Cathode ray tube having reduced convergence drift

Details Cathode ray tube having reduced convergence drift Cathode ray tube having reduced convergence drift

2000-10-16

2003-02-04

Kim, Robert H. (Department: 2882)

Electric lamp and discharge devices

Cathode ray tube

Envelope

C313S47700R

Reexamination Certificate

active

06515411

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cathode ray tube (CRT), and more particularly, to a CRT having reduced convergence drift, including a metal coating layer electrically connected to a built-in graphite layer on the inner surface of a neck portion.
2. Description of the Related Art
In general, if power is applied to a CRT, an electron gun emits electron beams from a cathode. The emitted electron beams pass through electron beam apertures of a plurality of electrodes are focused and accelerated. The accelerated electron beams are selectively deflected by a deflection yoke installed in the cone portion of a bulb and excite a phosphor layer coated on the inner surface of a panel which forms a screen, thereby producing a picture.
As shown in
FIG. 1
, a conventional CRT
10
includes a panel having a phosphor layer on its inner surface, a funnel
12
sealed in the panel
11
, and a shadow mask
13
inwardly spaced from the panel
11
.
The shadow mask
13
is coupled to a shadow mask frame
14
. The shadow mask frame
14
is fixedly positioned to a stud pin
15
on the inner surface of the panel
11
and a hook spring
16
connected to the stud pin
15
. Accordingly, the position of the shadow mask
13
in the panel
11
is determined.
An electron gun
17
for generating electron beams producing red (R), green (G) and blue (B) light, is inserted into a neck portion
12
a
of the funnel
12
. A deflection yoke
18
for deflecting the electron beams, is installed in a cone portion
12
b
of the funnel
12
.
An inner graphite layer
19
and an outer graphite layer
100
coated on inner and outer surfaces of the funnel
12
, respectively, and thus a high voltage applied to an anode can be stabilized by forming a condenser using the glass funnel
12
as an insulator.
As known very well, the electron gun
17
includes a triode consisting of a cathode, a control electrode and a screen electrode, a plurality of focusing electrodes opposed to the screen electrode, for forming a pre-focusing lens unit, and a final accelerating electrode opposed to the focusing electrodes, for forming a main focusing lens unit.
A shield cup
110
is installed in front of the electron gun
17
. A plurality of bulb spacers
120
are fixed on the outer circumference of the shield cup
110
. The bulb spacers
120
elastically contact the inner graphite layer
19
to supply a positive voltage to the final accelerating electrode.
The CRT
10
must optimize the convergence characteristic by which R, G and B electron beams emitted from the electron gun
17
converge onto a point throughout a screen, inclusive of the center and corners of the screen. In the CRT
10
, when the electron beams are deflected, they may be shifted from their proper positions, a phenomenon which is called convergence drift.
The convergence drift is divided into thermal drift and charge drift. Specifically, the charge drift is caused by a change in the potential of the neck portion
12
a
due to the condition of the outer surface of the neck portion
12
a
when a high voltage is applied to the CRT
10
. The initial potential of the neck portion
12
a
is attributed to accumulation of charge due to electron beam current, causing an increase in the convergence error.
To overcome the problem, U.S. Pat. No. 4,868,454 discloses a method of stabilizing the potential of the surface of the neck portion with a metallic mirror coating on the inner surface of the neck portion. However, according to this method, convergence drift is 0.2 mm or greater, that is, the effect of removing charge is weak and occurrence of arcing is highly probable.
U.S. Pat. No. 5,536,997 discloses that an enamel layer electrically contacting a conductive layer coating the inner surface of a neck portion. The formation of the enamel layer relatively reduces convergence drift. However, this method has the following problems. First, the process of manufacturing a CRT is relatively complex. In other words, a conductive layer made of graphite is applied to the inner surface of the neck portion and dried. Then, an enamel glass solution is placed in contact with the conductive layer. During this procedure, the conductive layer and the enamel layer are electrically connected. Thus, forming the enamel layer is further necessary. Second, the arcing characteristic is poor. In the course of sealing the electron gun into the neck portion, it contacts the inner surface of the neck portion. Here, since an enamel layer having a predetermined thickness is present on the inner surface of the neck portion, when the bulb spacer is mounted at a proper position it scratches the enamel layer, which provides for a path for discharge. Also, the particles of the scratched enamel layer float between the focus electrode and the final accelerating electrode, resulting in a discharge between electrodes.
SUMMARY OF THE INVENTION
To solve the above problems, it is an object of the present invention to provide a CRT which can definitely reduce convergence drift with a metal coating layer on the inner surface of a neck portion.
Accordingly, to achieve the above object, there is provided a cathode ray tube having reduced convergence drift including a panel having a phosphor layer on its inner surface, a funnel sealed in the panel and having an inner graphite layer and an outer graphite layer on inner and outer surfaces, respectively, an electron gun in a neck portion of the funnel and consisting of a cathode, a control electrode, a screen electrode, a plurality of focusing electrodes, a final accelerating electrode and a shield cup, and a metal coating layer electrically connected with the inner graphite layer on the inner surface of a neck portion and having the surface resistivity of 10
7
&OHgr;/□ or less.
Here, the metal coating layer is preferably formed on the inner surface of the neck portion higher than the top surface of the focusing electrodes.
Also, the metal coating layer is preferably electrically connected with the final accelerating electrode via the shield cup.
Also, the metal coating layer may be selectively formed on the inner surface of the neck portion adjacent to side electron beam apertures for red and blue electron beams.
Further, the metal coating layer is preferably a metal thin film having either iron or chrome as a main component. Also, the metal coating layer may be a metal thin film having both iron and chrome as main components.


REFERENCES:
patent: 4052641 (1977-10-01), Dominick et al.
patent: 4101803 (1978-07-01), Retsky et al.
patent: 4153857 (1979-05-01), Delsing et al.
patent: 4403170 (1983-09-01), Misono et al.
patent: 4868454 (1989-09-01), Paridaens
patent: 5536997 (1996-07-01), Van Hout
patent: 6-260112 (1994-09-01), None

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