Electric lamp and discharge devices – Cathode ray tube – Ray generating or control
Reexamination Certificate
2001-01-09
2002-01-01
Day, Michael H. (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Ray generating or control
C313S270000, C313S271000, C313S037000, C313S337000
Reexamination Certificate
active
06335590
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a cathode ray tube and particularly to a cathode ray tube provided with a heater having improved immunity against mechanical shock caused in the operation of welding the heater to heater supports in fabrication of an indirectly heated cathode structure and having improved immunity against adverse effects caused by thermal expansion in a manufacturing process of a cathode ray tube.
In general, color cathode ray tubes such as a color picture tube and a color display tube comprise an evacuated envelope (a glass bulb) formed of a panel portion having a faceplate, a neck portion and a funnel portion for connecting the panel portion and the neck portion, a phosphor screen formed on an inner surface of the faceplate including a multiplicity of phosphor elements of three colors, a shadow mask having a multiplicity of apertures therein and spaced from the phosphor screen in the panel portion, a three-beam in-line electron gun housed in the neck portion for generating three electron beams and projecting the electron beams through the shadow mask to the phosphor screen, an inner magnetic shield of generally truncated pyramidal shape extending from the interior of the funnel portion into the panel portion and having openings on the shadow mask side thereof and the electron gun side thereof, and a deflection device mounted in a vicinity of a transition region between the funnel portion and the neck portion.
Three electron beams emitted from the electron gun are deflected appropriately by the deflection device, travel through the inner magnetic shield, pass through beam apertures in the shadow mask, impinge upon the phosphor screen and excite the phosphor elements of desired colors to generate light and to display a desired image on the faceplate.
The three-beam in-line electron gun housed in the neck portion includes three indirectly heated cathodes arranged in a line, and the first, second, third, fourth, fifth and sixth grid electrodes arranged in axially spaced relationship in this order on the electron beam exit side of the three indirectly heated cathodes. Each indirectly heated cathode includes a metal sleeve, a cap-shaped base metal having an electron emissive material coating on an outer top surface thereof and fitted over one end of the metal sleeve, a heater positioned within the metal sleeve, and heater supports each having a cross section of the shape of a square bracket and welded to a leg portion of the heater.
FIGS. 3A
,
3
B and
3
C are schematic illustrations of a prior art indirectly heated cathode structure for a cathode ray tube,
FIG. 3A
being its cross sectional view,
FIG. 3B
being a plan view, partly broken away, of a heater of the cathode structure, and
FIG. 3C
being an enlarged view of a portion of the heater indicated by circle
60
in FIG.
3
B.
In
FIGS. 3A
,
3
B and
3
C, reference numeral
31
denotes a metal sleeve,
32
is a cap-shaped base metal,
33
is an electron emissive material coating,
34
is a heater,
35
is heater supports,
36
is a heating wire,
37
is an insulating coating,
38
is a dark color coating, and
39
is leg portions of the heater
34
.
The cap-shaped base metal
32
is fitted over one end of the metal sleeve
31
and is coated on its outer top surface with an electron emissive material layer
33
.
The heater
34
comprises the spirally wound heating wire
36
made of tungsten (W), the insulating coating
37
made of alumina (Al
2
O
3
) and covering the heating wire
36
and the dark color coating
38
made of fine tungsten powders and covering the insulating coating
37
.
The heater
34
is provided with a major heating portion formed of the heating wire
36
spirally wound and is inserted into the metal sleeve
31
. The leg portions
39
of the heater
34
comprise a covered portion
39
A covered with the insulating coating
37
and the dark color coating
38
and an exposed portion
39
B with the heating wire
36
being uncovered. The exposed portions
39
B are welded to one end of the two heater supports
35
, respectively.
The metal sleeve
31
is supported concentrically with and within an outer support sleeve
40
which in turn is supported by glass beads
41
.
The heater supports
35
are supported by the glass beads
41
via support studs
42
such that the major heating portion of the heater
34
is positioned within the metal sleeve
31
.
The major heating portion of the heater
34
is formed of the heating wire
36
spirally wound, and each of the leg portions
39
of the heater
34
is of the three-layer winding form in which the heating wire
36
is spirally wound in three layers by doubling back the heating wire
36
upon itself at each end of the leg portion
39
.
Fabrication of the three-layer winding structure of the heating wire
36
in the leg portion
39
comprises winding first the heating wire
36
spirally at a fine pitch from one end of the leg
39
to the other end thereof, then doubling back the heating wire
36
and winding it spirally at a coarse pitch from the other end thereof to the one end thereof, and again doubling back the heating wire
36
and winding spirally it at a fine pitch from the one end thereof to the other end thereof. This structure of multilayer winding of the heating wire is hereinafter referred to as the primary winding structure.
The heating wire
36
formed into the primary winding structure is again wound spirally to form the major heating portion of the heater
34
to be positioned within the metal sleeve
31
. This structure of the large-diameter spiral winding of the heating wire of the major heating portion is hereinafter referred to as the secondary winding structure.
The heating wire
36
having the secondary winding structure is coated with alumina (Al
2
O
3
) except for the exposed portion
39
B of the leg portions
39
of the heater
34
, is covered with fine tungsten (W) powders on the alumina coating, and then is fired at a high temperature, 1650° C., for example. The fired heating wire
36
is immersed in a mixed solution of hydrochloric acid (HCl) and nitric acid (HNO
3
) to dissolve molybdenum (Mo) having served as a mandrel for winding the heating wire and to complete the heater
34
. The heater as described above is disclosed in Japanese Utility model Publication No. Sho 57-34671, for example.
The heater
34
has sufficient resistance to sparks and mechanical shock because of its three-layer winding structure of the heating wire
36
in its leg portions
39
and has good workability in the operation of welding the exposed portions
39
B of the leg portion
39
to the heater supports
35
.
SUMMARY OF THE INVENTION
Although the prior art heater of the three-layer winding structure for the indirectly heated cathode structure has a sufficiently high resistance to sparks and mechanical shock, the increased strength of the heater leg portions easily causes damages such as cracks in the insulating coating made of alumina (Al
2
O
3
) during the operation of welding the exposed portions to the heater supports.
There is a problem in that the damages such as cracks caused in the insulating coating extend and a portion of the insulating coating comes off in flakes when the heater is turned on during the operation of manufacturing a cathode ray-tube.
The flakes from the insulating coating scatter within the evacuated envelope of the cathode ray tube and degrade the performance of the cathode ray tube. The flakes stuck between electrodes of the electron gun deteriorate withstand voltage characteristics of the cathode ray tube, and the flakes stuck in beam apertures in the shadow mask of the cathode ray tube prevent phosphor elements associated with the beam apertures from luminescing.
An object of the present invention is to solve the above-mentioned problems of the prior art and is to provide a cathode ray tube which is free from peeling of the insulating coating of its heater and degrading its performance when the heater is turned on, and which is low-cost and superior in mass productivity.
To accomplish th
Iwamura Norio
Koizumi Sachio
Antonelli Terry Stout & Kraus LLP
Day Michael H.
Hitachi , Ltd.
Williams Joseph
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