US2817782A - Cathode ray tube deflection apparatus - Google Patents

Description

2 Sheets-Sheet 1 D. P. OVER E'I'L CATHODE RAY TUBE DEFLECTION APPARATUS Dec. 24, 1957 Filed July 9, 1954 HND www Dec. 24, 1957 D. P. OVER s-rAl.A 2,817,782

cATHom-z RAY TUBE DEFLECTION APPARATUS Filed July `9, 1954 2 Sheets-Sheet 2 CATHoDE RAY rtian nnrLncrroN APPARATUS Donald P. Over, Barrington, and William H. Bjarkow,

Pennsauken, N. J., assignors to Radio Corporation of America, a corporation of Delaware Application July 9, 1954, Serial No. 442,412

Claims. (Cl. 313-76) For various facturing tolerances of the kinescope beam-forming electrodes and the deflection apparatus itself, there has long been present the need for effective beam-centering means.

In the case of electromagnetic deflection systems wherein two pairs of electromagnetic coils are furnished with suitable current wave forms of specific vertical and horizontal frequencies, several schemes have been proposed in the past for effecting proper centering of the electron 1 beam. One such scheme is that of supplying a unidirectional current to the deflection coils whereby to afford a fixed magnetic component to the varying magnetic field setup by the currents in the coils. While this proposal, known as D. C. centering, has been found to be quite effective in use, it is a costly system requiring the addition of components to the deection system which would other- Wise be unnecessary.

Another known arrangement for effecting beam centering in a cathode ray tube is that which provides permanently magnetized members associated with the path of the electron beam for providing a xed magnetic field In the case of wide angle kinescopes wherecomponent. in the beam may be deflected through 90, for example it is necessary to maintain the center of deflection as close to the llared portion of the kinescope as is practicable, in order to prevent the beam from striking the neck of the tube in a manner which causes an objectionable shadow effect.

It is a primary object of the present invention to prov vide simplified but effective electron beam centering means of the permanent magnet type.

A further object of the invention is the provision of electron beam centering means, which means does not undesirably shift the deflection center rearwardly of the kinescope neck.

In order to illustrate the utility and efhcacy of the present invention it will be described herein in connection with one popular form of commercial television receiver deflection arrangement which is of itself of simple and inexpensive construction. As is well-known to those skilled in the art, presently used electromagnetic deflection arrangements for providing the horizontal scanning energy are mainly of the so-called reaction scanning type in Jhichdellection during a portion of the scanning time is due to the action of a driver tube coupled through a transformer to the horizontal deflection coils, While the remaining portion of the scan is effected by the conduction of a damper tube connected in shunt with the coils. That is to say, the inductance of the coils and transformer forms, together with the inherent capacities of the system, a resonant circuit which furnishes the energy for rapid llyback of the beam to an extreme position on the tube screen. After one-half cycle of the free oscillation, the damper tube is rendered conductive and serves to damp the remaining cycles of the ringing. Normally, and as described andk claimed in U. S. Patent No. 2,598,134, granted May 27, 1952, to O. H. Schade for Power Conservation System, the current through the damper tube is stored in a large B-boost capacitor for increasing the available operating potential for the driver tube. Moreover, conventional systems often include a linearity inductance in circuit with the B-boost capacitor and damper tube for improving the linearity of the scan, as described and claimed in U. S. Patent No. 2,440,418, granted April 27, 1948 to S. I. Tourshou for Cathode Ray Beam Dellecting Circuit.

l The matter of beam-centering has been found to be particularly acute with low cost television deflection arrangements from which the linearity inductance is omitted in the interest of economy. That is to say, the omission of the linearity inductance has been found to result in the cramping of the righthand portion of the scanned raster,

lsuch that the geometric center of the distorted raster is actually not coincident with the physical center of the tube screen. While it would be possible to decenter the beam through the use of direct current centering energy applied to the deilection coils, such a solution is inimical tothe interests of economy.

Moreover, in the case of wide-angle deflection kinescopes, and as has been mentioned briefly supra, any permanent magnet centering device which is located any appreciable distance from (i. e. nearer the electron gun) 'the effective deilection center of the yoke is objectionable because of its production of the shadow effect resulting from the beams striking the tube neck.

Hence it is still another object of the present invention to provide simple and inexpensive, yet extremely elfective, permanent magnet beam-centering means, which means is particularly well-adapted for use with low-cost, wide angle deflection yokes.

In general, the present invention may be understood as follows: commercial television deflection yokes comprise a pair of arcuate horizontal coils surrounded by a similar pair of vertical coils disposed at right angles t-o the horizontal coils. In order to increase the efficiency of the coils, it is also common practice to provide a magnetic core surrounding the longitudinal conductors of the coils in such manner as to afford a low reluctance return path for the electromagnetic deflection llux. Such cores are normally made of a plurality of ferrite sections which together form a cylinder. A strap of steel or similar material surrounds the core sections and holds them securely about the deflection coils, thereby forming a completely self-contained and rigid structure. The present applicants have found that the amount of unidirectional magnetic centering energy necessary for correctly decentering the electron beam in a situation as set forth above may be had -by permanently magnetizing the steel strap aimsa which," as stated, is included in deilection'yokes for securing the multisection magnetic core about the coil. Since the trap is located at substantially the center of deflection of the yoke, it does not deleteriously move the center of deilection rearwardly as is the case with certain known permanent magnet rings, for example, which are normally disposed behind the yoke.

Additional objects and advantages of the present invention will become apparent to persons skilled in the art from a study of the following detailed description of the accompanying drawing, in which:

Fig. l illustrates, by way-'of yblock'andschematic circuit diagram, a television receiver of well-known'form in conjunction with which'the'presentiiinvention maybe advantageously employed;

Fig,- 2 is anexploded, isometric View` of'fan" electro'- magneticV deflection yoke `embodying the invention;

Fig. 3 ris a side elevational rview-off-the yoke of Fig. 2 in i-ts'assembled form;

Fig.4 is a horizontal sectional viewrl of a television receiver kinescope illustrating certainY beam'- pathsuseful in explaining the operation offtheinvention;` and Fig. 5 vis'a schematic view ofone-'suitable*arrangement-14 forV producing the apparatusl of 'thelinventiom Referring to the drawing, there is ill'ust'ratedin Fig. l l a complete television receiver of commercialform" andi of llow cost design, as 'will become apparent* Radit'r'fre' quency carrier waves-modulatedV by Video-*intelligence are interceptedby anantenna and applied toa televisiontuner y'12 which may, A for example,v comprise the lusualradiofrequency-(R. F.), mixer, intermediate frequency* (I.`F.) and second detector stages. V"The,detected'fvideoe information is amplified by v*aYstagef14whoseoutput terminal 16 is designated for connectionfto the b 'eamintensity controlling electrodeI -of the image-reproducing kinescopel. The detected-signals,"- whichdnormallyr in- 'Y clude synchronizing pulses '-offleld' and line rate Y(e. g. 60 fC. P. S. and"l5,'750'C. P. S.),are also* applied to af' sync'puls'e separating c'ircuit'Zl)` which applies'tlie ver-'5 tical-synchronizing pulses to thevertical scanning circuits 22 whose output terminals-Y--Y are adapted for connection to the vertical rdellection winding 24 associ-vv ated with the kinescope 18.

Horizontal or line ratesync'pulses-from-the separator circuit 20 are applied to a horizontal deflection oscillatorA 26 which produces a sawtooth signal 28 of the proper frequency. Thee-sawtooth waveform 28 is applied via a capacitor V30 and resistor 32 to :the control electrode 34 of ythe horizontal output or driver tube 36-`of conventional form. The anode -38fof thev driver'tube- 36: is connectedl to 'a'vr tap 40 von the horizontal outputtransformer-WIWf illustrated herein-as an -auto-transformer. f-Connected# across that portion of thetransformer T'between its taps42 and 44 are the horizontaldellection coils46,4 isolatedfrom'- the transformer direct current-wise by-means--- of the blocking capacitor 48.v 'A conventional balancing circuitV comprising thefcapacitors S0; 52- and 545l and thef resistor S6 is connected as shown for the purpose of1eliminating ringing inthe system, all in a wellknown man nerJA high voltage'or "llybackv step-up winding 58V applies flyback voltagelpulses to the anode of ya high voltage rectifier diode 62' which, in vconjunction with the filter capacitor 64, provides a high, unidirectional accelerating potential for application via the lead 66 'to the'high -voltage anode terminal 68of the kinescope 18. A damper diode 70 has its cathode 72 connected to the transformer T at point 74 and its anode connected tothe-B+ terminal-'76. A B-boost capacitor 78 is yconnected, as shown, between the terminal 76 and the terminal 44 at the bottom end ofthe'transformerwinding.

Since the above-described dellectioncircuitis--wellknown and does not per se constitutea part ofthe present invention, its operation need not be described in de-- tail. Briefly, however, it may be notedthat-,in accordance with conventional reaction scanning techniques, thej driver tube 36-furnishes a sawtoothy current :through:A thetransformer T'for` a portion of the go time of the horizontal line scan. At the end of the scanning the tube 36 is driven to cut-oil, thereby producing a llux col' lapse in the transformer and coils. The inductance of' the circuit and its capacities form a resonant circuit which oscillates at a predetermined frequency several times greater than the horizontal scanning rate, which oscillation results in the ilyback or retrace of the beam in the kinescope. At the end of one-half cycle of the free oscillation, the damper tube 70 is rendered conductive and serves to damp the remaining cycles of oscillation. Current through the damper tube also provides the energy for the deflection coils for approximately the first half of the succeeding line scan.l

As pointedout in the above-cited Schade patent, a

portion of the energy in the circuit is stored in the ca-V pacitor 78 which is charged as shown, whereby to boost the B+ potential applied to the driver tube. In more expensive horizontal deflection arrangements, a linearity inductance according to the above-cited Tourshou patent is ordinarily connected between the B| terminal 76 and the junction of thefcapacitor 78 and damper diode 70. The function of the linearity inductance is, in general, that of purposely distorting the sawtooth current in the dellection winding by rounding off the sawtooth to compensatef'for screen flatness of the cathode ray tube andthe like. 1That1 is, the linearity inductance causes a voltage variablevwith time to produce this effect in the yoke current. It has been found that omission of the linearity inductance as an economy measure results in an'undesirablefcra'mping of the raster produced on the screen'o'f the kinescope 18, which cramping` causes theL rasterto appear off-center.

In orderf'tofcornpensat'e for the off-center appearance which results yfrom the omission of a linearity inductance,

longitudinal conductors of the coils 24 and 26 in order to furnish a lowreluctance return path for the electromagnetic iluxk produced by the coils, thereby increasing their"efflciency. Since the core is, asI stated, actually a plurality'of separa-te, arcuate pieces, the usual practice is to'provid'e ay strap of lsteel for holding the core pieces together around the dellection coils. The present invention provides such a strap 82 of generally cylindrical shape having afraised bridge 83 which encloses a nutV (not shown)adapted'to be threadedly engaged by a bolt (notfshown) through the slot 84for the purpose of mounting the -yoke on the receiver chassis.

secured to each other as by means of one or more screws 86 which, when tightened, securely hold the sections of the core around the deflection windings.

In the past, any attempt to magnetize the strap 82 would have been considered highly undesirable, since such magnetization would necessarily have been looked upon as interfering with the action of the deflection windings 24 and 26. The present applicants, however, have found that by magnetizing the strap 82 permanently, as shown in Figs. 2 and 3 (i. e. transversely or diametrically), certain advantageous results are obtained, which may be better understood from the showing of Fig. 4.

In Fig. 4, which is a sectional view taken along that plane which yseparates the horizontal deflection coils 46 yfrom each other, the above-described cramping action of thelow cost deflection-circuit of Fig. 1 is illustrated. l The :kinescope 18- includes an electron gun y(not shown) for formingand directing an electron beam (not shown) along the center line 88 of thekinescope toward itsA luminescent phosphor Iscreen 90. The kinescope com-.-

The ends ofthe strap 82 diametrically opposite the bridge 83 arel prises generally a cylindrical neck portion 92 and a flared bulb portion 94. The deflection coils 24 and 46 surround the neck portion 92 adjacent the bulb 94 and, when respectively energized with vertical and horizontal deflection currents as described, cause the beam to scan a rectangular raster. Disregarding the vertical deflection action, the horizontal coils 46 cause the beam to scan across the screen 90 through the angle defined by the dotted lines AX and BX. As may be seen from the drawing, the cramping of the righthand side of the raster (i. e. the upper part as viewed in the drawing) results in a situation in which the apparent center line of the raster is at line OX which is angularly displaced from the vertical center line OX of the kinescope 18. Thus, as will be understood, the resultant raster appears decentered insofar as the physical center of the kinescope is concerned. Such decentering could, of course, be corrected by causing a unidirectional current to flow through the horizontal deflection coils 49 in such direction as to shift the beam by a fixed amount, but would require additional circuitry, thereby undesirably increasing the cost of the apparatus. Also, it would be possible to effect the necessary centering correction by means of a permanent magnet annulus around the kinescope neck 92 behind the yoke, as taught in the prior art. This latter possibility is, however, ruled out because it would necessarily shift the center of deflection of the yoke rearwardly, so that the beam path at its extremes of deflection would strike the neck of the kinescope at the points 96, thereby causing a shadow to appear in the image on the screen 90.

The present invention, on the other hand, by reason of its permanently magnetized strap 82, shifts the beam center so that deflection of the beam, through the combination of the fixed magnetic field from the strap and the sawtooth field from the windings 46, is defined by the angle AXB shown in solid lines. In order further to illustrate the operation of the invention, the reference line 100, drawn at right angles to the kinescope center line 8S, intersects the lines representative of the extreme deflection paths, from which it may be noted that the distance AO' is equal to the distance OB', where O lies on the physical center line 88 of the kinescope. Hence, it will be appreciated that, without distorting the raster, the permanent magnetic field from the strap 82 brings about that recentering of the electron beam which is necessary to effect coincidence of the raster center with the kinescope center.

Although the permanently magnetized strap 82 surrounds the ferrite core 80 (being separated therefrom by an insulating liner 102), so that the core 80 represents a magnetic shunt for the strap, enough of the flux from the strap passes through the core to furnish the requisite unidirectional field within the neck 92 of the kinescope. In one practical form of the invention, it has been found that magnetizing the strap 82 so that it has a strength (measured at the location of the screws 86) of approximately 30 Gauss is sufficient to produce an effective field within the kinescope neck of between 2 and 3 Gauss.

While any suitable means for effecting permanent magnetization of the strap 82 may be employed in producing the yoke of the present invention, one simple but effective arrangement is illustrated schematically in Fig. 5. That is to say, although the strap may be provided with its magnetization by passing it through a strong D. C.

v magnetic eld, a simpler method is that of passing direct current through the horizontal deflection winding 46 in the assembled yoke (Fig. 3), which current results in the permanent magnetization of the strap 82. The arrangement of Fig. 5 includes a source of direct current of, for example, 40() volts illustrated as a battery 104 and a large capacitor 106. A switch 108 is adapted for connection to either of the terminals 110 and 112, as shown. With the switch 108 in contact with terminal 110, the capacitor 106 will be charged by the battery 104 with the polarity estarse shown. The rotation of the switch 108 into contact with the terminal 112 causes the capacitor 106 to discharge through the horizontal deflection coils 46, current flow being indicated by the arrow 114. Once the capacitor 106 has been discharged through the horizontal coils, the strap 82 will be magnetized in such amount as to produce a fixed flux field within the kinescope neck of, for example, approximately 4 to 6 Gauss. The magnetization of the strap is stabilized by operating the yoke with deflection currents of the usual frequency, whereupon flux density is decreased by about 50%, after which it remains stable.

From the foregoing, those skilled in the art will recognize that the present invention requires no additional parts for effecting the beam centering action necessary for proper image reproduction. The apparatus of the present invention is, therefore, capable of low cost production in keeping with commercial aims.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent is:

1. An electromagnetic deflection yoke for use with a cathode ray tube, said yoke comprising: a first pair of deflection coils; a second pair of deflection coils arranged at right angles to said first pair of coils, such that the magnetic fields produced by said first and second pair of coils are substantially normal; magnetic core means cornprising a plurality of longitudinal sections adapted to form a generally continuous low reluctance magnetic path around said coils; and a strap surrounding said core sections in such manner as to hold said sections securely around said coils, said strap being permanently magnetized transversely so that its flux lines pass between said longitudinal core sections and lie in planes parallel to the magnetic fields of said coils.

2. An electromagnetic deflection yoke for use in association with a kinescope, said yoke comprising: a first pair of arcuate coils adapted to surround a kinescope neck; a second pair of arcuate coils surrounding said first pair of coils and arranged at right angles thereto; a generally cylindrical magnetic core means comprising a plurality of longitudinal sections of a cylinder surrounding said first and second pairs of coils; and a strap surrounding said core means, said strap being permanently magnetized along a diameter whereby flux lines produced by said strap pass between said longitudinal core sections to provide a unidirectional magnetic field transversely of said yoke.

3. An electromagnetic defiection yoke for use with a cathode ray tube, said yoke comprising: a first pair of deflection coils; a second pair of deflection coils arranged at right angles to said first pair of coils, such that the magnetic fields produced by said first and second pair of coils are substantially normal; magnetic core means comprising a plurality of longitudinal sections adapted to form a generally continuous low reluctance magnetic path around said coils; and a strap tightly surrounding said core sections in such manner as to hold said sections securely around said coils, said strap being permanently magnetized transversely so that its flux lines pass between said longitudinal core sections to lie in planes parallel to the magnetic fields of said coils, whereby to provide a xed electron beam-centering field.

4. An electromagnetic deflection yoke for use in association with a kinescope, said yoke comprising: a first pair of arcuate coils adapted to surround a kinescope. neck; a second pair of arcuate coils surrounding said first pair of coils and arranged at right angles thereto; a generally cylindrical magnetic core means comprising a plurality of longitudinal sections of a cylinder surrounding said first and second pairs of coils; and a strap surrounding said core means for holding said plurality of sections together around said first and second pairs of coils, said strap being permanently magnetized along a diameter whereby flux lines produced by said strap pass between said core sections to provide a unidirectional magnetic field transversely of said yoke,

7 8 5. The invention as defned'by claim 1 wherein said 2,553,039 Gray May 15, 1951 permane'ntlyil magnetzedvstrap Yis disposed'- rotationally,v 2,555,831 Tourshou.; lune 5, 19511 aboutsaid core seations sothat itsx1ines`ofuxfarersub 2,565,331 Tqrschi Aug. 21, 1951 stan'tillyi'para-llelltmthe- 'eld'lin'es produced"'by-one ofl 2,578,342 Ekvall Dec. 111, 1951 saidpair'sffcoilsz- 5 2,639,314?! Bryani.. .,May 19,` 1953 2,686,2763'1- Anderson E .Aug..10; 1954 References Cited 'f inthe-- l lof"A thisf'patent UNITED STATES; PATENTS 4765941v4 25.135929? Gethmanm- ...Iu1y ,4, 19509510 EOREIGN PATENTS GreatpBIitain DecT 20,1937.

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