The present invention relates to a color cathode ray tube used in a picture display device such as a television receiver or a computer display and a method for manufacturing the same.
FIG. 5 shows an example of a conventional cathode ray tube. This cathode ray tube includes a bulb 7, a shadow mask structure 9 and an electron gun 10 as main elements.
The bulb 7 includes a panel 2 and a funnel 6. The panel 2 has a phosphor screen 1 on its inner surface. The funnel 6 has a conductive layer 4 on its inner wall 3 and an anode button 5 for applying a high voltage to the conductive layer 4. The conductive layer 4 includes a conductive layer 4a located between the anode button 5 and the electron gun 10, a conductive layer 4b located on the side of the panel 2 and a conductive layer 4c located on the side of a neck portion 6a. The shadow mask structure 9 has a shadow mask 8 facing the phosphor screen 1 on the inner surface of the panel 2. The neck portion 6a of the funnel 6 encloses the electron gun 10.
The shadow mask structure 9 is provided with the first spring 11. The first spring 11 has a contact portion 11a. The contact portion 11a contacts the conductive layer 4 on the inner wall 3 of the funnel, thereby electrically connecting the shadow mask structure 9 to the conductive layer 4. A final electrode 110 of the electron gun 10 is provided with the second spring 12. The second spring 12 has a contact portion 12a. The contact portion 12a contacts the conductive layer 4 on the inner wall 3 of the funnel, thereby electrically connecting the final electrode 110 to the conductive layer 4.
The cathode ray tube described in Publication of Japanese Unexamined Patent Application (Tokkai) No. Sho 59-171439, having a configuration such as shown in FIG. 5, is configured so that the conductive layer 4a located between the anode button 5 and the electron gun 10 has a specific resistance of 0.1 to 10 xcexa9cm, and the conductive layer 4b located on the side of the panel 2 and the conductive layer 4c located on the side of the neck portion 6a have a specific resistance of 0.1 xcexa9cm or less. The above configuration reduces a maximum instantaneous current generated between electrodes in the bulb at the time of a spark, and thereby prevents individual circuit components in TV sets from malfunctioning and breaking.
In such a color cathode ray tube, a sequence of the conductive layer 4c, the conductive layer 4a and the conductive layer 4b is formed on the inner wall 3 of the funnel, in the direction of an electron beam emitted from the electron gun 10. Therefore, a junction portion A of the conductive layer 4a and the conductive layer 4c becomes step-wise, as does a junction portion B of the conductive layer 4a and the conductive layer 4b. In other words, both edge portions of the conductive layer 4a are formed over different planes rather than on the same plane. One edge portion of the conductive layer 4a is formed on two different planes of the inner wall 3 of the funnel and the conductive layer 4c. The other edge portion of the conductive layer 4a is formed on two different planes of the inner wall 3 of the funnel and the conductive layer 4b. Consequently, the junction portions A and B have had problems such as poor conductivity, clogs of apertures of the shadow mask 8 due to shedding off of layers or electric discharges in the tube. Also, the conductive layers 4a, 4b and 4c having different specific resistance are formed extensively on planes with different shapes in the inner wall 3 of the funnel 6, resulting in the complexity of the manufacturing steps.
In addition, in recent years, there has been a concern that leakage electric field emitted from a TV set having a color cathode ray tube might be harmful to the human body. Accordingly, VLEF (Very Low Electric Field) standards have been adopted for regulations (the standardized electric field value is up to 1.0 V/m in a horizontal deflection frequency of 2 to 400 kHz).
It is an object of the present invention to provide a color cathode ray tube and a method for manufacturing the same that reduces a leakage electric field and a maximum instantaneous current generated in a bulb at the time of electric discharge and realizes a stable connection of conductive layers with different specific resistance.
The color cathode ray tube according to the present invention is an improvement of a color cathode ray tube including a bulb having a panel with a phosphor screen disposed on an inner surface thereof and a funnel, a shadow mask structure having a shadow mask that is provided in opposition to the phosphor screen on the inner surface of the panel, an electron gun enclosed in a neck portion of the funnel, a conductive layer provided on an inner wall of the funnel, an anode button provided in the funnel and used for applying high voltage to the conductive layer, a first spring supported by the shadow mask structure and having a contact portion that is biased against the conductive layer, and a second spring supported by a final electrode of the electron gun and having a contact portion that is biased against the conductive layer. The conductive layer includes a first conductive layer and a second conductive layer having a specific resistance lower than that of the first conducive layer. The first conductive layer is formed on an entire range of the inner wall of the funnel to be provided with the conductive layer. The second conductive layer is formed on the first conductive layer within a range from the anode button to the contact portion of the first spring. The contact portion of the first spring contacts the second conductive layer, thereby electrically connecting the shadow mask structure to the second conductive layer. The contact portion of the second spring contacts the first conductive layer, thereby electrically connecting the electrode to the first conductive layer.
With this configuration, since the second conductive layer is formed on a single surface of the first conductive layer, the connection of conductive layers with different specific resistance is firm. In addition, by setting a specific resistance of the second conductive layer lower than that of the first conductive layer, a current pulse, which is generated when an electron beam collides with the shadow mask, easily runs from the anode button to the contact portion of the first spring via the second conductive layer as a lower resistance portion. Consequently, the emission of electric field is suppressed, thus reducing the leakage electric field. In addition, with the first conductive layer that forms a higher resistance portion being connected to the contact portion of the final electrode, the maximum instantaneous current generated between electrodes in the bulb at the time of a spark can be reduced.
In the above configuration, it is desirable that the first conductive layer has a specific resistance of 1 to 3 xcexa9cm.
It is also desirable that the second conductive layer has a specific resistance of 0.05 to 0.2 xcexa9cm.
Furthermore, it is desirable that the first conductive layer is made of a material mainly containing graphite and titanium oxide and the second conductive layer is made of a material mainly containing graphite.
In accordance with the present invention, a method to manufacture the color cathode ray tube with the above configuration includes forming the first conductive layer on the inner wall of the funnel, applying a conductive coating with a specific resistance lower than that of the first conductive layer on the first conductive layer between the anode button and the contact portion of the first spring, and drying the conductive coating to form the second conductive layer.
With this method, since the second conductive layer is formed on a single surface of the first conductive layer, a stable connection between the first and second conductive layers can be obtained and the manufacturing steps of forming each conductive layer can be simplified.