1. Field of the Invention
The present invention relates to a cathode ray tube and, in particular, a cathode ray tube for applying a predetermined voltage to a corresponding electrode via a resistor unit which is disposed in the neck of a cathode ray tube.
2. Description of the Related Art
Generally, a color CRT is known as a CRT which is supplied with high voltage. The color CRT, usually, comprises an envelope 3 comprising a panel 1, a funnel 2 and a neck 6, as shown in FIG. 1. A phosphor screen (target) 5 is formed on the inner surface of the panel 1 and a shadow mask 4 is provided opposite to the phosphor screen (target) 5 which is composed of a three-color phosphor layer for emitting R (red), B (blue) and G (Green) light. At a time of use, a deflection yoke 20 is mounted near a boundary between a funnel 2 and a neck 6.
An electron gun assembly 7 is located in the neck 6 to emit three electron beams 9. The electron gun assembly 7 is composed of a plurality of electrodes, such as a cathode serving as an electron beam generating section, an electrode for controlling the generation of the electron beams 9 emitting from the cathode, and an electrode for focusing the electron beams toward the phosphor screen at accelerated speed. It is necessary to supply a high anode voltage of about 25 to 30 KV and medium voltage of about 5 to 8 KV (focusing voltage) to the corresponding electrodes.
A voltage which is to be applied to the associated electrode in the electron gun assembly 7 is applied there via a corresponding stem pin 17 which extends through a stem section 6a of the neck 6 in airtight fashion, noting that anode voltage is applied via an inner conductive film 16 which is formed on the inner surface of an anode terminal 8 and funnel 2. Supplying a medium voltage, such as a focusing voltage, via the stem section 6a poses a "arcing or flashover" problem as involved at a supply section such as a socket which is connected to the stem pin 17. This causes a complex structure.
A way for obtaining a requisite medium voltage through the division of anode voltage which is made by a resistor unit located within the CRT is disclosed in Japanese Utility Model Disclosure (KOKAI) Nos. 48-21561 and 55-38484 and U.S. Pat. Nos. 3,932,786 and 4,413,298. However, there is no adequate space for the resistor unit to be arranged within the CRT. For this reason, the resistor unit is located in a small space in the neck 6 such that it is situated near the electron gun assembly 7.
FIG. 2 is one form of an electron gun assembly having a resistor unit arranged in it. In an arrangement shown in FIG. 2, reference numeral 7 denotes electron gun assembly 10a, 10b, 10c (10b, 10c hidden from view in FIG. 2), heaters; 11a, 11b, 11c (11b, 11c hidden from view in FIG. 2), cathodes; G1, G2, G3, G4 and G5, first, second, third, fourth and fifth grids, respectively; 12, a shield cup; 13a, 13b, a pair of insulating support rods; 15, a spacer; 16, an inner conductive film and 17, a stem pin.
In the electron gun assembly 7, a resistor unit 14 is located at the back surface of the insulating support rod 13a.
The resistor unit 14 is formed as shown in FIG. 3. In the arrangement shown in FIG. 3, 18 denotes an insulating board; 19, a high resistance section; T1 . . . T4, voltage pickup terminals; and CN, a connector.
If the resistor unit 14 is arranged in a narrow space in the neck 6 such that it is located near the electron gun assembly 7, a relatively complex potential distribution is created in the space in the neck of the CRT, which is caused by a potential on each electrode in the electron gun assembly 7 and on the inner conductive film 16. For this reason, a problem occurs as set out below.
That is, since the surface of the neck 6 and those of the insulating support rods 13a, 13b and resistor unit 14 are formed with an insulating material, electrons leaking from an "electrode side" opening of the electron gun assembly 7 as well as electrons emitted from the electrode in the presence of a strong electric field are accelerated from a low to a high potential zone. Upon the collision of electrons on the insulating material as set forth above, many secondary electrons are generated, moving toward the high potential section while increasing in number. As a result, a greater discharge occurs, sometimes destroying a drive circuit for the CRT, the resistor unit 14, insulating support rods 13a, 13b and so on.
Even in the case where no greater discharge takes place, a tiny steady discharge may occur between the aforementioned material and the electrode. At that time, bluish white light is observed as a discharge, causing a variation in the potential on the insulating material as set forth above and in a potential distribution around the insulating material. This variation exerts an adverse effect upon an electron lens, thus degrading an electron beam spot configuration on the phosphor screen 5 and hence reducing image quality.
As a solution to the problem as set out above, Japanese Patent Disclosure (KOKAI) 57-119437 discloses the technique of using a metal ring for surrounding such an insulating support rod against a low or a medium potential electrode. Even in the arrangement shown in FIG. 2, a metal ring SR is placed at that location of the third grid G3 as near to an electrode pickup terminal T3 as possible to surround the insulating support rods 13a, 13b and resistor unit 14 with it. The metal ring SR is heated to form an evaporated matter on the inner wall of the neck 6. In FIG. 2, reference numeral 101 denotes a metal evaporation film, that is the evaporated matter.
In the arrangement using such a technique, an electric field still stays strong in the area of the resistor unit 14 which is situated near an electrode pickup terminal T2. A tiny discharge is developed between an involved location near to the electrode pickup terminal T2 and the metal deposition film 101 on the inner wall of the neck and between that and the insulating support rods 13a, 13b, causing a variation in a division voltage on the resistor unit 14. The variation of the division voltage fails to exhibit a given performance of an electronic lens. It is, therefore, not possible to prevent a deterioration in an electron beam spot pattern on the phosphor screen 5 and in an image quality.
In the case where a given voltage is applied to a corresponding electrode on the electron gun assembly 7 through a given division resistance on the resistor unit 14 which is located near the electron gun assembly 7 in the narrow space of the neck 6, if such a metal ring SR is used so as to prevent the occurrence of a discharge in the neck 6, there is less beneficial result in the event of the resistor unit's voltage pickup terminal being higher in voltage than the metal ring SR, failing to achieve complete prevention of a discharge in the neck 6 of the color CRT, that is, to achieve a normal operation of the color CRT.