Abstract:
A method and apparatus for improving magnetic shielding in mask-frame-assemblies. The system includes providing a cathode ray tube, a mask frame assembly disposed within the cathode ray tube and at least one magnetic shield disposed on the edge of the mask frame assembly. These magnetic shields provide shielding for the horizontal component of the terrestrial magnetic field and thus reduces distortion in the picture displayed on the tube.

Description:
[0001]    This invention generally relates to color picture tubes and, more particularly, to a method and apparatus that provides magnetic shielding in color picture tubes.  
         BACKGROUND OF THE INVENTION  
         [0002]    A color picture tube includes an electron gun for generating and directing three electron beams to a screen of the tube. The screen is located on the inner surface of the faceplate of the tube and is made up of an array of elements of three different color-emitting phosphors. A color selection electrode, also referred as a shadow mask, is interposed between the gun and the screen to permit each electron beam to strike only the phosphor elements associated with that beam. A shadow mask is a thin sheet of metal, such as steel, that is usually contoured to somewhat parallel the inner surface of the tube faceplate.  
           [0003]    The geometries of tension shadow masks typically include the following: (1) a near cylindrical profile with a near circular profile along the x-axis and a linear profile along the y-axis; or (2) a planar or near planar profile with a linear or near linear profile along the x-axis and a linear profile along the y-axis.  
           [0004]    The new generation of tensioned shadow masks has a gap on the short sides of the mask between the mask and the mask frame. The gap is the product of the tension mask being mounted to a set of termination bars that are affixed to standoffs that connect the mask to the mask frame. This gap allows a horizontal component of the earth&#39;s magnetic field to effect the propagation of the electron beam. The magnetic field causes a depreciation in performance in the horizontal component areas of the screen of the television set. The amount of magnetic field penetration is known as shielding efficiency. Therefore, improved shielding is needed to eliminate or, at least, mitigate the intrusive magnetic field to produce a commercially viable television tube.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention provides a method and apparatus for providing magnetic shielding in a tension mask of a color picture tube and in particular, a tube with a flat front profile. The apparatus comprises two magnetic shields that are affixed to the short sides of a mask frame assembly. These shields are formed of sheet metal and are rectangular in shape. Each of the shields contains a hole and several slots disposed through the material to aid in aligning and mounting the shields to a mask frame. The shields have a specific length, width, height and thickness, where the thickness of the sheet metal is very small compared to both the length, width and height of the shield.  
           [0006]    The method comprises mounting the aforementioned shields to a mask frame. A shield is placed on each of the two short sides of a mask frame. The shields cover a portion of the mask frame that runs from the top of the short sides of the mask frame to the bottom of the short sides of the mask frame. A set of retaining clips are affixed to the mask frame and guide the shield into correct alignment. The retaining clips pass through slots that are disposed in the shield and temporarily secure the shields to the mask frame assembly. A rivet is placed through a hole formed in the shield and a hole formed in the mask frame to permanently affix the shield to the mask frame. The shield is affixed such that it covers the gap produced by the tension mask being mounted to standoffs affixed to the mask frame. Each of the shields covers the gap between the mask frame and the tension mask and extend slightly above or in front of the tension mask assembly. By covering the gap formed between the tension mask and the mask frame, a magnetic field protection system is realized that provides a higher shielding efficiency for the horizontal component of the terrestrial magnetic field. This invention improves shielding efficiency upwards of 40%.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a side view, partially in the axial section, of a color picture tube, including a tension mask-frame assembly in which the mask is planar or otherwise known as true flat;  
         [0008]    [0008]FIG. 2 is a perspective view of a tension mask of the present generation of tension masks to which the invention relates;  
         [0009]    [0009]FIG. 3 is a perspective view of the magnetic shielding of the present invention;  
         [0010]    [0010]FIG. 4 is a cross-sectional view of the magnetic shield clips as attached to the magnetic shield;  
         [0011]    [0011]FIG. 5 is a cross-sectional view of the tension focus mask of FIG. 6 along line  5 - 5 ; and  
         [0012]    [0012]FIG. 6 is a perspective view of the tension mask of FIG. 2 incorporating the magnetic shielding features of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]    [0013]FIG. 1 shows a cathode ray tube  10  having a glass envelope  12  comprises a rectangular face plate panel  14  and a tubular neck  16  connected by a rectangular funnel  18 . The funnel  18  has an internal conductive coating (not shown) that extends from an anode button  20  to a neck  16 . The panel  14  comprises a viewing face plate  22  and a peripheral flange or sidewall  24  that is sealed to the funnel  18  by a glass frit  26 . A three-color phosphor screen  28  is carried by the inner surface of the face plate  22 . The screen  28  is a line screen with the phosphor lines arranged in triads, each triad including a phosphor line of each of the three colors. A mask frame assembly is removably mounted in a predetermined spaced relation to the screen  28 . An electron gun  32  (schematically shown by the dashed lines in FIG. 1) is centrally mounted within the neck  16  to generate three in-line electron beams, a center beam and two side beams, along convergent paths through the mask frame assembly  30  to the screen  28 .  
         [0014]    The tube  10  is designed to be used with an external magnetic deflection yoke, such as the yoke  34  shown in the neighborhood of the funnel to neck junction. When activated, the yoke  34  subjects the three beams to magnetic fields that cause the beams to scan horizontally and vertically in a rectangular raster over the screen  28 .  
         [0015]    The mask frame assembly  30 , shown in greater detail in FIG. 2, includes a mask frame  322  comprising two long sides  36  and  38  and two short sides  302 A and  302 B. The two long sides  36  and  38  of the mask parallel a central major axis, x, of the tube and likewise, the two short sides  302 A and  302 B parallel a central minor axis y of the tube. The mask frame assembly  30  includes a set of conductive lines. These are otherwise known as strands  44 . They parallel the central minor axis y. The strands  44  are attached to a set of terminating bars  312  which are attached to the two short sides  302 A and  302 B of the mask frame assembly  30 . In one embodiment, the strands  44  are flat strips that extend vertically, having a width of about 13 mils and a thickness of about 2 mils.  
         [0016]    [0016]FIG. 2 depicts a mask frame assembly  30  that comprises two main sub assemblies, the tension mask  308  and the mask frame  322 . The tension mask  308  is a sheet of material that has been etched to form strands  44  and is attached to a set of termination bars  312 . The tension mask  308  is first tensioned and then attached to the mask frame  322  by a set of four standoffs  314 . A noticeable gap  320  is created by the standoffs  314  between the tension mask  308  and the mask frame  322 . One of the goals of the invention is to close off this gap  320  from the effects of the terrestrial magnetic field.  
         [0017]    A set of four spring clips  310 A,  310 B,  310 C and  310 D are disposed around the outer periphery of the short sides  302 A and  302 B of the mask frame  322  to provide an engagement mechanism that attaches the mask frame  322  to the magnetic shields of the present invention (shown as  400  in FIG. 3). The four spring clips  310 A- 310 D have a general U-shape form. These spring clips  310 A- 310 D may be formed of any heat resistant resilient material such as steel or INVAR®. The spring clips  310 A- 310 D may be affixed to the mask frame  322  by spot, tack or seam welding. A small aperture  316  is disposed in the short sides  302 A and  302 B of the mask frame  322 . The aperture  316  is formed through the short sides  302 A and  302 B, respectively. The apertures are formed in a circular shape so as to accept the rivet-type fastener  616  as shown in FIG. 6.  
         [0018]    [0018]FIG. 3 is a perspective view of the magnetic shield  400  according to the present invention. The magnetic shield  400  has a front  402 , a back  404 , four sides  412 ,  418 ,  414  and  420 , a respective thickness  406 , a width  408  and respective height  424 . The magnetic shield  400  may be formed of a heat resistant fero magnetic material such as steel, aluminum or copper. Features of the magnetic shield  400  include a plurality of slots  410  disposed through the magnetic shield  400  and an aperture  416  that is also disposed through the thickness of the material. The length of the magnetic shield  400  is such that its length  424  extends the length of the short sides  302 A and  302 B of the mask frame assembly  30 . The width  408  of the magnetic shield  400  covers the gap  320  between the tension mask  308  and the mask frame  322  as well as extending above the standoff height of the tension mask  308 .  
         [0019]    The slots  410  in the magnetic shield  400  are formed to cooperate with spring clips  310 A- 310 D that are attached to the mask frame  322 . The slots  410  are designed to allow a portion  311  of the spring clips  310  to pass through the magnetic shield  400  and temporarily affix the shield  400  in place so as to align apertures  416  and  316 . FIG. 4 is a cross-sectional view of the magnetic shield clips  310 A as attached to the magnetic shield  400 . A portion  311  of spring clip  310 A protrudes through the aperture  410  of the front  402  of the magnetic shield  400 . The magnetic shield  400  is held in place by detents  313  formed in the spring clip  310 A. The aperture  416  is formed through the entire thickness  406  of the material of the magnetic shield  400 . The aperture  416  is formed to accommodate a rivet type fastener  616  shown in FIG. 6.  
         [0020]    [0020]FIG. 5 is a cross-sectional view of mask frame assembly  30  taken along line  55  as depicted in FIG. 6. FIG. 5 depicts the mask frame assembly  30  with the magnetic shield  400  installed. The magnetic shield  400  can be seen attached to the mask frame assembly  30  by spring clips  310 C and  310 D projecting through slots  410  disposed through the magnetic shield  400 . As is clearly shown, the magnetic shield  400  covers gap  320  formed between mask frame  322  and tension mask  308 . Further, magnetic shield  400  extends a distance  422  above tension mask  308 . This overhang area  422  is approximately between 0.2 and 0.5 millimeters in width. This portion  422  of the magnetic shield  400  provides a significant amount of magnetic shielding in the mask frame assembly  30  that prevents the distortive effects of terrestrial magnetic fields from reaching the phosphor screen  28  of the cathode ray tube  10 .  
         [0021]    [0021]FIG. 6 is a perspective view of a mask frame assembly  30  as seen in FIG. 2 combined with the magnetic shielding  400  of FIG. 3. As can be seen in FIG. 6, the magnetic shielding  400  has been applied to the short sides  302 A and  302 B of the mask frame assembly  30 . Clearly shown are the spring clips  310 A- 310 D extending through slots  410  disposed on magnetic shielding  400 . Also shown is a rivet  616  disposed through magnetic shielding aperture  416  and aperture  316  of the short side  302 A of the mask frame  322 . The magnetic shield  400  is mounted to the spring clips  310 A- 310 D and closes the gap between mask frame  322  and tension mask  308 . While the magnetic shield  400  is close to the tension mask  308 , the magnetic shield  400  does not come in direct contact with the tension mask  308 . The tension mask  308  does not physically touch either of the magnetic shields  400  and is electrically isolated from the mask frame  322 .  
         [0022]    As the embodiments that incorporate the teachings of the present invention have been shown and described in detail, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings without departing from the spirit of the invention.