Field of the Invention

This invention generally relates to cathode ray tubes (CRTs) and, more particularly, to a shielding arrangement for a tensioned mask-frame assembly comprising microphonic dampers.

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 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.

A tension mask comprises a set of strands or tie bar slitted apertures in a membrane that are tensioned onto a mask frame to reduce their propensity to vibrate at large amplitudes under external excitation. The vibration response of a CRT tension mask to mechanical excitation, such as a blow to the TV cabinet or the operation of the TV's speakers, is generally referred to as microphonics. Such vibrations would cause gross electron beam misregister on the screen and would result in objectionable image anomalies to the viewer of the CRT. To overcome this vibration, tension masks are usually very stiff in the directions perpendicular to the CRT's axial centerline. However, existing tension masks are generally susceptible to small amplitude motion in the direction approximately parallel to the CRT's

axial centerline. This motion is undesirable because it changes the landing locations on the CRT's phosphor screen of the electron beams that are directed through the mask's apertures. One approach to mitigate the microphonics effect is to avoid resonant frequencies of the mask that are coincidental with those excitations that are of high energy and/or are frequently occurring. Another approach is to limit the maximum amplitude of the mask vibration with a mechanical stop, so as to limit the maximum excursion of the beam landing variation. Another source of electron beam misregister and beam motion is residual magnetism within the CRT. Td remove this residual magnetism, a degaussing process is performed. One of the controlling parameters for optimizing magnetic performance of a tube is degauss recovery. Good degauss recovery manifests itself in low beam motion with the tube located in the external earth magnetic field and in good register of the electron beam with the phosphor element on the screen, after the tube has undergone a degaussing process to set up balancing fields in the IMS, mask, and frame components inside the CRT. With the introduction of true flat CRT's that use tension masks, including focus tension masks, optimization of magnetic shielding by degaussing has become more difficult.

During tube degaussing, existing IMS's must achieve effective magnetic field coupling with the mask through an intervening frame. In tension mask CRT designs, the mask is attached to a rigid frame. In order to maintain tension in the tension mask, the frame has to have high yield stress, which is usually accompanied by poor magnetic properties, i.e., high coercive force and low permeability. This makes degaussing the frame difficult, provides poor flux coupling during the degaussing process, and leaves very high residual magnetic fields inside the CRT. These residual magnetic fields cause the CRT to have very high electron beam misregister, poor purity and poor picture quality.

It is desirable to develop a tensioned mask-frame assembly with microphonic damping. It is also desirable to provide a mask-frame assembly that allows tension masks and frames to be uniformly degaussed.

Summary of the Invention

The present invention therefore provides a cathode ray tube (CRT), comprising a tension mask mounted on a tension mask frame in the CRT and a vibration damper operatively associated with the tensioned mask to dissipate vibration thereof through friction under a spring load. In an advantageous exemplary embodiment, the damper forms a part of, or is attached to, an internal magnetic shield to provide magnetic coupling of the internal magnetic shield and the mask-frame assembly.

Brief Description of the Drawings

The invention will now be described by way of example with reference to the accompanying figures of which:

Figure 1 is a sectional plan view of a cathode ray tube according to an embodiment of the present invention;

Figure 2 is a front view of a tension mask/frame assembly from the cathode ray tube of Figure 1, showing a partial cut-away of the tension mask; Figure.3 is a perspective sectional view of an existing tension mask/frame assembly and internal magnetic shield arrangement;

Figure 4 is a perspective sectional view of a tension mask/frame assembly and internal magnetic shield arrangement according to an exemplary embodiment of the present invention;

Figure 5 is a perspective sectional view of a tension mask/frame assembly and internal magnetic shield arrangement according to an alternative exemplary embodiment of the present

invention;

Figure 6 is a perspective sectional view of a tension mask/frame assembly and internal magnetic shield arrangement according to another alternative exemplary embodiment of the . present invention;

Figure 7 is a plan view partially in section of a tension mask/frame assembly of Figure 6; and

Figure 8 is a perspective sectional view of a tension mask/frame assembly and internal magnetic shield arrangement according to another alternative exemplary embodiment of the present invention.

Detailed Description of the Invention

Figure 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 panel 3 and to the neck 4. The panel 3 comprises a substantially cylindrical or a rectangular 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 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 color selection tension mask assembly 10 is removably mounted in predetermined spaced relation to the screen 12. An electron gun 13, shown schematically by dashed lines in Figure 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 assembly 10 to the screen 12.

The tube 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 assembly 10, as shown in Figure 2, has a metal frame 20 that includes two long sides 22 and 24, and two short sides 26 and 28. The two long sides 22, 24 of the frame are parallel to a central major axis, X, of the tube; and the two short sides 26, 28 parallel a central minor axis, Y, of the tube. Although the tension mask assembly 10 is shown here diagrammatically as a sheet for simplicity, it includes an apertured shadow mask 30 that contains a plurality of metal strips (not shown) having a multiplicity of elongated slits (not shown) therebetween that parallel the minor axis of the shadow mask 30. The long sides 22, 24 have a cantilever edge 25 extending toward the screen 12.

In an existing arrangement of a tension mask assembly 10 and an internal magnetic shield (IMS) 50, as shown in Figures 3, the tension mask 30 is attached to the cantilever edge 25 of the long sides of the tensioned mask frame, hereafter frame 20. The attachment may be performed, for example, by welding. The IMS 50 is attached to the long sides of the frame 20 at a location removed from the tension mask 30. In the embodiment illustrated in Figure 3, the long sides of the frame 20 comprise triangle shaped bars with two legs at a right angle to each other and the cantilever edge 25 on the end of one leg and the IMS 50 attached to the other leg. Thus, the existing shielding arrangement provides magnetic flux coupling through the tensioned mask frame 20.

Li this arrangement, the IMS 50, tension mask 30 and frame 20 are made from low carbon steel or iron-nickel alloys. Magnetic shielding and degaussing ability of the tension mask 30, tensioned mask frame 20, and IMS 50 system are improved if each of the components has high anhysteretic permeability (i.e. at least 3000) and low coercivity.

However, the tensioned mask frame 20 must have high yield stress to provide the strength . necessary for proper function. This high yield stress is usually accompanied by poor magnetic properties, e.g., high coercivity and low permeability. Even if the coercivity of the tension mask 30 and the IMS 50 are low, indicating good magnetic properties, the overall performance of the tube is deteriorated if the coercivity of the tensioned mask frame 20 is high, indicating poor magnetic properties. Having high magnetic reluctance, the tensioned mask frame 20 increases the reluctance of the IMS/frame/mask assembly. Additionally, a residual magnetic field is retained after degaussing at the interface of the tension mask 30 and the tensioned mask frame 20 that is difficult to remove and leads to beam misregister. Conventional degaussing is performed using a special degaussing coil placed close to the IMS 50, and will degauss the IMS 50 adequately. Conventional degaussing, however, will do very little to remove residual magnetic fields from the high coercivity tensioned mask frame 20 and the tension mask 30 behind it. The tensioned mask frame 20 causes the earth magnetic field to be distorted and concentrated at particular points, which can magnetize the tension mask 30 and IMS 50 when the tube is degaussed. In addition, a residual magnetic field exists due to the high coercivity tensioned mask frame 20 that is difficult to remove and leads to beam misregister. hi an exemplary embodiment of the present invention, as shown in Figure 4, the tension mask 30 is attached to a cantilever end 25 of the frame 20 on the screen side of the frame (only on long sides), such as by welding. A damper 40 is wrapped partially around the frame 20, extending over at least part of the inside face 201 and bottom face 2OB of the frame 20. The damper 40 is formed into a spring 41 at its inside edge, such as by looping the material back on itself as shown in Figure 4. The damper is positioned such that the spring 41 is contacted by the tension mask 30 during vibration of the tension mask.. The damper 40 is a part of the IMS 50 or is attached to the IMS 50 to provide magnetic coupling of the IMS 50

and the mask-frame assembly. For example, as illustrated in Figure 4, the internal magnetic shield (IMS) 50 may be placed over the damper 40 at the bottom face 2OB of the frame 20, and both the IMS 50 and damper 40 are then attached to the frame 20, such as by a fastener 60. Vibration of the tension mask 30 adjacent to its attachment to the frame 20 causes frictional sliding between the tension mask 30 and the spring 41 of the damper 40 and/or within the damper 40, itself. This sliding friction extracts vibrational energy, damping the tension mask motion and reducing misregister of the electron beams on the phosphor screen due to microphonics. The spring force of the spring 41 maintains frictional contact between the tension mask 30 and the damper 40.

The damper 40 may comprise a material having a magnetic permeability that is equal to or greater than the tension mask 30 and the IMS 50. The high permeability damper 40 is attached to or in contact with both the tension mask 30 and the IMS 50. Thus, the damper 40 provides magnetic coupling of the tension mask 30 and the IMS 50, reducing misregister of the electron beams on the phosphor screen due to residual magnetism at the frame 20.

An advantage of the present invention is that the damper 40 may be attached to the frame 20 before the tension mask 30 is attached to the frame 20 to form the mask-frame assembly. Thus, the tension mask 30 is not disturbed during damper installation procedures, . such as welding.

An alternative embodiment of the present invention is shown in Figure 5, wherein the damper 40 extends over the entire outside edge 2OC, essentially enclosing the frame 20.

In this exemplary embodiment, the damper 40 comprises a clad-type foil made from a ferromagnetic metal.

In another alternative embodiment of the present invention, as shown in Figures 6 and 7, damper 40 comprises a frame shield 44 that wraps around the frame 20 and a plurality of spring fingers 45. These spring fingers 45 may be integral with the frame shield 44. Alternatively, they may be separate pieces attached to the frame shield 44 or to the frame 20, as shown in Fig. 6. As shown in Figure 7, the spring fingers 45 are spaced along the long sides 24,-26 of the frame and-positioned under the mask 30 to provide contact surfaces for the mask 30.

In another alternative embodiment of the present invention, as shown in Figure 8, the frame 20 may comprise a profile that is not triangular. For example, as shown in Figure 8, the frame profile may be an angle or "L" shaped. It should be understood that the present invention may be adapted to various frame profiles.

The foregoing illustrates some of the possibilities for practicing the invention. Many other embodiments are possible within the scope and spirit of the invention. It is, therefore, intended that the foregoing description be regarded as illustrative rather than limiting, and that the scope of the invention is given by the appended claims together with their full range of equivalents.