WO2000036627A1

WO2000036627A1 - Cathode-ray tube internal magnetic shield structure

Info

Publication number: WO2000036627A1
Application number: PCT/EP1999/009768
Authority: WO
Inventors: Renzo Incagli; Silvio Santovincenzo
Original assignee: Videocolor Spa
Priority date: 1998-12-16
Filing date: 1999-12-10
Publication date: 2000-06-22
Also published as: ITMI982703A1; IT1304107B1

Abstract

Cathode-ray tube (8) comprising a glass envelope consisting of a front part forming the picture screen and a rear part of substantially conical shape, a frame (32)/shadow mask assembly suspended inside the envelope, a magnetic screen (52), and means for mechanical connection between the frame and the screen along the direction of the longitudinal axis Z of the tube, the frame having, in addition, means (62) for exerting a mechanical constraint on the magnetic screen, at the parts of the screen in contact with the frame, substantially in the direction of the longitudinal axis Z.

Description

CATHODE-RAY TUBE INTERNAL MAGNETIC SHIELD STRUCTURE

The invention relates to a cathode-ray tube having an internal magnetic screen and, more particularly, to the means for fixing the said screen on the frame intended to support it inside the tube.

A colour cathode-ray tube comprises a glass envelope composed of a front slab forming the picture screen and a rear part of substantially conical shape. An electron gun delivers one or more electron beams, each electron beam is deviated by a magnetic deflection system so as to create a picture on the screen with a predetermined colour, the superposition of three primary colours forming a colour picture on the internal surface of the picture screen. The electron beams converge on a colour-selection shadow mask which lies close to the screen and whose function is to allow each beam to illuminate on the screen the phosphors with the corresponding colour which are deposited on it. The earth's magnetic field can affect the trajectories, with the consequence that the points of impact of the beams on the screen no longer correspond to the positions which they should have in order to reproduce the colours faithfully. A magnetic screen intended to reduce the effect of the earth's magnetic field on the trajectories of the electron beams which scan the tube screen is generally arranged around the trajectories of the said beams .

The magnetic screen may be arranged either inside or outside the tube. However, in order not to burden the size of the tube, the magnetic screen is generally arranged inside the tube, as close as possible to the substantially conically shaped glass envelope which constitutes its rear part.

The magnetic screen is generally made of thin (100 to 200 urn) steel plates. In order to ensure good _ 1 -

magnetic coupling with the frame which supports it, the magnetic: screen should be solidly fixed to the said frame; this can be done using elastic clips which are inserted through openings made in the frame and the shadow mask. In a second fastening method, illustrated in American Patent US 5,327,043, the magnetic screen is fixed by lugs which are made directly on the frame and partially cover a peripheral portion of the screen.

So as not to damage the magnetic screen and to allow it to be fastened to the frame, the said fastening using elastic clips should be performed when the openings in the shadow mask and in the frame are perfectly aligned, which makes it difficult to automate this operation. Furthermore, the structure proposed in American Patent US 5327043 describes fastening means comprising overlap of the corners of the frame by the magnetic screen, which means that a process for configuring the screen is complicated and fitting on the frame is difficult to automate, the small thickness of the material and the shape of the corners of the screen making it particularly fragile. This structure is essentially designed for tubes whose frame is held by suspensions lying in the corners. Moreover, a form of connection between the frame and the screen which does not rigidly immobilize the assembly can allow the magnetic screen to vibrate at its natural mechanical frequency in response to electric or acoustic signals present in the environment of the tube during operation, and induce vibrations of the colour-selection shadow mask which will interfere with the reproduction of the picture on the screen of the tube. In addition, these mechanical vibrations can give rise to metal particles which interfere with proper operation of the tube and, for example, obstruct the openings in the colour-selection shadow mask.

In order to avoid the drawbacks listed above, a cathode-ray tube according to the invention comprises a glass envelope consisting of a front part forming the picture screen and a rear part of substantially conical shape, a frame/shadow mask assembly suspended inside the envelope, and a magnetic screen, characterized in that the frame has, in addition, means for exerting a mechanical constraint on the magnetic screen, at the parts of the screen in contact with the frame, with a direction substantially directed at the longitudinal axis Z of the tube. The invention will be understood more clearly from the following description and the drawings in which:

- Figure 1 represents a sectional view of a tube according to the invention.

- Figure 2 illustrates the prior art. - Figure 3 shows an embodiment of a frame according to the invention.

- Figure 4 shows an embodiment of the invention in section.

- Figure 5 illustrates another embodiment of the invention.

- Figure 6 is a sectional view of the frame and the magnetic screen level with the means for exerting a mechanical constraint on the magnetic screen. Figure 1 shows a cathode-ray tube 8 comprising a glass envelope composed of a substantially rectangular front face 12 and a conically shaped rear part 16 ending in a cylindrical neck 14. The front face 12 has a part 18 forming the display screen and a peripheral sleeve 20 allowing the front face to be connected by sealing to the rear part 16. The front face 12 is defined by two orthogonal axes X and Y, a major axis X passing through the middles of the short sides of the said front face and a minor axis Y passing through the middles of the long sides of the same face. The axes X and Y are perpendicular to the longitudinal axis Z of the tube passing through the centre of the neck 14 and the centre of the front face 12. A screen 22, formed by a mosaic of phosphor in three primary colours, is deposited on the internal surface of the front face. The screen may be composed of lines or points of phosphor. A shadow mask 24, for colour selection, is arranged at a predetermined distance from the screen 22. An electron gun 26 is arranged inside the neck 14 and generates three electron beams which converge on the screen through the holes in the shadow mask 24 in order to excite the phosphors on it and create a coloured picture.

A deflection device 28, arranged on the conical rear part, generates magnetic fields intended to deviate the electron beams output by the gun 26, in order to make them scan the surface of the screen 22. The shadow mask 24 is held in position inside the tube using a frame 32, which is itself held inside the front face by elastic supports arranged either in the middle of the sides of the frame or in the corners, as illustrated by Figure 2. The frame 32 is substantially rectangular, with two opposite sides 36 parallel to the X axis, and two shorter opposite sides 38 parallel to the Y axis. As illustrated by Figures 1, 4 and 5, the sides 36 and 38 are generally L-shaped and have two mutually perpendicular parts 40, 42. The edge 40 of the frame has a surface substantially perpendicular to the Z axis, whereas the edge 42 extends substantially in the direction of the longitudinal axis Z, in the direction of the screen 22. Beside the screen 22, at a short distance from it, the colour-selection shadow mask 24 is arranged, the edges of which are curved in towards the rear of the tube so as to match the shape of the part 42 of the frame to which it is generally welded. A magnetic screen 52 is attached to the surface

40 of the frame 32 perpendicular to the longitudinal axis Z. The screen 52 has a peripheral flange 56 which matches the surface 40 of the frame.

The screen 52 is in the conventional way, illustrated by Figure 2, fixed to the frame using two series of means:

- tabs 54 arranged on the surface 40 of the frame make it possible, by sliding part of the surface of the flange 56 under the said tabs, to immobilize the screen along the direction of the Z axis. - pressed parts 60, lying in the corners of the screen and extending in a direction parallel to Z while matching the shape of the frame, make it possible to immobilize the frame along the X and Y directions.

In view of the small thickness of the material of the screen, which thickness makes it mechanically very fragile, it is difficult to automate the fitting and fastening of the screen 52 on the frame 32.

The invention proposes to create, on the frame itself, means for exerting a mechanical constraint on the screen along a direction perpendicular to the central axis of the tube: in this way, the screen will no longer be liable to move along the X or Y directions.

Figures 3 and 4 illustrate an embodiment of the invention. In an XY plane, Figure 3 shows a frame according to the invention and, more particularly, the surface 40 of the frame on which the magnetic screen comes to bear. Figure 4 is a section, in the zone A marked in Figure 3, of the frame/magnetic screen assembly on the plane passing through B-B and parallel to Z. Lugs 62 are created on the frame's surface 40 intended to support the internal magnetic screen; these lugs are made either by pressing the surface 40 or by cutting a portion of the metal forming the surface 40, or by adding metal welded to the said surface 40. There are for example four of these lugs 62, at least one on each of the opposite sides of the frame; the screen is arranged after configuration so that its periphery comes into contact with these lugs. These lugs are arranged in such a way as to exert a mechanical constraint on the screen, the forces of which mechanical constraint are substantially directed at the Z axis; the magnetic screen is elastically constrained so that the dimensions of its periphery along the X and Y directions are reduced in such a way that after putting it down en the surface 40 of the frame and relaxing the constraint has been relaxed, the periphery 58 of the screen will abut on the lugs, against which it will remain compressed.

As illustrated in Figure 6 which, at two lugs arranged on opposite sides of the frame, and on the plane passing through C-C marked in Figure 3 and parallel to Z, represents a section of the frame/magnetic screen assembly, the dimensions of the screen between two lugs 62 are reduced by 2Δ after insertion against the lugs. In view of the small thickness of the material forming the screen, experience has shown that, in order to obtain a good compromise between preserving the integrity of the said screen and elastically deforming the periphery of the magnetic screen, the value Δ could be between 1/1000 and 1/100 of the length of the diagonal of the front face 18 of the tube.

Advantageously, and for better distribution of the mechanical constraint, it is preferable to arrange two lugs on each of the long sides 36 of the frame, as illustrated in Figure 3. The lugs 62 arranged on the short sides of the frame may either lie in the middle of the said sides if the frame/shadow mask/screen assembly is suspended in the front face 18 at the corners, or be offset from the middles of the said sides, as in the case illustrated by Figure 3 when the frame/shadow mask/screen assembly is suspended level with the middles of the sides 36, 38. In an advantageous embodiment, as illustrated by

Figure 5, it is possible to do without auxiliary means for immobilizing the magnetic screen along the direction of the Z axis by arranging for the constraint means 62 to fulfil this function of immobilizing the screen along the direction of the Z axis as well. To that end, taking the plane of the edge 40 of the frame as a reference, the tab-shaped constraint lugs 62 make an angle Θ of less than 90°, preferably between 0° and 45°, with the plane of the edge 40, at the level of the contact with the screen. The screen 52 is then fastened to the frame by compressing the edges of the screen 52 so as to allow the flange 56 to be inserted under the lugs 62, relaxation of the said compression having the effect that the periphery 58 of the screen will abut on the lugs, thus immobilizing the screen 52 against the surface of the frame along the three directions X, Y, Z. Although the frame/screen assembly according to the invention and illustrated by Figures 4 and 5 shows that the constraint on the screen is exerted at the contact between the periphery of the said screen and the lugs 62 secured to the surface of the frame, the invention is not limited to this embodiment, and the flange 56 may for example be pierced with openings to allow the lugs 62 to pass through and the periphery of these openings may abut against the said lugs.

Claims

1. Cathode-ray tube (8) comprising a glass envelope consisting of a front part forming the picture screen (18) and a rear part of substantially conical shape (16), a frame/shadow mask assembly (32/24) suspended inside the envelope, and a magnetic screen (52), characterized in that the frame has means (62) for exerting a mechanical constraint on the magnetic screen, at the parts of the screen in contact with the frame, with a direction substantially directed at the longitudinal axis Z.

2. Cathode-ray tube according to Claim 1, characterized in that means (62) for exerting a mechanical constraint in the direction of the axis Z also fulfil the function of means (54) for mechanical connection along the direction of the longitudinal axis Z.

3. Cathode-ray tube according to one of the preceding claims, characterized in that the means (62) for exerting a constraint in the direction of the axis Z exert constraint forces at the contact between the periphery (58) of the screen and the said means (62).

4. Cathode-ray tube according to one of the preceding claims, characterized in that the connection means and/or the means for exerting a constraint are lugs produced by cutting or pressing the surface of the frame.

5. Cathode-ray tube according to the preceding claim, characterized in that the lugs (62) are in the form of tabs making an angle of between 0° and 45° with the part (40) of the frame on which a peripheral surface (58) of the screen bears.

6. Cathode-ray tube according to one of the preceding claims, characterized in that the means (62) exert an elastic deformation Δ with a value between 1/1000 and 1/100 of the diagonal of the front face of the tube, on the periphery (58) of the screen.