Cathode-ray tube having an internal magnetic shield

Abstract: A cathode-ray tube (10) has a faceplate panel (12) joined to a funnel (14) along a sidewall (16) of the panel, and an inter­ nal magnetic shield (42) disposed proximate an inner surface (22) of the funnel and connected along one end (44) thereof to a back portion (28) of a shadow-mask frame (30) oriented or­ thogonally to a central axis (Z) of the tube and supported adja­ cent the sidewall. An apertured shadow mask (36) is connect­ ed along an edge (38) thereof to a front portion (40) of the shadow-mask frame opposite the back portion. Along the di­ rection of the central axis, the one end of the magnetic shield overlaps the corresponding edge of the shadow mask around substantially all of the shadow-mask frame.

Description: This invention pertains to a cathode-ray tube having an internal magnetic shield attached to a shadow-mask frame therein. A shadow-mask cathode-ray tube typically has a magnetic shield to reduce the influence of magnetic fields on electron beam trajectories as a cathodoluminescent screen of the tube is scanned. In particular, the angles of incidence of the electron beams at every point on the shadow mask must not change significantly from the design values, or the beams will move away from their intended landing positions on the screen. Such a cathode-ray tube is shown in GB-A-2136200. The magnetic shield may be disposed either outside the tube as an external magnetic shield, or inside the tube as an internal magnetic shield. GB-A-2079043 shows a cathode ray tube having a one-piece shadow mask and frame with an internal magnetic shield attached thereto. However, the internal magnetic shield is usually made of 0.10 to 0.18mm thick cold-rolled steel and is fastened to a shadow-mask frame by resilient clamping pins. The frame is supported by springs that engage mounting studs that extend inwardly from a glass rectangular faceplate panel of the tube. During tube fabrication, the internal magnetic shield is fastened to the frame prior to the steps of frit sealing a sidewall of the faceplate panel to a glass funnel of the cathode-ray tube. The internal magnetic shield is designed to fit into the funnel and to be as close to the funnel wall as possible. However, it should not touch the funnel, to avoid any friction between the shield and a conductive anode coating on the inner surface of the glass funnel. Such an arrangement is shown in US-A-4571521. In order to be effective, a magnetic shield must be thoroughly demagnetized (degaussed), in position, by subjecting the magnetic shield to the field from a degaussing coil energized by alternating current of progessively reduced amplitude. Degaussing is normally expressed in terms of ampere turns; typically, for an internal magnetic shield, it would be in the order of 1500A turns. This procedure effectively reorients magnetic domains in the shield and tends to leave it magnetized so as to nullify the field within the shield. The degaussing coil is typically built into the receiver, and the alternating current is automatically reduced from a high value to zero every time the receiver is turned on. This insures against deterioration of color purity and white uniformity caused by changing magnetic field environments. After degaussing, the extent to which an electron beam strikes the cathodoluminescent screen closer to its intended landing position, measured in micrometers of residual misregister, is an indication of the effectiveness of degaussing recovery. Using the same amount of degaussing current, the degaussing recovery for a cathode-ray tube with an additional external magnetic shield is usually better than that for a cathode-ray tube with an internal magnetic shield only. However, an external magnetic shield adds to the manufacturing cost. Consequently, in order to achieve a comparable degree of color purity using only an internal magnetic shield, it is necessary to improve its inherent degaussing recovery. In accordance with the present invention, a cathode-ray tube has a faceplate panel joined to a funnel along a sidewall of the panel, and an internal magnetic shield disposed proximate an inner surface of the funnel and connected along one end thereof to a back portion of a shadow-mask frame oriented orthogonally to a central axis of the tube and supported adjacent the sidewall, a separately formed multi-apertured shadow mask being connected along an edge thereof to a front portion of the shadow-mask frame opposite the back portion in which said one end of said magnetic shield overlaps, along the direction of the central axis, the corresponding edge of the shadow mask around substantially all of the shadow-mask frame. Embodiments of the invention provide for an internal magnetic shield showing a significant improvement in residual misregister after degaussing. Embodiments of the present invention, given by way of non-limitative example, will now be described with reference to the accompanying drawings, in which: FIGURE 1 is a cross-sectional view illustrating a cathode-ray tube having a prior-art internal magnetic shield disposed therein. FIGURE 2 is a cross-sectional view illustrating an overlapping internal magnetic shield embodying the present invention. FIGURE 3 is a cross-sectional view illustrating a second overlapping internal magnetic shield embodying the present invention. FIGURE 1 shows a cathode-ray tube 10 having a faceplate panel 12 joined to a funnel 14 thereof along a sidewall 16 of the panel 12. A cathodoluminescent screen 18 is disposed on the inner surface of the panel 12, and a conductive coating 20 is disposed on the inner surface 22 of the funnel 14 which serves as the anode for the tube 10. A prior-art internal magnetic shield 24 is disposed within the tube 10, with one end 26 thereof proximate the sidewall 16, and extends backward the inner surface 22 of the funnel 14. The magnetic shield 24 is connected along the one end 26 to a back portion 28 of a shadow-mask frame 30 oriented orthogonally to a central axis of the tube 10, shown as dotted line Z. The shield 24 is fastened to the frame 30 by resilient clamping pins 32, which are inserted through aligned apertures disposed in both the shield 24 and the frame 30. The shadow-mask frame 30 is supported adjacent the sidewall 16 by mounting studs 34 which extend inwardly from the faceplate panel 12. Since the sidewall 16 of the faceplate panel 12 is generally rectangularly shaped, the typical internal magnetic shield 24 has four corners. A multi-apertured shadow mask 36 is connected along an edge 38 thereof to a front portion 40 of the shadow-mask frame 30 opposite the back portion 28, as shown in FIGURE 1. In addition to the internal magnetic shield 24, the shadow mask 36 itself makes a significant contribution to the total shielding of the cathode-ray tube 10. The shadow mask 36 is typically made of 0.15 mm thick cold-rolled steel, and is welded to the frame 30. The mask 36 may be welded to the inside of the frame 30, as shown in FIGURE 1, to form a MIFA (Mask Inside Frame Assembly), or the outside of the frame 30 to form a MOFA (Mask Outside Frame Assembly). Using either the MIFA or MOFA, a small gap, shown as distance G in FIGURE 1, is created between the edge 38 of the shadow mask 36 and the one end 26 of the internal magnetic shield 24 around the perimeter of the shadow-mask frame 30. Heretofore, it was assumed that the frame 30, since it was also made of a magnetic material, albeit different from that of the magnetic shield 24, also performed a shielding function along the gap G. FIGURE 2 shows one embodiment of an overlapping internal magnetic shield 42 which provides for a significant improvement in degaussing recovery. It has been discovered that the residual misregister after degaussing is improved substantially by overlapping, along the direction of the central axis Z, the one end 44 of the internal magnetic shield 42 with the corresponding edge 38 of the shadow mask 36 around substantially all of the shadow-mask frame 30. Preferably, the one end 44 of the internal magnetic shield 42 is extended forward to overlap the edge 38 of the shadow mask 36 along the sides of the frame 30 except in the corners thereof. FIGURE 2 shows a MIFA structure wherein the shadow mask 36 is connected to the inside of the frame 30. Since the internal magnetic shield 42 is connected to the outside of the frame 30, the mask 36 and shield 42 overlap but do not actually contact each other, as shown in FIGURE 2. FIGURE 3 shows a second embodiment of the overlapping internal magnetic shield 42 incorporated into a MOFA structure. Since the magnetic shield 42 is also connected to the outside of the frame 30, the shield 42 actually contacts the shadow mask 36, although such contact is not necessary to achieve the benefits of the present invention. The one end 44 of the internal magnetic shield 42 may also be extended around the inside of the shadow-mask frame 30 in order to overlap the shadow mask 36 in either the MIFA or MOFA configuration. It is not necessary that the magnetic shield 42 comprise one integral piece. The overlapping portion of the shield 42 may in fact comprise two or more pieces, particularly in the embodiment where the shield 42 extends around the inside of the frame 30. It is important that the one end 44 of the magnetic shield 42 actually overlap the corresponding edge 38 of the shadow mask 36. This overlapping distance, along the direction of the central axis Z, should preferably be at least twice the thickness of the shadow-mask frame 30 in order to achieve satisfactory magnetic coupling and, hence, improved magnetic recovery. Preferably, the one end 44 of the internal magnetic shield 42 extends substantially to the front of the shadow-mask frame 30, as shown in FIGURES 2 and 3. The TABLE below shows data values for residual misregister recorded in tests performed on RCA 27V SP cathode-ray tubes with different types of magnetic shielding, including that of the present invention. The first row in the TABLE represents a cathode-ray tube having an external magnetic shield and a prior-art internal magnetic shield. The second row represents a tube having a prior-art internal magnetic shield but no external magnetic shield. The third row represents a tube having the present overlapping internal magnetic shield. In each row, data values for residual misregister at the ends of the diagonal, major and minor panel axes, and also at the panel center, were recorded after changes in the relative magnetic field along the vertical Y axis (100 mG), central Z axis (250 mG) and X axis (250 mG) directions. The recorded data values show that the overlapping internal magnetic shield (row 3) provides a significant improvement in residual misregister over the prior-art internal magnetic shield (row 2). For a 250 mG change in magnetic field along the central Z axis direction, the overlapping shield provides even better degaussing recovery time than that achieved with an external magnetic shield (row 1). It is hypothesized that the shadow-mask frame 30 presents a relatively high magnetic reluctance during degaussing, thereby creating a gap in the magnetic shielding which degrades the residual misregister in the cathode-ray tube 10. It has been discovered that this residual misregister after degaussing may be improved substantially by overlapping the one end 44 of the magnetic shield 42 with the edge 38 of the shadow mask 36 around substantially all of the shadow-mask frame 30. The magnetic shield 42 has an improved degaussing recovery which, under similar operating conditions, achieves a level of color purity heretofore unattainable with the prior-art internal magnetic shield, thereby providing an acceptable alternative to the more expensive addition of an external magnetic shield.

Claims: A cathode-ray tube (10) having a faceplate panel (12) joined to a funnel (14) along a sidewall (16) of said panel, and an internal magnetic shield (42) disposed proximate an inner surface (22) of said funnel and connected along one end (44) thereof to a back portion (28) of a shadow-mask frame (30) oriented orthogonally to a central axis (Z) of said tube and supported adjacent said sidewall, a separately formed multi-apertured shadow mask (36) being connected along an edge (38) thereof to a front portion (40) of said shadow-mask frame opposite the back portion; characterized in that the said one end of said magnetic shield overlaps, along the direction of said central axis, the corresponding edge of said shadow mask around substantially all of said shadow-mask frame. A cathode-ray tube (10) as defined in Claim 1, characterized in that the one end (44) of said magnetic shield (42) extends forward to overlap the edge (38) of said shadow mask (36). A cathode-ray tube (10) as defined in Claim 1, characterized in that said shadow mask (36) is connected to the outside of said shadow-mask frame (30), and said magnetic shield (42) is connected to the outside of said shadow-mask frame, in contact with said shadow mask. A cathode-ray tube (10) as defined in Claim 1, characterized in that said shadow mask (36) is connected to the inside of said shadow-mask frame (30), and said magnetic shield (42) is connected to the outside of said shadow-mask frame. A cathode-ray tube (10) as defined in Claim 3 or Claim 4, characterized in that the overlapping distance along the direction of said central axis (Z) comprises at least twice the thickness of said shadow-mask frame (30). A cathode-ray tube (10) as defined in any one of the preceeding Claims, characterized in that the said one end (44) of said magnetic shield (42) extends substantially to the front of said shadow-mask frame (30). A cathode-ray tube (10) as defined in any one of the preceeding Claims, characterized in that said shadow-mask frame (30) is rectangular shaped, and the said one end (44) of said magnetic shield (42) overlaps the edge (38) of said shadow mask (36) along the sides of said frame except in the corners thereof.

Inventors: MORRELL ALBERT MAXWELL, MORRELL, ALBERT MAXWELL
Applicants: RCA THOMSON LICENSING CORP, RCA THOMSON LICENSING CORPORATION
Application year: 1987
Main classification: H01J29
IPC codes: H01J31, H01J29