjustment for the electron beam can be made by rotating the cylindrical magnets 112 as illustrated in FIG. 3. 
     Also, in the present invention, since the magnetic shield plate 108 substantially covers the portion of the tube 101 where the beam alignment correction is required, the electron beam of the pickup tube 101 will not be influenced or disturbed by external magnetic fields. 
     Thus, according to the invention the mounting of one or more cylindrical magnets between the outside of the pickup tube and the inside of the tubular magnetic shield plate allows alignment adjustments to be made by rotating the magnets around their axes. 
     Also, since the magnetic shield plate 108 covers the outside of that portion of the tube where the beam alignment correction is required which comprises the space between the grids 103 and 104 of the pickup tube, the electron beam will not be subjected to external magnetic fields. 
     Instead of installing two cylindrical shaped magnets, the rear bobbin may be provided with only a single cylindrical magnet if the rear bobbin 105a is mounted relative to the main bobbin 105 so that it can be rotated through an angle of 180° around the tubular axis of the pickup tube. When the required alignment direction is known before mounting the rear bobbin 105a, it would be attached to the main bobbin 105 at the desired position which is known beforehand in the case of tubes utilizing magnetic focusing and static deflection which are designated as mixed field type pickup tubes. In such arrangements, a single cylindrical magnet 112 with a holder 113 can be installed so that the cylindrical axis of the magnet 112 is aligned with the central axis of the pickup tube 101. 
     Although the invention has been described with respect to a pickup tube, it is to be realized that it can be utilized in other cathode ray tubes such as storage tubes. Although the shield plate 108 has been described as being made of permalloy, it can of course be made of other suitable magnetic shielding materials. 
     Although the invention has been described with respect to preferred embodiments, it is not to be so limited as changes and modifications may be made which are within the full intended of the invention as defined by the appended claims.