Note: Descriptions are shown in the official language in which they were submitted.
~13-53Z~
RCA 73,479
TI1TED UNITARY DEGAUSSING COIL ARRANGEMENT
This invention relates to an arrangement for
degaussing color picture tubes having magnetic shields by
use of a unitary degaussing coil in a tilted or skewed
position.
Color television displays are often accomplished
by means of shadow-mask picture tubes. In such tubes, a
plurality of electron beams are directed from slightly
different points of origin towards a screen coated with
fluorescent phosphors. The phosphors are grouped in
triads or groups of three, each member of which
fluoreseces in a different primary color when illuminated
by an electron beam. Each member of each triad is made
responsive to a single one of the plurality of electron
beams by means of a shadow-mask. The shadow-mask is a
thin conductive shield with a multitude of perforations
precisely located with respect to each triad of color
phosphors. The shadow-mask ideally allows the phosphor
of a particular color to be illuminated only by an
electron beam originating at a particular location. Thus,
~5 with three electron beams originating in slightly different
locations, each of the color phosphors of a trio is pure,
i.e. t iS illuminated by only one electron beam, and
gradations of color may be achieved by appropriate control
of the source of electrons.
In the past, color television displays were
sensitive to the influence of the earth's magnetic field.
The magnetic field passing through the kinescope deflects
the electron beams away from their intended paths and
changes the apparent source of the electron beams arriving
at the shadow-mask in a manner dependent upon the
orientation of the picture tube relative to the earth's
magnetic field, thereby impairing color purity. Early
attempts to correct this problem involved the use of field
neutralizing coils, as described in United States Patent
~3~2~ .
1 -2- RCA 73,479
No. 2,921,226 issued January 12, 1960 to Vasilevskis. In
the field neutralization arrangement, a direct current is
passed through one or more coils arranged about the
kinescope in such a manner as to balance to zero or
neutralize the earth's field. This required adjustment by
an expert, and required readjustment if the television
was moved to a new location.
Another approach to correcting color purity
problems resulting from the earth's magnetic field
involves the use of magnetic shields. In such an
arrangement, a hollow shield in the form of the frustum
of a cone surrounds the region through which the electron
beam travels before reaching the shadow-mask and screen.
The high permeability of the shield diverts extraneous
magnetic fields away from the electron beam. It was
found, however, that the permeable magnetic shield was
capable of becoming magnetized and, when so magnetized,
itself perturbed the color purity. Degaussing arrangements
were then devised to demagnetize the shield and the shadow-
mask. The degaussing is accomplished by passing an
alternating current of initially large magnitude through
one or more coils arranged about the shadow-mask and
magnetic shield as described in United States Patent
No. 2,962,621 issued November 29, 1960 to Fernald. The
magnitude of the alternating current is then gradually
reduced to zero and the magnetization of the shield and
shadow-mask is reduced. A coil disposed along the periphery
of the shadow-mask as illustrated by Fernald tends to
produce a relatively small magnetic field near the center
of the shadow-mask and screen. Thus, effective degaussing
with such a coil requires currents which may be undesirably
large.
In order to increase the magnetic flux through
the magnetic shield and through the screen without
excessive coil currents, arrangements such as those
described by Norley in United States Patent No. 3,322,998
issued May 30, 1967 and by Matsushima et al., in United
~13~ -'CA 73,479
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States Patent No. 3,872,347 issued March 18, 1975 were
devised. In the Norley and Matsushima arrangements, a
pair of coils are arranged symmetrically on the picture
tube, with each coil having a portion lying along the
periphery of the shadow-mask and a portion extending towards
the neck of the tube. The two-coil arrangements produce
a magnetic field the lines of which are transverse to or
across the longitudinal axis of the tube. "`his "cross
axial" degaussing field approach using a pair of coils
provides effective degaussing. However, the two coils
require additional labor for mounting as compared with a
single coil, and their interconnection may give rise to
incorrect wiring. Furthermore, the two-coil arrangement
tends to require a greater length of conductor than the
single-coil type, and may respond to the deflection yoke
field with currents which perturb color purity.
It is desirable to have a degaussing arrangement
using a unitary coil requiring a relatively short conductor
length which provides degaussing performance substantially
equivalent to that of the two-coil system, combined with
the simplicity and short conductor length of the single-
coil type.
In a preferred embodiment, the present invention
includes a degaussing arrangement for a color television
picture tube. The picture tube comprises an envelope having
a flared portion with large and small ends, a faceplate
portion adjacent the large end, and a neck portion coaxial
with the flared portion and joined to the small end. The
picture tube further comprises a shadow-mask enclosed
within the envelope adjacent the faceplate portion and an
electron gun assembly enclosed within the neck portion of
the envelope. A flared magnetic shield is enclosed within
the envelope and has large and small ends. The shield is
3~ adjacent a segment of the flared portion of the envelope.
he degaussing arrangement further comprises a single
degaussing coil encircling the flared portion of the
envelope, with the axis of the degaussing coil tilted relative
to the axis of the flared envelope portion in such manner as
to dispose a first segment of the circumference of the coil
~1353~ RCA 73,479
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in a position overlying a region of the envelope adjacent a
portion of the large end of the shield. A second segment of
the circumference of the coil, diametrically is disposed
oppositely to the first segment, in a position which is
- contiguous with a region of the envelope adjacent a portion
of the small end of the shield and which is located more
remotely from the axis of the flared envelope portion than
is the periphery of the opening at the small end of the
shield. Means are also provided for supplying a variable
alternating current solely to the single degaussing coil
to effect degaussing of the shadow-mask and the magnetic
shield.
- In the Drawings:
FIGURES la and lb illustrate in perspective views
a picture tube with a degaussing arrangement embodying the
invention;
FIGURES 2a and 2b illustrate in respective rear
and side views the arrangement of FIGURE la and lb;
Z FIGURE 3 is a cross-sectional view of the picture -~
tube of the preceding FIGURES;
FIGURE 4billustrates magnetic field distributions
associated with the invention and FIGUR~S 4a and 4c illustrate
magnetic field distributions associated with the prior art;
and
FIGURE 5 is a semipictorial diagram containing
block, schematic and pictorial portions depicting the
arrangement of the invention as used with a television
receiver.
Referring to the drawings:
In FIGURES la and lb, a picture tube designated
generally as 10 includes an envelope consisting of three
distinct portions. A frusto-conical or pyramidal portion
12a of the envelope is joined at the small end of the frusto-
35 conical section with a neck portion 12b of the envelope. A
flattened faceplate portion 12c closes the large end of
frusto-conical portion 12a. A base 14 at the end of neck
section 12b remote from the frusto-conical portion supports
pins by which connections are made to the electron gun
40 assembly mounted within neck portion 12b. A high-voltage,
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~35~Z~ RCA 73,479
1 -5-
anode or ultor connection button 16 is located on the surface
of frusto-conical section 12a. ~ degaussing coil designated
generally as 20 is located on the exterior surface of
envelope 12. Coil 20 has a generally rounded triangular
shape. One leg 20a of coil 20 is adjacent the periphery of
the junction of faceplate 12c and frusto-conical section 12a
of the envelope. i~dditional legs 2Ob and 20c of coil 20
form a vertex extending towards and around the junction of
the neck and frustum portions of the envelope. A pair of
leads 22 are taken from coil 20 for connection to a source,
not shown, by means of which a current of variable
amplitude is passed through coil 20 for degaussing.
FIGURES 2a and 2b illustrate in respective rear
and side views the arrangement of FIGURES la and lb. The
rear view of FIGURE 2a illustrates the somewhat rectangular
outline of the large end of the frusto-conical section which
is common in the picture tubes currently in use. r~he neck
portion, however, is circular as seen in this rear view for
ease in mounting deflection yokes and other neck components.
The top cross-sectional view of kinescope 10 in
FIGURE 3 illustrates a phosphor screen 330 adjacent the
inside of faceplate 12c of the envelope. A shadow-mask 332
is mounted near phosphor layer 330 by means of mounting
brackets 334 and 336. ~lounting brackets 334 and 336 also
support a hollow magnetic shield 338 formed of a magnetically
permeable material. Shield 338 preferably has the general
form of frusto-conical portion 12a of the envelope of picture
tube 10. Both the large and small ends of shield 338 are
open to allow the passage of one or more electron beams from
an electron gun assembly 340 mounted within neck portion 12b
to phosphor layer 330.
l`he direction of magnetic field distribution
generated by a degaussing coil mounted concentric with the
35 picture tube is illustrated in cross-sectional view in
FIGURE 4a. In a test of such an arrangement, the magnitude
of the field available for degaussing at the center of screen
332 was 0.2 gauss with a coil producing a maximum of 20 gauss.
The coil also produced between 0.3 and 0.6 gauss at the
40 periphery of the opening at the small or entrance end of
~ 1 3S 3 Z~ RCA 73,479
1 -6-
magnetic shield 338.
By contrast, the inventive arrangement has as its
core more of magnetic shield 338, and produces a magnetic
field distribution somewhat as shown in FIGURE 4b. The
center of the screen is at a region of greater field
strength. Using a degaussing coil producing the same
maximum field strength as that of FIGURE 4a, the degaussing
field at the center of the screen was 0.5 gauss, which is
almost double that of the prior art arrangement. The
corresponding field strengths at the entrance end of
magnetic shield 338 were 7.5 gauss at the side nearest the
coil, l gauss at the side farthest from the coil and 4 gauss
at the other two sides. Thus, the fields available for
16 degaussing the magnetic shield are much greater in the inven-
tive arrangement than in the arrangement of FIGURE 4a when
the coils have the same number of ampere-turns.
FIGURE 4c illustrates the magnetic field distribu-
tion associated with two-coil arrangements such as those of
Norley and ~latsushima. In the arrangement of FIGURE 4c, the
combined effect of the pair of coils causes a substantial
magnetic field flux transverse to the axis of the picture
tube in the region of magnetic shield 338. At the faceplate
and shadow-mask end of the tube, the magnetic field lines
are parallel with the shadow-mask and tend to have a uniform
strength through the center of the shadow-mask. The
degaussing provided by the inventive arrangement is substan-
tially equivalent to that provided by the two-coil arrange-
ment of FIGURE 4c in that it provides degaussing for the
internal magnetic shield as well as for the mask and frame
assembly. The inventive degaussing arrangement is less
costly by virtue of requiring less conductor length than
either the single-coil arrangement of FIGURE 4a or the two-
coil arrangements. The inventive arrangement is easier to
assemble than the two-coil arrangements and cannot be wired
incorrectly.
FIGURE 5 illustrates a circuit by which the
degaussing coil may be energized with an alternating current
having an initially large amplitude which decreases to a
substantially zero amplitude. In FIGURE 5, a portion of a
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- ~35Q~Z~ ~CA 73,479
television receiver includes a plug 510 adapted to be
connected to the alternating power mains. A switch 512 and
fuse 514 couple the mains across primary winding 516a of a
power transformer 516. A secondary winding 516b of trans-
former 515 produces alterna~ing current which is utilized by
a power supply illustrated as a block 518 for energlzing the
remainder o~ the television receiver, illustrated as a block
520. The alternating-current mains are also coupled through
a thermally-response resistor 530 to leads 222 of coil 220.
Thermally-responsive resistor 530 has a positive temperature
characteristic, 50 that the resistance of the thermistor is
low at room temperature and increases with increasing
temperature.
At room temperature, the resistance of thermistor
530 is low. When switch 512 is closed in order to energize
the television receiver, power is applied through transformer
516 to power supply 518 to energize the receiver. At the
same time, an alternating voltage is applied across the
series combination of thermistor 530 and coil 220. Because
the resistance of thermistor 530 is initially low, a relative-
ly large alternating current flows through coil 220. The
current flow through thermistor 530 causes the thermistor to
heat, and its resistance to rise. The increasing resistance
decreases the current flow resulting from the applied voltage
and partially compensates for the reduced power dissipated in
the thermistor at the decreased current. The temperature of
the thermistor continues to rise, resulting in a rapid
reduction in the current to a small limiting value as the
30 resistance of the t~m~tor reaches a maximum. A circuit
arrangement using temperature-responsive resistors is des-
cribed in United States Patent 4,~24,427 issued to Belhomme
on ~lay 17, 1977.
'hile the arrangement as described uses an
35 internal magnetic shield, those skilled in the art will
recognize that the principles of the invention are equally
applicable to a kinescope fitted with an external shield.
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