Patent Publication Number: US-6700319-B2

Title: Cathode-ray tube having a tension mask with microphonics control

Description:
FIELD OF THE INVENTION 
     This invention relates generally to cathode-ray tubes having tension mask assemblies and, more particularly, to tension mask assemblies having vibration damping means. 
     BACKGROUND OF THE INVENTION 
     A color cathode ray tube, or CRT, includes an electron gun for forming and directing three electron beams to a screen of the tube. The screen is located on the inner surface of the faceplate panel of the tube and is made up of an array of elements of three different color-emitting phosphors. A shadow mask, which may be either a formed mask or a tension mask having strands, is located between the electron gun and the screen. The electron beams emitted from the electron gun pass through apertures in the shadow mask and strike the screen causing the phosphors to emit light so that an image is displayed on the viewing surface of the faceplate panel. 
     One type of CRT has a tension mask comprising a set of strands that are tensioned onto a mask support frame to reduce their propensity to vibrate at large amplitudes under external excitation. Such vibrations would cause gross electron beam misregister on the screen and would result in objectionable image anomalies to the viewer of the CRT. 
     Vibrations causing gross electron beam misregister may be created by microphonic sources within the TV set. Although it is difficult to control the amount of vibration generated by these microphonic sources, it is necessary to have the tension mask within the CRT be adaptable to this environment in which such microphonic vibrations exist. While tensioning the mask strands reduces electron beam misregister, further reductions are necessary. It is therefore desirable to develop a mask frame assembly having vibration damping characteristics for further minimizing the undesirable result of electron beam misregister on the screen caused by vibration sources within the TV set. 
     SUMMARY OF THE INVENTION 
     A cathode-ray tube having a luminescent screen on a faceplate panel and a tension mask assembly, wherein the assembly comprises a pair of support blades for supporting a tension mask. The tension mask has a pair of mask borders each being fixed to a respective support blade and a plurality of strands extending between the mask borders. The mask further comprises cross wires extending generally perpendicular to the strands on a screen facing side and at least one shield assembly extending between and attached to the mask borders. The shield assembly has a sheath extending along one side of the mask in contact with at least one strand and a shield attached to the cross wires of the mask in overlapping relation to the sheath. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of example with reference to the accompanying figures of which: 
     FIG. 1 is a cross sectional view of a CRT showing a tension mask frame assembly. 
     FIG. 2 is a perspective view of the tension mask frame assembly. 
     FIG. 3 is a partial perspective view of the corner section shown in FIG. 2 having a damping mechanism according to the invention. 
     FIG. 4 is a cross sectional view of the damping mechanism. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a cathode ray tube (CRT)  1  having a glass envelope  2  comprising a rectangular faceplate panel  3  and a tubular neck  4  connected by a funnel  5 . The funnel  5  has an internal conductive coating (not shown) that extends from an anode button  6  toward the faceplate panel  3  and to the neck  4 . The faceplate panel  3  comprises a viewing faceplate  8  and a peripheral flange or sidewall  9 , which is sealed to the funnel  5  by a glass frit  7 . A three-color phosphor screen  12  is carried by the inner surface of the faceplate panel  3 . The screen  12  is a line screen with the phosphor lines arranged in triads, each of the triads including a phosphor line of each of the three colors. A tension mask support frame assembly  10  is removably mounted in predetermined spaced relation to the screen  12 . An electron gun  13 , shown schematically by dashed lines in FIG. 1, is centrally mounted within the neck  4  to generate and direct three inline electron beams, a center beam and two side or outer beams, along convergent paths through the tension mask frame assembly  10  to the screen  12 . 
     The CRT  1  is designed to be used with an external magnetic deflection yoke  14  shown in the neighborhood of the funnel-to-neck junction. When activated, the yoke  14  subjects the three beams to magnetic fields which cause the beams to scan horizontally and vertically in a rectangular raster over the screen  12 . 
     The tension mask support frame assembly  10 , as shown in FIG. 2, includes two long sides  22 ,  24 , and two short sides  26 ,  28 . The two long sides  22 ,  24  of the tension mask support frame assembly  10  are parallel to a central major axis, X, of the tube; and the two short sides  26 ,  28  are parallel to a central minor axis, Y, of the tube. The sides  22 ,  24 ,  26 ,  28  are preferably formed of rectangular tubular material. The two long sides  22 ,  24  and two short sides  26 ,  28  preferably form a continuous mask support frame  20  in which the long sides  22 ,  24  lie in a common plane generally parallel to a tension mask  30 . A tension mask support blade member  40  is mounted to each of the long sides  22 ,  24  for supporting the tension mask  30 . The tension mask  30  is attached to each support blade member  40  as will described below in further detail. 
     Referring to FIGS. 3 and 4, the vibration damping features of the invention will now be described in greater detail. The tension mask  30  is terminated to a respective support blade member  40  along the lower edge  32  in a conventional manner such as by welding. The tension mask  30  has a plurality of strands  62  extending from a border  66 . It should be understood that although only one edge  32  along the long side  24  is shown, on the opposite edge near the long side  22 , similar border and attachment features are located. A plurality of cross wires  60  extend generally perpendicular to and over the strands  62 . The cross wires  60  are electrically insulated from the strands  62  and extend over the strands  62  on a screen facing side. The cross wires  60  are terminated to a bus bar  64  extending along the short side  26 . A sheath assembly  50  is provided inside of the bus bar  64 . The sheath assembly  50  consists of a sheath  54  which is positioned on the gun facing side of the tension mask  30 . A shield  70  is located over the sheath  54  on the opposite or screen facing side of the tension mask  30  such that both the strands  62  and the cross wires  60  lie between the sheath  54  and the shield  70 . Also the sheath  54  lies beneath at least one strand  62 . The sheath  54  has a pair of spaced apart mask engaging surfaces  52 ,  56 . Extending from each mask engaging surface  52 ,  56  is a respective angled portion  57 ,  58 . The angled portions  57 ,  58  are connected by a flat section  55 . Near both the bottom and the top of the sheath  54 , a pair of tabs  68  extend outward from the flat section  55  through apertures  72  in the mask border  66  and are bent to engage the screen facing side of the tension mask  30 . The sheath assembly  50  is positioned such that the sheath  54  is positioned to have its mask engaging surface  56  in contact with at least one or several strands  62 . Similarly, the shield  70  is positioned to overhang and be in precise alignment with at least one or several of the strands  62 . This precision alignment ensures that the last mask aperture column will be useful for precisely printing the last phosphor triads on both sides of the screen. Additionally, the precision alignment ensures that the electron beam landing during to-tube operation on the last phosphor triads will have the proper clipping and leaving tolerances. Additionally, FIG. 4 shows a conductive adhesive  74  for attaching the cross wires  60  to the shield  70 . 
     A vibration damping means can be applied along an edge of the sheath assembly  50 . FIG. 4 shows one such vibration damping means, wherein vibration damper  80  is provided along an edge of the sheath assembly  50 . The vibration damper  80  consists of an arm  82  extending from an edge of the shield  70 . A ring  84  is loosely connected to the arm  82  for further absorbing vibrations along the mask  30  and sheath assembly  50 . The ring  84  is allowed to freely slide in an aperture of the arm  82 . When the tension mask  30  is excited by audio or any other energy, this energy which is transferred to the tension mask  30  is, in turn, transferred to the arm  82  thereby driving the ring  84  to slide in the aperture resulting in the energy being scrubbed away. Each sheath assembly  50  is assembled to the tension mask  30  and support blade member  40  as follows. First, the mask borders  66  are attached to the support blade member  40  under tension by conventional techniques such as welding. An insulative layer (not shown) is then applied over the screen facing side of the tension mask  30  to cover each of the strands  62 . Cross wires  60  are then applied over the strands and terminated to the bus bar  64  such that they are insulated from the strands  62  and are positioned generally perpendicular to the strands  62 . The sheath assemblies  50  are then applied by inserting the tabs  68  through the apertures  72  and bending the tabs  68  over the screen facing surface of the mask border  66 . The tabs  68  are then preferably welded to the mask border  66 . The shield  70  is then applied over the sheath  54  utilizing an adhesive such as a glass frit which is cured during tube processing. Both the sheath  54  and shield  70  are positioned such that the mask border  66  is in frictional contact with at least one mask strand  62  to damp vibrations in the tension mask  30 . 
     Advantageously, the vibration damping features of the invention as described above serve to damp vibrations within the tension mask  30  which would otherwise cause undesirable gross electron beam misregister on the screen. As the embodiments that incorporate the teachings of the present invention have been shown and described in detail, those skilled in the art can devise other varied embodiments that incorporate these teachings without departing from the spirit of the invention.