Display tube having a detachable getter

Abstract: A display tube has a getter which is clamped in an aperture of the screening cap. The getter and the screening cap may be of a simple construction and the getter can be replaced in a simple manner so that the display tube can be easily repaired. The getter is supported by the cone and the inner wall of the screening cap.

Description: The invention relates to a display tube comprising a neck, a cone and a display window, an electron gun being accommodated in the neck, an internal magnetic screening cap being arranged between the gun and the display window, an electrically conductive layer being present on the inner wall of the cone and the display tube comprising a detachable getter which electrically contacts said conductive layer. Display tubes are used, inter alia , in television receivers. Such a display tube is known from United States Patent Specification US 4,182,974. Within the scope of the invention, the term "getter" is to be understood to mean a part of the display tube which comprises at least a getter holder with getter material. The getter is heated during the manufacture of a display tube. In this process getter material is vaporized. Said vaporization causes the pressure in the display tube to decrease. Sometimes, however, a display tube exhibits a defect which can be remedied by repairing the display tube. The display tube under repair is opened, i.e. the neck is opened and the vacuum in the display tube is interrupted. After the display tube has been repaired, the neck is closed and the display tube is evacuated. Subsequently, getter material is again vaporized. It is advantageous to replace the getter when the display tube is under repair. In the known display tube, the getter is secured to a projection on an anode contact located in the cone. The getter is secured in a manner such that it can be replaced. To replace the getter, the screening cap is provided with a groove which gives access to the anode contact. Within the scope of the invention it has been recognized that said groove adversely affects the magnetic screening and mechanical rigidity of the screening cap. A reduction of the magnetic screening leads to picture errors. A reduction of the mechanical rigidity may lead to problems relating to microphonics. It is an object of the invention to provide a display tube of the type mentioned in the opening paragraph, in which the getter can be replaced in a simple manner and the screening cap is improved. To this end, a display tube according to the invention is characterized in that the screening cap comprises an inwardly extending portion at the end facing the neck, in that the getter projects from an aperture in said portion and is clamped in the aperture, a portion of the getter pressing against the cone and electrically contacting the electrically conductive layer, and a portion of the getter pressing against the inner wall of the screening cap, the aperture being situated between said portions of the getter. In said portion of the screening cap there is an aperture from which the getter projects and in which the getter is clamped. Such an aperture can be made in a simple manner and has no or hardly any influence on the magnetic screening and mechanical rigidity of the screening cap. By virtue thereof, the magnetic screening by the magnetic screening cap and the mechanical rigidity of the screening cap are improved relative to the known display tube. A further advantage is that the anode contact does not have to be provided with a projection. A getter can be replaced as follows: the getter is pulled out of the aperture or, in a preferred embodiment in which the getter is clamped in the aperture by means of resilient elements which can be moved towards each other, the resilient elements are moved towards each other after which the getter is detached. The getter is then removed from the display tube and a new getter is introduced into the display tube via the neck and inserted into the aperture and secured to the screening cap. Said getter can be inserted by means of a device for inserting the getter into the aperture via the neck. The resilient elements press against the edge of the aperture, causing the getter to be clamped in the aperture. A part of the getter presses against the internal conductive layer and an electric contact between the getter and the conductive layer is formed. Another part presses against the inner wall of the magnetic screening cap. The getter may be of a simple construction. By virtue of said parts and the aperture between said parts, the position of the getter in the display tube is accurately defined. An accurate position of the getter is important, inter alia, when the getter material is vaporized by inductively heating the getter holder by means of an induction coil. Said induction coil is situated outside the display tube. The efficiency with which this process is carried out depends on the position of the getter holder and the induction coil relative to each other. The efficiency with which the getter material is vaporized increases as the position of the getter holder is more accurately defined. A preferred embodiment of the display tube according to the invention is characterized in that the portion facing the gun is a flat portion which extends approximately parallel to the display window, in that the screening cap comprises a conical portion and the aperture is situated in the flat portion near the transition between the conical portion and the flat portion. The getter can be accurately positioned by providing the aperture in the screening cap at the indicated location. A further embodiment of the display tube according to the invention is characterized in that the screening cap is provided with guide means for guiding the getter to the aperture. This simplifies the insertion of the getter into the aperture. When the getter is introduced into the tube, the guiding ridges guide the getter to the aperture. A preferred embodiment of the display tube according to the invention is characterized in that the resilient elements are parts of a resilient bracket and are provided with indentations. Said indentations allow interlocking between the bracket and the aperture. The invention will be explained in greater detail by means of an exemplary embodiment of the display device according to the invention and with reference to the accompanying drawings, in which Fig. 1 is a longitudinal cross-sectional view of a display device in accordance with the invention; Figs. 2A, 2B and 2C are a front view, rear view and side view, respectively, of a getter for a display tube in accordance with the invention; Fig. 3 is an elevational view of a screening cap of a display tube in accordance with the invention, viewed from the neck; Fig. 4 is a sectional view of a detail of a display tube in accordance with the invention; Fig. 5 is a sectional view of a detail of a further example of a display tube in accordance with the invention; Figs. 6a up to and including 6c show a device for inserting a getter via the neck. The Figures are not drawn to scale. In the Figures, corresponding parts generally bear the same reference numerals. A display tube, in this example colour cathode ray tube 1, has an evacuated envelope 2 comprising a display window 3, a cone 4 and a neck 5. In the neck there is situated an electron gun 6 for generating three electron beams 7, 8 and 9. A display screen 10 having phosphor elements luminescing in red, green and blue is situated on the inside of the display window. On their way to the display screen 10 the electron beams 7, 8 and 9 are deflected across the display screen 10 by means of deflection unit 11 and pass through shadow mask 12 which comprises a thin plate having apertures 13. The electron beams 7, 8 and 9 pass through the apertures 13 at a small angle and each electron beam impinges on phosphor elements of only one colour. The shadow mask is suspended in the display tube by means of suspension means 14. The display tube comprises a magnetic screening cap 15. Said screening cap may, for example, be secured to the shadow mask at a supporting edge or supporting corners for the shadow mask or to suspension means. By virtue of said magnetic screening cap, the influence of disturbing magnetic fields, such as the earth's magnetic field, on the deflection of the electron beams is reduced. A getter 16 is secured to the screening cap. A conductive layer 17 is applied to the inner wall of the cone and, inter alia , electrically contacts the electron gun, for example, via springs 18 and electrically contacts the getter 16. An electric potential can be applied to the conductive layer via an anode contact 19. The screening cap 15 comprises an inwardly extending portion 20 at the end facing the neck 4. As shown in this example, said portion may extend approximately transversely to the axis of the cone. In a process step in the manufacture of the display tube, the getter holder is heated so as to vaporize the getter material. This causes the pressure in the display tube to be reduced. When the display tube is under repair the neck is opened. After the display tube has been repaired the neck is closed and the display tube is evacuated again. Subsequently, getter material is vaporized again. A getter which has not been used works better than a getter which has already been used. Consequently, it is advantageous to replace the getter of the display tube under repair. In the display tube known from US 4,182,974, the anode contact is provided with a projection to which the getter is secured. The screening cap has a groove which extends from the end of the screening cap facing the electron gun to beyond the anode contact. As a result thereof the magnetic protection provided by the screening cap in the vicinity of the groove is reduced. The mechanical strength and rigidity of the screening cap are reduced by the groove. The invention aims at providing a display tube having an improved magnetic protection and a stronger screening cap. Figs. 2A, 2B and 2C are a front view, rear view and side view, respectively, of a getter which can suitably be used for a display tube in accordance with the invention. Getter 16 has a getter holder 21 containing getter material. Said getter holder 21 is attached to a resilient bracket 23 by means of a rod 22. The resilient bracket 23 comprises two portions 24 and 25 which can move relative to one another. Said portions comprise indentations 26. The bracket further comprises limbs 27 which ensure that the getter holder 21 is kept at some distance from the cone. Fig. 3 is an elevational view of a screening cap viewed from the neck. The screening cap 15 has an inwardly extending flat surface 32 which corresponds to portion 20 in Fig. 1, a cone portion 33 and an edge 34. An aperture 31 is situated in the flat surface near the transition between the flat surface and the cone portion. In this example the distance between the aperture and the transition ranges between approximately 1 and 10 mm. By virtue of the position of the aperture, the aperture 31 has no or substantially no adverse effect on the strength of the flat surface 32. A weakened flat surface may lead to a variation in the position of the aperture, causing the accuracy with which the getter is positioned to be reduced. The getter is secured to the screening cap by introducing the bracket 23 into the aperture 31 until the edge of the aperture and the indentations interlock. When the getter is fed through the neck, the aperture is not visible from the neck. Within the scope of the invention it has been recognized that this could cause problems. In this example, guiding ridges 35 are formed in the flat surface 32 in the vicinity of the aperture 31. During inserting the getter, said ridges guide the getter to the aperture 31. By virtue of such means the getter can be secured more easily. The guiding ridges may consist of two ridges projecting from surface 32. In an embodiment, portion 36 of surface 32, which is located between the ridges, 35 is recessed relative to portions 37 and 38 which are located on either side of the ridges, and ridges 35 are formed by transitions between portion 36 and portions 37 and 38. Fig. 4 is a sectional view of a detail of a display tube in accordance with the invention. Movable parts 24 and 25 (not shown) are clamped in aperture 31 of the screening cap 15 causing the getter 16 to be secured to the screening cap in a self-clamping manner. The edge of aperture 31 and the indentations 26 (not shown) interlock. By means of said indentations the getter can be accurately positioned. The getter lies against the cone with limbs 27. The inside of the cone is provided with a conductive layer 17 which is in electric contact with the getter via the limbs 27. In operation, said layer carries high voltage. The movable parts 24 and 25 of the getter lies against the inner wall 33 of the screening cap 15. The aperture is situated between the portions 27 and movable parts 24 and 25 of the getter. The position of the getter is accurately defined by limbs 27, movable parts 24 and 25 and aperture 31. Getter 16 can be removed in a simple manner. The getter can be pulled out of aperture 31 by introducing a hook into the display tube through the neck, hooking the hook between the portions 24 and 25 and pulling it. Preferably the getter is detached without exerting forces on the screening cap by moving the movable parts 24 and 25 towards each other. In order not to change the position of the screening cap, the getter is preferably detached without exerting a large force on the screening cap by moving the movable parts 24 and 25 towards each other. By virtue thereof, the clamping force is eliminated and the getter becomes detached. Also in detaching the getter, the guide means, in this example guiding ridges, are useful; they can lead a device which is used to detach the getter to said getter. Fig. 5 is a sectional view of a detail of an embodiment of a display tube in accordance with the invention. This embodiment differs from the one shown in Fig. 4 in that bracket 23 is replaced by a spring 50 comprising two movable portions 51 and 52. The portions 51 and 52 may have indentations which interlock with the edge of the aperture. The spring is bent toward the cone 4. Aperture 31 is situated between portion 27 of the getter which lies against the inner wall of the cone and portion 52 which, in point 53, lies against the inner wall 33 of screening cap. One aspect of the invention is formed by the insight that the getter can be replaced, preferably, in a simple manner via the neck. If a used getter has been removed, a replacement getter can be secured to the screening cap by means of, for example, an insertion device as diagrammatically shown in Figs. 6a up to and including 6c. The insertion device 70 comprises a central portion 71, collapsible portions 72 and 73, levers 75 and 76, handle 77 and a flange 78. Getter 16 is secured in portion 73. The insertion device is introduced into the neck of a display tube until flange 78 contacts the edge of the neck. By pressing-in lever 75, portion 73 is caused to swing out (Fig. 6b). By further pressing-in lever 75, portion 72 is caused to swing out. As a result thereof, getter 16 is inserted into aperture 31 of the screening cap. Subsequently, lever 76 is pressed-in causing the getter 16 to become detached from portion 73. Portions 72 and 73 then collapse and the insertion apparatus 70 is removed from the neck. It will be obvious that within the scope of the invention many variations are possible to those skilled in the art. In the example given, the screening cap has a triangular aperture. In other embodiments the aperture may be of a different shape, for example, diamond-shaped or slit-shaped. In the display tube in accordance with the invention, the getter can be secured before or after the display window and the cone are bonded together. This enables both frittable and non-frittable getters to be used. The difference between these two types of getters resides in that a frittable getter can withstand the temperature prevailing during bonding together the display window and the cone, whereas a non-frittable getter cannot withstand said temperature. Within the scope of the invention, the term "getter" is to be understood to mean a part of the display tube which comprises at least a getter holder for getter material and a connection between the getter holder and another part of the display tube. Within the scope of the invention, the expression "conductive layer" is to be understood to mean also a resistive layer. The inwardly directed part may extend, as shown in the example, transversely to the tube axis but it may also make an angle with said axis.

Claims: A display tube (1) comprising a neck (5), a cone (4) and a display window (3), an electron gun (6) being accommodated in the neck (5), an internal magnetic screening cap (15) being arranged between the gun (6) and the display window (3), an electrically conductive layer (17) being present on the inner wail of the cone (4) and the display tube (1) comprising a detachable getter (16) which electrically contacts said conductive layer (17), characterized in that the screening cap (15) comprises an inwardly extending portion (20) at the end facing the gun, in that the getter (16) projects from an aperture (31) in said portion and is clamped in the aperture, a portion (27) of the getter pressing against the cone (4) and electrically contacting the electrically conductive layer (17), and a portion (24,25) of the getter pressing against the inner wall (33) of the screening cap, the aperture (31) being situated between said portions (27; 24,25). A display tube as claimed in Claim 1, characterized in that the getter is clamped in the aperture (31) by means of resilient elements (24,25) which can be moved towards each other. A display tube as claimed in Claim 1 or 2, characterized in that the portion of the screening cap (15) facing the gun (6) is a flat portion (20) which extends approximately parallel to the display window (3), in that the screening cap (15) further comprises a conical portion (33) and the aperture is situated in the flat portion (20) near the transition between the conical portion and the flat portion. A display tube as claimed in Claim 1, 2 or 3, characterized in that the screening cap (15) is provided with guide means (35) for guiding the getter (16) to the aperture (31). A display tube as claimed in Claim 2, characterized in that the resilient elements (24,25) are parts of a resilient bracket and are provided with indentations (26).

Inventors: OVERBEEK JOHANNES JOSEPH MARIA, VENNIX JOHANNES ADRIANUS, OVERBEEK, JOHANNES JOSEPH MARIA, VENNIX, JOHANNES ADRIANUS
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1992
Main classification: H01J7
IPC codes: H01J29, H01J7