Abstract:
A color picture tube has a tensioned mask supported by a support frame mounted within the tube. The mask has a significantly lower coefficient of thermal expansion than the frame. The mask has an active apertured portion formed by a plurality of parallel vertically extending strands, through which electron beams pass during operation of the tube, and two opposite side border portions outside the active apertured portion. The two opposite side border portions have tie bars that extend between the vertical strands of the mask. The tie bars accommodate expansion of the frame, while substantially maintaining the positions of the vertical strands in the active portion of the mask.

Description:
This invention relates to color picture tubes having tension masks, and particularly to a tube having means for connecting a tension mask, that is made of a material having a relatively low coefficient of thermal expansion material, to a support frame, that has a significantly higher coefficient of thermal expansion. 
     BACKGROUND OF THE INVENTION 
     A color picture tube includes an electron gun for generating and directing three electron beams to the screen of the tube. The screen is located on the inner surface of a faceplate of the tube and is made up of an array of elements of three different color emitting phosphors. A color selection electrode, which may be either a shadow mask or a focus 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. 
     One type of color picture tube has a tensioned shadow mask mounted within a faceplate panel thereof. In order to maintain the tension on the mask, the mask must be attached to a relatively massive support frame. Although such tubes have found wide consumer acceptance, there is still a need for further improvement in tube types to reduce the weight and cost of the mask-frame assemblies in such tubes. 
     It has been suggested that a lighter frame could be used in a tension mask tube if the required tension on a mask is reduced. One way to reduce the required mask tension is to make the mask from a material having a low coefficient of thermal expansion. However, a mask from such material would require a support frame of a material having a similar coefficient of thermal expansion to prevent any mismatch of expansions during thermal processing that is required for tube manufacturing, and during tube operation. Because the metal materials that have low coefficients of thermal expansion are relatively expensive, it is relatively costly to make both the mask and frame out of identical or similar materials. Therefore, it is desirable to use the combination of a low expansion tension mask with a higher expansion support frame. The present invention provides a solution to the problem that exists when there is a substantial mismatch in coefficients of thermal expansion between a tension mask and its support frame. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improvement in a color picture tube having a tension mask supported by a support frame mounted within the tube. The tension mask has a significantly lower coefficient of thermal expansion than that of the frame. The mask includes an active apertured portion formed by a plurality of parallel vertically extending strands, between which are elongated apertures through which electron beams pass during operation of the tube. Two opposite side border portions, outside the active apertured portion, have tie bars that extend between the vertical strands of the mask. The tie bars accommodate expansion of the frame, while substantially maintaining the positions of the vertical strands in the active portion of the mask. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view, partly in axial section, of a color picture tube embodying the invention. 
     FIG. 2 is a front view of a tension shadow mask. 
     FIG. 3 is a front view of a small section of a border portion of the mask of FIG.  2 . 
     FIG. 4 is a perspective view of a corner of a tension shadow mask-frame assembly. 
     FIGS. 5 through 11 are front views of small sections of six different alternative embodiments of tension mask border portions. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a color picture tube  10  having a glass envelope  11  comprising a rectangular faceplate panel  12  and a tubular neck  14  connected by a rectangular funnel  15 . The funnel  15  has an internal conductive coating (not shown) that extends from an anode button  16  to the wide portion of the funnel and to the neck  14 . The panel  12  comprises a substantially flat external viewing faceplate  18  and a peripheral flange or sidewall  20 , which is sealed to the funnel  15  by a glass frit  17 . A three-color phosphor screen  22  is carried by the inner surface of the faceplate  18 . The screen  22  is a line screen with the phosphor lines arranged in triads, each triad including a phosphor line of each of the three colors. A color selection tension shadow mask  24  is removably mounted in predetermined spaced relation to the screen  22 . An electron gun  26 , shown schematically by dashed lines in FIG. 1, is centrally mounted within the neck  14  to generate and direct three inline electron beams, a center beam and two side beams, along convergent paths through the mask  24  to the screen  22 . 
     The tube  10  is designed to be used with an external magnetic deflection yoke, such as the yoke  30  shown in the neighborhood of the funnel-to-neck junction. When activated, the yoke  30  subjects the three beams to magnetic fields which cause the beams to scan horizontally and vertically in a rectangular raster over the screen  22 . 
     The tension shadow mask  24 , shown in FIGS. 2 and 3, includes two long sides  32  and  34 , and two short sides  36  and  38 . The two long sides  32  and  34  of the mask parallel a central major axis, X, of the mask; and the two short sides  36  and  38  parallel a central minor axis, Y, of the mask. The tension shadow mask  24  includes an active apertured portion  40  that contains a plurality of parallel vertically extending strands  42 . A multiplicity of elongated apertures  44 , between the strands  42 , parallel the minor axis Y of the mask. The electron beams pass through the apertures  44  in the active portion  40  during tube operation. Each aperture  44  extends continuously from a border portion  46  at a long side  32  of the mask to another border portion  48  at the opposite long side  34 . 
     A frame  50 , for use with the tension shadow mask  24  is partially shown in FIG.  4 . The frame  50  includes four sides: two long sides  52 , substantially paralleling the major axis X of the tube, and two short sides  54 , paralleling the minor axis Y of the tube. Each of the two long sides  52  includes a rigid section  56  and a compliant section  58  cantilevered from the rigid section. The rigid sections  56  are hollow tubes, and the compliant sections  58  are metal plates. Each of the short sides  54  has an L-shaped cross-section upper portion  60  parallel to and separated from a flat bar-shaped lower portion  62 . The two long sides  32  and  34  of the tension mask  24  are welded to the distal ends of the compliant sections  58 . Although the present invention is described in embodiments using the frame  50 , it is to be understood that many other types of tension frames could also be used with the present invention. 
     The strands  42  are continued from the active portion  40  into the border portions  46  and  48 , where they are connected by tie bars  49 , which interrupt the continuation of the apertures  44 , as shown in FIGS. 3 through 10. The tie bars  49  in adjacent columns are vertically offset from each other, so that no two tie bars in adjacent columns have horizontally aligned centerlines. The strands  42  also extend beyond the border portions  46  and  48 , where they are individually welded to the frame. 
     The purpose of the tie bars  49  in the border portions  46  and  48  of the mask  24  is two-fold. First, the tie bars  49  accommodate undesirable strand positioning errors that occur when individual strands are welded to the compliant section  58 . Such individual strand attachment is required to avoid inelastic deformations that would be produced during thermal processing of mask-frame systems, wherein the mask and frame have considerably different coefficients of thermal expansion and the mask has a solid border in the weld zone, as is known in the art. When a low-thermal expansion mask with a solid border is affixed to a high-thermal expansion compliant section  58 , thermal processing of the tube, which can reach temperatures as high as 450° C., can cause the mask to be inelastically stretched in the solid border region, and upon cool-down the mask wrinkles. In the absence of a solid border region, tie bars  49  assure that the desired strand spacing is maintained during welding. 
     The second purpose for the tie bars  49  is to accommodate the greater expansion of a high expansion frame  50  compared to that of a low expansion mask  24 , without causing appreciable relocation of the mask strands  42  through permanent deformation of the mask border portion. The tie bars  49 , together with the strand sections they interconnect, generally achieve this result by elastically stretching near the active portion of the mask. A key objective of all border treatments disclosed herein, wherein masks and frames are constructed of dissimilar materials, is to provide strand-to-strand spacing means when welding individual strands or small groups of strands, such that the mask can withstand the customary thermal processing of the tube without the formation of inelastic deformations that would result in errors in strand-to-strand openings. 
     Other embodiments of mask borders having different tie bar patterns are shown in FIGS. 5 through 11. In a mask  64 , shown in FIG. 5, tie bars are omitted near the compliant section  58 . In a mask  66 , shown in FIG. 6, the tie bar positions are repeated every third column instead of every second column, as is done in the pattern of FIG.  3 . FIG. 7 shows a mask  68 , wherein the spacing between tie bars is increased. FIG. 8 shows a mask  70 , wherein the repeat distance between tie bars is varied between adjacent columns. FIG. 9 shows a mask  72 , wherein a large tie bar is inserted in every other column. FIG. 10 shows a mask  74 , wherein every fourth column includes a large tie bar and the three intermediate columns do not have any tie bars near the compliant section  58 . FIG. 11 shows a mask  76  having a border portion similar to that shown in the mask  24  of FIG. 3, but an active portion  40  having widely spaced tie bars  78  connecting the strands therein. Alternatively, the vertical spacing between the tie bars  78  in the active portion  40  could be the same as the vertical spacing between tie bars in the border portion  46 . Also possible are additional mask embodiments, which have border portions that include tie bars. Preferably, the centerlines of tie bars in adjacent apertures are vertically offset. 
     In different embodiments, the vertical spacings between tie bars are in the range of 2.03 mm (80 mils) to as much as 76.96 mm (3030 mils). However, vertical spacings of tie bars of 2.54 mm (100 mils) to 4.06 mm (160 mils) are preferred. Generally, tie bar thickness of about 0.38 mm (15 mils) is preferred, although thicknesses of 1.02 mm (40 mils) are used in some embodiments. Preferably, the mask is made from Invar material that is 0.10 mm (4.0 mils) thick, and the frame is made of AK steel. 
     All known commercially used tension shadow mask tubes have had solid border portions at the mask-to-frame weld points. This was acceptable when the mask and frame were made from similar expanding materials. However, when a mask and frame differ greatly in coefficients of thermal expansion, such solid border portions will deform, thereby permanently deforming the active portion of the mask during thermal processing of the tube. Individual attachment of the mask strands to the frame, in combination with the novel border portion of the present invention, prevents substantial distortion in the active portion of the mask by providing a “mechanical filter” that accommodates any individual strand attachment error or movement during processing or tube operation.