• Automatic snap-in raster orientation
• Push-on axial purity positioning
• Internal magneto-static beam alignment
• Hi-Bi gun with quadrupole cathode lens
• Self-aligning, self-converging assembly with low power consumption, when combined with deflection unit AT 1870
• North-south pin-cushion distortion-free
110 deflection
Hi-Bri screen
Pigmented phosphors: enhanced contrast
Phosphor lines follow glass contour
In-line gun
Standard 36,5 mm neck
Soft-Flash technology
Slotted shadow mask optimized for minimum moire
Fine pitch over entire screen
Quick-heating cathodes
Internal magnetic shield
Anti-crackle coating
Reinforced envelope for push-through mounting.
FLASHOVER PROTECTION
High electric field strengths are present between the gun electrodes of picture tubes. Voltages between gun electrodes may reach values of 20 kV over approx. 1 mm. As a result of the Soft-Flash technology these flashover currents are limited to approx. 60 A offering higher set reliability, optimum circuit protection and component savings. Primary protective circuitry using properly grounded spark gaps and series isolation resistors (preferably carbon composition) is still. necessary to prevent tube damage and damage to the circuitry which is directly connected to the tube socket. the spark gaps should be connected to all picture tube electrodes at the socket according to the figure below; they are not required on the he;iter pins. No other connect- ions between the outer conductive coating and the chassis are permissible. The spark gaps should be designed for a breakdown voltage at the focusing electrode (g3) of 10,5 kV, and at the other electrodes of 1,5 to 2 kV. The values of the series isolation resistors
should be as high as possible (min 1,5 kQ) without causing deterioration of the circuit performance. The resistors should be able to withstand an instantaneous surge of 20 kV for the focusing circuit and 12 kV for the remaining circuits without arcing.
DEGAUSSING
The picture tube is provided with an internal magnetic shield. This shield and the shadow mask with its suspension system may be provided with an automatic degaussing system, consisting of two coils covering top and bottom cone parts. For proper degaussing an intial magnetomotive force (m.m.f.) of 300 ampere-turns is required in each of the coils. This m.m.f. has to be gradually decreased by appropriate circuitry. To prevent beam landing disturbances by line-frequency currents induced in the degaussing coils, these coils should be shunted by a capacitor of sufficiently high value. In the steady state, no significant m.m.f. should remain in the coils(.;;;; 0,3 ampere-turns). If single-phase power rectification is employed in the TV circuitry, provision should be included to prevent asymmetric distortion of the a.c. voltage applied to the degaussing circuit due to high d.c. inrush currents.
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GENERAL OPERATIONAL RECOMMENDATIONS
INTRODUCTION
Equipment design should be based on the characteristics as stated in the data sheets. Where deviations from these general recommendations are permissible or necessary, statements to that effect will be made. If applications are considered which are not referred to in the data sheets of the relevant tube type, extra care should be taken with circuit design to prevent the tube being overloaded due to unfavourable operating conditions.
SPREAD IN TUBE CHARACTERISTICS
The spread in tube characteristics is the difference between maximum and minimum values. Values not qualified as maximum or minimum are nominal ones. It is evident that average or nominal values, as well as spread figures, may differ according to the number of tubes of a certain type that are being checked. No guarantee is given for values of characteristics in settings substantially differing from those specified in the data sheets.
SPREAD AND VARIATION IN OPERATING CONDITIONS
The operating conditions of a tube are subject to spread and/or variation. Spread in an operating condition is a permanent deviation from an average condition due to, e.g.. component value deviations. The average condition is found from such a number individual cases taken at random that an increase of the number will have a negligible influence. Variation in an operating condition is non-permanent (occurs as a function of time). e.g .. due to supply voltage fluctuations. The average value is calculated over a period such that a prolongation of that period will have negligible influence.
LIMITING VALUES
Limiting values are in accordance with the applicable rating system as defined by IEC publication 134. Reference may be made to one of the following 3 rating systems. Absolute maximum rating system. Absolute maximum ratings are limiting values of operating and environmental conditions applicable to any electronic device of a specified type as defined by its published data, and should not be exceeded under the worst probable conditions. These values are chosen by the device manufacturer to provide acceptable serviceability of the device, taking no responsibility for equipment variations, environmental variations, and the effects of changes in operating conditions due to variations in the characteristics of the device under consideration and of all other electronic devices in the equipment. The equipment manufacturer should design so that, initially and throughout life, no absolute maximum value for the intended service is exceeded with any device under the worst probable operating condit- ions with respect to
supply voltage variation, equipment components spread and variation, equipment control adjustment, load variations, signal variation, environmental .conditions, and spread or variations in characteristics of the device under considerations and of all other electronic devices in the equipment.
Design-maximum rating system.
Design-maximum ratings are limiting values of operating and environ- mental conditions applicable to a bogey electronic device* of a specified type as defined by its pub- lished data, and should not be exceeded under the worst probable conditions. These values are chosen by the device manufacturer to provide acceptable serviceability of the device, taking responsibility for the effects of changes in operating conditions due to variations in the charac- teristics of the electronic device under consideration. The equipment manufacturer should design so that, initially and thoughout life, no design-maximum value for the intended service is exceeqed with a bogey device under the worst probable operating conditions with respect to supply-voltage variation, equipment component variation, variation in char- acteristics of all other devices in the equipment, equipment control adjustment, load variation, signal variation and environmental conditions.
Design-centre rating system.
Design-centre ratings are limiting values of operating and environmental conditions applicable to a bogey electronic device* of a specified type as defined by its published data, and should not be exceeded under average conditions. These values are chosen by the device manufacturer to provide acceptable serviceability of the device in average applications, taking responsibility for normal changes in operating conditions due to rated supply-voltage variation, equipment component spread and variation, equipment control adjustment, load variation, signal variation, environmental conditions, and variations or spread in the characteristics of all electronic devices. The equipment manufacturer should design so that, initially, no design-centre value for the intended service is exceeded with a bogey electronic device* in equipment operating at the stated normal supply voltage.
If the tube data specify limiting values according to more than one rating system the circuit has to be
designed so that none of these limiting values is exceeded under the relevant conditions.
In addition to the limiting values given in the individual data sheets the directives in the following
paragraphs should be observed.
HEATER SUPPLY
For maximum cathode life and optimum performance it is recommended that the heater supply be designed at the nominal heater voltage at zero beam current. Any deviation from this heater voltage has a detrimental effect on tube performance and life, and should therefore be kept to a minimum. Jn any case the deviations of the heater voltage must not exceed+ 5% and -10% from the nominal value at zero beam current. Such deviations may be caused by:
• mains voltage fluctuations;
• spread in the characteristics of components such as transformers, resistors, capacitors, etc.;
• spread in circuit adjustments;
• operational variations.
• A bogey tube is a tube whose characteristics have the published nominal values for the type. A bogey tube for any particular application can be obtained by considering only those characteristics which are directly related to the application.
CATHODE TO HEATER VOLTAGE
The voltage between cathode and heater should be as low as possible and never exceed the limiting values given in the data sheets of the individual tubes. The limiting values relate to that side of the heater where the voltage between cathode and heater is greatest. The voltage between cathode and heater may be d.c., a.c., or a combination of both. Unless otherwise stated, the maximum values quoted indicate the maximum permissible d.c. voltage. If a combination of d.c. and a.c. voltages is applied, the peak value may be twice the rated Vkf; however, unless otherwise stated, this peak value shall never exceed 315 V. Unless otherwise stated, the Vkf max. holds for both polarities of the voltage; however, a positive cathode is usually the most favourable in view of insulation during life. A d.c. connection should always be present betweeh heater and cathode. Unless otherwise specified the maximum resistance should not exceed 1 M.Q; the maximum impedance at mains frequency shou Id be less than 100 k.OHM.
INTERMEDIATE ELECTRODES (between cathode and anode)
In no circumstances should the tube be operated without a d.c. connection between each electrode and the cathode. The total effective impedance between each electrode and the cathode shou Id never exceed the published maximum value. However, no electrode should be connected directly to a high energy source. When such a connection is required, it should be made via a series resistor of not less
than 1 k.OHM.
CUT-OFF VOLTAGE
Curves showing the limits of the cut-off voltage as a function of grid 2 voltage are generally included in the data. The brightness control should be so dimensioned that it can handle any tube within the limits shown, at the appropriate grid 2 voltage. The published limits are determined at an ambient illumination level of 10 lux. Because the brightness of a spot is in general greater than that of a raster of the same current, the cut-off voltage determined with the aid of a focused spot will be more negative by about 5 Vas compared with that of a focused
raster.
LUMINESCENT SCREEN
To prevent permanent screen damage, care should be taken: - not to operate the tube with a stationary picture at high beam currents for extended periods; - not to operate the tube with a stationary or slowly moving spot except at extremely low beam currents; - if no e.h.t. bleeder is used, to choose the time constants of the cathode, grid 1, grid 2, and deflection circuits, such that sufficient beam current is maintained to discharge the e.h.t. capacitance before deflection has ceased after equipment has been switched off.
EXTERNAL CONDUCTIVE COATING
The external conductive coating must be connected to the chassis. The capacitance of this coating to the final accelerating electrode may be used to provide smoothing for the e.h.t. supply. The coating is not a perfect conductor and in order to reduce electromagnetic radiation caused by the line time base and the picture content it may be necessary to make multiple connections to the coating.
See also 'Flashover'.
METAL RIMBAND
An appreciable capacitance exists between the metal rimband and the internal conductive coating of the tube; its value is quoted in the individual data sheets.To avoid electric shock, a d.c. connection should be provided between the metal band and the external conductive coating. In receivers where the chassis ,can be connected directly to the mains there is a risk of electric shock if access is made to the metal band. To reduce the shock to the safe limit, it is suggested that a 2 Mil resistor capable of handling the peak voltages be inserted between the metal band and the point of contact with the external con- ductive coating. This safety arrangement will provide the necessary insulation from the mains but in the event of flashover high voltages will be induced on the metal band. It is therefore recommended that the 2 Mil resistor be bypassed by a 4, 7 n F capacitor capable of withstanding the peak voltage determined by the voltage divider formed by this capacitor and the capacitance of the metal rimband
to the internal conductive coating, and the anode voltage. The 4, 7 n F capacitor also serves to improve e.h.t. smoothing by addingthe rimband capacitance to the capacitance of the outer conductive coating.
FLASHOVER
High electric field strengths are present between the gun electrodes of picture tubes. Voltages between gun electrodes may reach values of 20 kV over approx. 1 mm. Although the utmost precautions are taken in the design and manufacture of the tubes, there is always a chance that flashover will occur. The resulting transient currents and voltages may be of sufficient magnitude to cause damage to the tube itself and to various components on the chassis. Arcing terminates when the e.h.t. capacitor is discharged. Therefore it is of vital importance to provide protective circuits with spark gaps and series resistors, which should be connected according to Fig. 1. No other connections between the outer conductive coating and the chassis are permissible. As our picture tubes are manufactured in Soft-Flash technology, the peak discharge currents are limited to approx. 60 A, offering higher set reliability, optimum circuit protection and component savings (see also Technical Note 039). However this limited value of
60 A is still too high for the circuitry which is directly connected to the tube socket. Therefore Soft-Flash picture tubes should also be provided with spark gaps.
IMPLOSION PROTECTION
All picture tubes employ integral implosion protection and must be replaced with a tube of the same type number or recommended replacement to assure continued safety.
HANDLING
Although all picture tubes are provided with integral implosion protection, which meets the intrinsic protection requirements stipulated in the relevant part of IEC 65, care should be taken not to scratch or knock any part of the tube. The tube assembly should never be handled by the neck, deflection unit or other neck components. A picture tube assembly can be lifted from the edge-down position by using the two upper mounting lugs. An alternative lifting method is firmly to press the hands against the vertical sides of the rimband. When placing a tube assembly face downwards ensure that the screen rests on a soft pad of suitable material, kept free from abrasive substances. When lifting from the face-down position the hand should be placed under the areas of the faceplate close to the mounting lugs at diagonally opposite corners of the faceplate.
When lifting from the face-up position the hands should be placed under the areas of the cone close
to the mounting lugs at diagonally opposite corners of the cone.
In all handling procedures prior to insertion in the receiver cabinet there is a risk of personal injury as a result of severe accidental damage to the tube. It is therefore recommended that protective clothing shou Id be worn, particularly eye shielding. When suspending the tube assembly from the mounting lugs ensure that a minimum of 2 are used; UNDER NO Cl RCUMSTANCES HANG THE TUBE ASSEMBLY FROM ONE LUG. If provided the slots in the rimband of colour picture tubes are used in the mounting of the degaussing coils. it is not recommended to suspend the tube assembly from one or more of these slots as permanent deformation to the rimband can occur. Remember when replacing or servicing the tube assembly that a residual electrical charge may be carried by the anode contact and also the external coating if not earthed. Before removing the tube assembly from the equipment, earth the external coating and short the anode contact to the coating.
PACKING
The packing provides protection against tube damage under normal conditions of shipment or handling. Observe any instructions given on the packing and handle accordingly. The tube should under no circumstances be subjected to accelerations greater than 350 m/s2.
MOUNTING
Unless otherwise specified on the data sheets for individual tubes there are no restrictions on the position of mounting. The tube socket should not be rigidly mounted but should have flexible leads and be allowed to move freely. It is very desirable that tubes should not be exposed to strong electrostatic and magnetic fields.
DIMENSIONS
In designing the equipment the tolerances given on the dimensional drawings should be considered. Under no circumstances should the equipment be designed around dimensions taken from individual tubes.
Abstract
30AX is a new in-line color TV display system with 110 deflection angle and interchangeable tubes and yokes. It is based on the production experience gained with the 20AX system introduced in 19741,2,3) and the results of further investigation in the field of tube technology and deflection yoke design. For the tube, this meant a new reference system, an internal magnetic correction ring and an improved gun design. For the yoke, the most important elements are a new "flangeless" winding technology, a change in the shape of the windings at the screen side of the line deflection coil and the use of field shapers embedded in the deflection coil.
A deflector for a cathode ray tube (called herein "CRT"), and more particularly a stator type deflector in which a plurality of slots for windings are formed in the inner surface of a tubular core and deflecting coils are positioned in these slots. The deflection Joke is a HIGH PRECISION MONO TOROIDAL TYPE.
The invention relates to a deflection unit for a color television display tube having a neck portion, a display screen, and a flared outer surface portion therebetween, said deflection unit comprising a field deflection coil and a line deflection coil each formed by a pair of diametrically oppositely positioned coil portions, and an annular core of a magnetically permeable material surrounding at least the line deflection coil, each line deflection coil portion being in the form of a saddle coil and having conductors wound to produce first and second side members, a front end and a rear end which together define a window, with the front end forming a flange, the front end of the coil portions of said line deflection coil, when said deflection unit is mounted on a display tube, being closer to the display screen than are the rear ends, with said front ends substantially surrounding a part of the flared portion of the display tube and the flanges, lying at an angle to the longitudinal axis of said display tube.
Such a deflection unit is commonly used for deflecting the electron beams in color television display tubes. In this known unit, the two coil portions which form the field deflection coil and the two coil portions which form the line deflection coil are both adapted, as regards their shape, to the flared profile of the display tube for which the deflection unit is destined. This means that the individual conductors of the coils engage the glass of the display tube as closely as possible when the deflection unit is mounted on the display tube for which it is intended. This applies in particular to the line deflection coil, since the sensitivity of the line deflection system is an important parameter with respect to the quality of a deflection device. For that purpose it is usual to make the front ends of the coil portions of the line deflection coil arc-like in shape such that they closely follow the contour of the display tube at its flared portion. This contour is often rotationally symmetrical so that the front ends in that case are of circular shape.
More rectangular shapes of this contour are also known, involving a corresponding shape for the front end so that in that case also they optimally conform to the contour of the display tube.
Parameters, known so far which are suitable to spatially shape the magnetic field of a deflection coil of the saddle type and which fully satisfy the requirements with respect to an optimum sensitivity, are provided by the wire distribution of notably the two substantially axially extending parts of each coil portion of which parts the front end forms the connection. Known techniques for this purpose are profiling of the space in the winding mould, profiling of the press die and the insertion of pins in the mould during the winding process. Furthermore it is known that the shape of the soft-magnetic core may also be used as a parameter to some extent.
It is known that in general a color television display system may present errors which may be distinguished as coma, astigmatism, raster defects and linearity defects. For so-called "three in-line guns" display systems it has proved generally possible, by using the above-mentioned design parameters, to make deflection coils by which astigmatism defects are sufficiently minimised.
Coma can also be minimised often in a corresponding manner. The situation is different for the raster defects and the linearity defects. The raster defects are divided into the North-South and the East-West defects. In "in-line" systems the North-South raster defect produces horizontal lines at the lower and upper edges of the picture which show a slight undulating distortion, while the East-West raster defect produces a strong-pin-cushion-like distortion which may be typically between 8 and 14%. Corrections for raster defects and linearity defects are obtained in general by suitable modulations of the line and field deflection currents. In addition, static magnets may alternatively be used for the correction of the undulating distortion.
A known disadvantage of modulating deflection currents, however, is that complicated electronic deflection circuits are required, which moreover consume additional energy and hence provide an expensive solution. In addition to a higher cost-price, the disadvantage of the use of static correction magnets is that, when the correction has to be larger than a few mm, problems arise with regard to the color purity.
It is an object of the invention to provide a deflection unit and a color display tube/deflection unit combination which reduces at least one of the above distortions.
According to one aspect of the invention there is provided a deflection unit as described in the opening paragraph of this specification, characterized in that the front ends of the line deflection coil portions together define a path whose length is greater than the length of a path around the flared portion of the display tube at the part thereof which said front ends are intended to surround.
The invention also provides a color display tube in combination with a deflection unit as described above.
The invention is based on the use of a real coil design parameter by means of which the undulating distortion and the pin cushion-like East-West raster defect, respectively, can be favorably influenced, and is achieved by the shape of the front end of the line deflection coil being no longer made as short as possible, as has been usual so far. As a result of this, the resulting sensitivity of the line deflection coil is slightly less than in conventional designs having the shortest possible length of front end, but, since, compared with designs in which the defects are removed by means of modulation of the deflection currents, the modulation becomes less, the electronic deflection circuits may be simpler which results in a lower overall energy consumption than that required with line deflection coils having a minimum front end length. The simplification of the circuits and their lower overall energy consumption both result in a lower cost-price. When, for the correction of any remaining "undulation effect," a static magnet is required, a weaker magnet may be used than would otherwise be necessary. Furthermore the sensitivity loss is at a minimum if the front end is bent towards the screen over such a distance as to engage the flared part of the display tube.
When using the shape of the front end as a design parameter, it has proved particularly efficacious to shape the profile of the front end along a path which encloses a polygon. In particular if this path according to a preferred form of the deflection unit according to the invention encloses a trapezium, the frame defects as mentioned above prove to be correctable effectively. (In this case the longer of the two parallel sides of the trapezium should be deemed to be nearest to the tube axis).
The above and other features of the invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which:
FIG. 1 is a diagrammatic longitudinal sectional view of a display tube having a deflection unit.
FIG. 2 shows part of a line deflection coil of a known type for use in the deflection unit shown in
FIG. 3 shows diagrammatically the location of the front end of the coil shown in FIG. 2 when mounted on a display tube.
FIG. 4 shows a part of a line deflection coil for use in a deflection unit according to the invention.
FIG. 5 shows diagrammatically the location of the front end of the coil shown in FIG. 4 when mounted on a display tube.
FIG. 6 shows in principle the errors to be corrected by the invention.
FI
G. 1 is a longitudinal sectional view through a color television display tube 1 having a longitudinal tube axis Z, a display screen 2 and three electron guns 4 situated in one plane. An electromagnetic deflection unit 5 is mounted on the tube neck 3. The deflection unit 5 comprises a pair of saddle coils 8 which form the coil portions of the field deflection coil for the field deflection, a pair of saddle coils 7 which form the coil portions of the line deflection coil for the line deflection, and a magnet core 6 surrounding the coils in the form of a ring. The saddle coils 7 and 8 shown are of the so-called sherl type, which means that their end sections adjacent the electron guns are not situated in a plane perpendicular to the tube axis 6, as are the end sections on the screen side, but are situated in a plane parallel to the tube axis Z. However, the invention is not restricted to the use of this type of saddle coil.
FIG. 2 shows a saddle shaped coil 9 of a conventional type having an arcuate shaped front end section 10, an arcuate shaped rear end section 11 and substantially axially extending intermediate sections 12 and 13 which sections together define a window 14. The profile of the front end section 10 follows a path 15 which is accurately adapted to the contour of the outer surface of the display tube 1 for which the coil 9 is destined. FIG. 3 which is a diagrammatic sectional view of the coil 9 at the area of the front end section 10 illustrates this. Up till now, pairs of such coils 9 have been used as the line deflection coil in conventional deflection units.
FIG. 4 shows a saddle shaped coil 16 which is used in a line deflection coil in a deflection unit according to the invention. The coil 16 consists of a front end section 17, a rear end section 18 and substantially axially extending conductors 19 and 20 which sections and conductors define a window 21. In this case the profile of the front end section 17 is formed along a path 22 which is longer than a path which is adapted to the contour of the outer surface of the display tube 1 for which the coil 16 is destined. All this is illustrated in FIG. 5 which is a diagrammatic sectional view of the coil 16 at the area of the front end section 17 and in which the contour of the outer surface of the display tube is denoted by 23. The path 22 in this case encloses a trapezium shaped space the longest parallel side of which faces the tube axis Z, but in general the space to be enclosed may be in the form of a polygon. In this case the rear end section 18 is shown to be horizontal, that is to say it does not lie in a plane which is at an angle to the tube axis as does the front end section 17. This coil shape is sometimes referred to as "shell" coil, but the invention is not restricted to this shape of coil.
The favorable effect of the use of this shape of the front end section 17 to correct raster defects may be considered as follows. It is known that raster defects are sensitive to variations of coil parameters on notably the screen side of the deflection unit, while the sensitivity to changes of parameters in the center of the deflection unit and on the gun side is directly reduced. However astigmatism is sensitive in particular to coil parameters in the center and on the screen side of the deflection unit and coma is influenced in particular by coil parameters on the gun side.
In coils of a "conventional" shape of the front end section where the enclosed path length is a minimum, the raster defects are produced as follows. Primarily the deflection coil is designed so that certain minimum requirements as regards astigmatism and possibly also coma are satisfied (in as far as this latter error is not corrected for by means of provisions in the display tube). This means that the coil parameters in the center of the deflection coils are controlled optimally with respect to the astigmatism. With respect to the raster defects no further parameter variations are possible and these errors are then to be taken as they present themselves following the astigmatism control.
In coils in which the shape of the front end section may be freely chosen, extra design parameters are available by which the astigmatism and also the raster defects can be influenced.
It has been found that several combinations of the coil parameters in the center of the deflection coils and of the front end section shape are possible which result in an acceptable level of astigmatism while the raster defects are always different. In this manner it is possible to find a front end shape - coil parameter combination with which the ultimate raster defects, for example, the "undulation effect" has fully disappeared or has been greatly reduced or that the pin-cushion distortion in the East-West direction has been reduced by a few percent, while it is even possible to deal with both types of errors simultaneously.
FIG. 6 shows diagrammatically, with reference to a display screen 24, the raster defects on the upper and lower sides of the display screen to be corrected by a deflection unit according to the invention having line deflection coils of the type shown in FIG. 4. The raster lines 25 shown have an undulating variation which is a frequently occurring shortcoming of in-line display systems. By using line coils of the type shown in FIG. 4 it was found that the raster lines were influenced so that they formed a straight line in the desired manner.
1. Field of the Invention
The invention relates to a deflection unit for a cathode ray tube having a neck portion and a display screen, the deflection unit being arranged between the neck portion and the display screen and around the flared portion of the tube connecting the neck portion and the display screen, the deflection unit comprising a field coil system and a line coil system for deflecting an electron beam produced in the neck portion in mutually orthogonal directions; the field coil system having a pair of diametrically opposite saddle type field deflection coils located on either side of a vertical axis and the line coil system having a pair of diametrically opposite saddle type line deflection coils located on either side of a horizontal axis extending at right angles to the vertical axis; each coil having a front end segment, a rear end segment and conductors extending between the front and the rear end segments.
2. Description of the Related Art
A deflection unit of the above described type is known from U.S. Pat. No. 4,229,720, issued Oct. 21, 1980, which corresponds to Netherlands patent specification No. 170,573 corresponding to U.S. Pat. No. 4,229,720, issued Oct. 21, 1988 and from the magazine "Funkschau" No. 23, 1980, pages 88-92 published in West Germany by Fanzis-Verlag GmbH published in West Germany.
In a deflection unit of this type the line deflection coils which generate a vertical magnetic field for the horizontal deflection must be arranged at right angles to the field deflection coils which generate a horizontal magnetic field for the vertical deflection. In the case of mutually orthogonal positions the magnetic coupling between the coil pairs is equal to zero so that no voltage is induced in the field deflection coils as a result of the magnetic field generated by the line deflection coils.
However, in practice it may occur that due to mechanical inaccuracies and/or manufacturing tolerances of the components during assembly the line deflection coils are not arranged exactly at right angles to the field deflection coils. In such a case a voltage will be induced in the field deflection coil as a result of the magnetic field of the line deflection coils. Detrimental consequences thereof are:
(a) the induced voltage reaches the field deflection circuit and the high voltage thus generated will disturb the operation of this field deflection circuit,
(b) the induced voltage produces a current through the field deflection coil via the field deflection circuit so that a rotation of the horizontal lines of the raster with respect to the horizontal axis becomes visible on the display screen. The convergence is also affected (twist errors).
It is an object of the invention to provide a means which provides correction in a simple manner for the possibility that in a deflection unit the line deflection coils and the field deflection coils may not be arranged exactly at right angles.
According to the invention this is achieved by providing two plate-shaped parts of a soft magnetic material near the front end segments of the two line deflection coils in positions which coincide with two diametrically opposite vertices of a rectangle whose diagonals intersect each other at least substantially on the longitudinal axis of the deflection unit and at which positions a portion of the front end segment of a line deflection coil overlaps a portion of the front end segment of a field deflection coil.
By providing the soft-magnetic plate-shaped parts in the above described manner the field lines are locally bundled in such a manner that the flux through the field deflection coils, and hence the coupling between the field deflection coils and the line deflection coils, is influenced so that the drawback mentioned above under (a) is eliminated and the drawback mentioned under (b) is greatly reduced.
The invention will now be described in greater detail with reference to the accompanying Figures wherein:
FIG. 1 is a diagrammatic cross-section (taken on the y-z plane) of a cathode ray tube with a deflection unit mounted thereon;
FIG. 2 is a diagrammatic perspective view of the field deflection coils and line deflection coils, shown at a distance from each other, of the deflection unit of the cathode ray tube-deflection unit combination shown in FIG. 1;
FIG. 3 is a front elevation on a larger scale of a deflection unit consisting of the field deflection coils and line deflection coils,
FIG. 4 is a diagrammatic cross-sectional view of the conductors taken on the line IV--IV in FIG. 3 showing the arrangement of a plate-shaped part with respect to the conductors and;
FIG. 5 is an elevational view of the display screen of the cathode ray tube of FIG. 1, showing a rotation to be corrected by means of the invention of the horizontal lines of the raster relative to the horizontal axis X.
FIG. 1 is a cross-sectional view of a display device comprising a cathode ray tube 1 having an envelope 6 extending from a narrow neck portion 2 in which an electron gun system 3 is mounted to a wide cone-shaped portion 4 which is provided with a display screen. A deflection unit 7 is mounted on the tube at the transition between the narrow and the wide portion. This deflection unit 7 has a support 8 of insulating material with a front end 9 and a rear end 10. Between these ends 9 and 10 there are provided on the inside of the support 8 a system of deflection coils 11, 11' for generating a line deflection magnetic field for deflecting electron beams produced by the electron gun system 3 in the horizontal direction, and on the outside of the support 8 a system of deflection coils 12, 12' for generating a field deflection magnetic field for deflecting electron beams procuced by the electron gun system 3 in the vertical direction. The systems of deflection coils 11, 11' and 12, 12' are surrounded by an annular core 14 of a magnetisable material. The separate coils 12, 12' of the system of field deflection coils, as well as the coils 11, 11' of the system of line deflection coils are of the saddle-type with rear end segments positioned flat against the tube wall. Deflection coils of the saddle type are self-supporting coils comprising a number of conductors which are wound to form longitudinal first and second side packets, an arcuate front end segment and an arcuate rear end segment together defining a window aperture. In such deflection coils the rear end segments may be flared with respect to the profile of the display tube (the original type of saddle coil) or they may be arranged flat against the tube wall (in this type of saddle coil the rear end segments follows, as it were, the tube profile).
As has been shown in greater detail in FIGS. 2 and 3, the deflection unit 7 has two line deflection coils 11 and 11' which are diametrically opposite to each other and are arranged on either side of a horizontal axis H, and two field deflection coils 12 and 12' which are located diametrically opposite to each other and are arranged on either side of a vertical axis V extending at right angles to the horizontal axis H.
Each line deflection coil consists of a front end segment 15, a rear end segment 16 and conductors 17 connecting the front end segment 15 and the rear end segment 16. Similarly, a field deflection coil 12 consists of a front end segment 18, a rear end segment 19 and conductors 20 connecting the front end segment 18 and the rear end segment 19.
As explained and shown in the Netherlands patent specification No. 170,573 mentioned in the preamble, the coils constituting the deflection device are arranged in conventional manner around a trumpet-shaped portion of a colour television display tube, which trumpet-shaped portion connects a display screen of the television display tube to a neck portion of the relevant television display tube. The arrangement is such that the longitudinal axis of the deflection unit which is constituted by the coils coincides with the longitudinal axis of the display tube, whilst the front end segments 15 and 18 of the line and field deflection coils are located at the end of the deflection unit facing the display screen.
In the following elaboration the quadrant in FIG. 3 located above the horizontal axis H and to the right of the vertical axis V will be denoted the frist quadrant, the quadrant located below the horizontal axis H and to the right of the vertical axis V will be denoted the second quadrant, the quadrant located below the horizontal axis H and to the left of the vertical axis V will be denoted the third quadrant and the quadrant located above the horizontal axis H and to the left of the vertical axis V will be denoted the fourth quadrant.
Assuming that the current flows through the line deflection coils as is indicated by the arrows I and the line and field deflection coils are arranged exactly at right angles to each other, line deflection flux will enter the first quadrant in the field deflection coil, which flux is equal to the line deflection flux leaving the field deflection coil in the second quadrant, so that the net line deflection flux in the field deflection coil is equal to zero in this case. The same applies to the line deflection coil located in the third and fourth quadrants.
If, however the symmetry plane of the two line deflection coils 11, 11' has been slightly rotated clockwise with respect to the horizontal axis H (for example, as a result of manufacturing tolerances or the like) the line flux entering the field deflection coil 12 in the first quadrant will slightly decrease and the flux leaving the second quadrant will slightly increase, so that there is a net line deflection flux leaving the field deflection coil 12. Correspondingly, a net line deflection flux is obtained entering the field deflection coil 12' located in the third and fourth quadrants.
The (unwanted) result is that the horizontal lines of the raster present a rotation with respect to the horizontal (x) axis on the display screen 5 as shown in FIG. 5.
In order to counteract this effect, plate-shaped parts 21, 21' manufactured from a soft magnetic material are provided near the transition of the front end segments 15 into the conductors 17, on diagonal D which extends through the longitudinal axis of the deflection unit and across those ends of the front end segments 15 of the line deflection coils 11, 11' which are located furthest away from the horizontal axis H as a result of the rotation in the direction of the arrows C. Such plate-shaped parts, as shown in FIG. 4, may have a L-shaped structure and whose long limbs extend along the a portion of the front end segments 15 of the line deflection coils which overlaps a portion of the front end segments 18 of the field deflection coils. The length of these limbs corresponds with the width of the front end segment 15 at this region. The short limbs of the L-shaped plate-shaped parts extends over the edge of the relevant front end segments of the line deflection coils towards the front end segment 18 of the field deflection coil.
By providing these plate-shaped parts or field conductors manufactured from a soft magnetic material, the line deflection flux entering the field deflection coil is intensified in the first quadrant and the line deflection flux leaving the field deflection coil in the third quadrant is intensified, so that the above described effect caused by the rotation of the line deflection coils in the direction of the arrows C is counteracted.
It will be evident from the foregoing that in the case of a rotation of the symmetry plane of the line deflection coils in an anti-clockwise direction relative to the horizontal axis the plate-shaped parts have to be provided on the line deflection coils at two diametrically opposite points located on the diagonal D'.
A rotation of the line deflection coils with respect to their desired position is mentioned above as an example. However, the field deflection coils may deviate from their symmetrical location, or both the line deflection coils and the field deflection coils may have a deviating location. In all these cases the present invention provides a correction by arranging two plate-shaped soft magnetic parts near the front end segments of the two line deflection coils in positions which coincide with two diametrically opposite vertices of a rectangle whose diagonals intersect each other at least substantially on the longitudinal axis of the deflection unit and in which positions a portion of a front end segment of a line deflection coil overlaps a portion of the front end segments of a field deflection coil. And in all these cases the explanation given for their operation remains valid.
In one embodiment parts 21, 21' were manufactured from an Si Fe alloy having a thickness of 0.35 mm and a width of 3 mm, which in a deflection unit as described in the article mentioned in the preamble resulted in a coupling influence of 9 mV at a voltage of 1 V across the line deflection coils.
The influence of spreading, if not corrected, is, for example, 6 mV in the case of an incorrect arrangement, which results in a total range of between -18 mV and +18 mV.
In this case this will be reduced to ±9 mV by using the correction means according to the invention.
In practice the position of the correction means (the plates 21, 21'), and hence the choice of the correct diagonal, can be determined by measuring the phase of the voltage produced across the field deflection coil with respect to the voltage applied across the line deflection coil.
es of which are parallel, if the beams enclose a given angle with the gun axes. This enables an easy manufacture of the electrodes and an accurate assembly of the guns. In these guns the focusing of the beams is independent of the convergence.
1. An electric discharge tube comprising an envelope having a main axis, a display screen and an electron gun system for producing a plurality of electron beams and converging the beams on the display screen, the electron gun system comprising first electrode means for generating the electron beams, the first electrode means being situated along axes parallel to the main axis of said tube; second electrode means situated along the path of the electron beams between the first electrode means and the display screen, said second electrode means comprising respective last electrodes situated on the side toward the dislay screen and an associated preceding electrode, with electrodes in use constitute a lens field which focuses the electron beams symmetrically; and third electrode means between the first and the second electrode means for forming an asymmetric lens field to coverge the electron beams on the display screen, characterized in that the axes of the electrodes of all electrode means are parallel to said axes of the first electrode means; and
the last electrodes (76, 96, 106), situated on the side toward the display screen, of those second electrode means which are situated eccentrically with respect to the main axis of the tube, have axes (54) which are situated eccentrically with respect to the axes (55) of the associated preceding electrodes (75, 95, 105) and to the axes (62) of the associated first electrode means, the axes (55) of said preceding electrodes (75, 95, 105) having a smaller distance to the main axis of the tube than the axes (54) of the associated last electrodes (76, 96, 106) situated on the side toward the display screen, said axes (54) of said last electrodes in turn having a smaller distance to the main axis of the tube than the axes (62) of the associated first electrode means (71, 72, 73, 91, 92, 93).
2. An electric discharge tube as claimed in claim 1, characterized in that all said axes are situated in one plane, the axes of one of the first electrode means and the associated second electrode means coincide with the main axis of the tube, and the axes of two other first and second electrode means are situated symmetrically with respect to the main axis of the tube.
The invention relates to a colour display tube comprising first electrode means to generate plurality of electron beams, situated along axes parallel to the main axis of said tube; a display screen on which said electron beams converge; second electrode means situated along the path of the electron beams between the first electrode means and the display screen, which second electrode means form a lens field which focuses the electron beams symmetrically; and third electrode means between the first and the second electrode means with which, if desired in cooperation with the first electrode means, an asymmetric lens field is formed to converge the electron beams on the display screen.
Such a colour display tube is disclosed in U.S. Pat. No. 2,957,106. Such display tubes are used inter alia as tubes to display coloured pictures, as oscilloscope tubes, etc. In such tubes it is desired for the electron beams to be converged in one point on the display screen. In U.S. Pat. No. 2,957,106 an asymmetric electron lens is provided in the path of the electron beams which do not coincide with the main axis of the tube between the triode part of the electron gun formed by the cathode, the first and second grids, and the focusing lens, so that the beams are deflected towards each other and converge on the display screen. The focusing lens is formed by a lens field between two electrodes. These electrodes consist of curved electrode plates having apertures therein. The plates are curved so as to be always perpendicular to the electron path. By applying a potential difference between the plates an electron lens is formed which is symmetrical for the electron beams and which has a focusing effect and focuses each electron beam on the display screen. It is very difficult to manufacture such very accurately curved electrode plates and assemble them with respect to each other. Electrodes of such electron guns are assembled by means of assembly pins which have to enclose a very accurate angle with respect to each other. In order to be able to remove the guns from the assembly pins it is necessary for these pins to be connected detachably in a jig as a result of which their mutual angle becomes less accurate as a result of detrition, diurt, bending an breaking of the pins.
This problem is recognized in U.S. Pat. No. 3,906,279 and a solution to this problem is given. This patent teaches a construction for the convergence of three electron beams from three assembled electron guns whch operate independently of each other and the axes of which are parallel and hence parallel assembly pins can be used. This construction is characterized in that of each electron gun which is situated eccentrically with respect to the main axis of the tube, the last electrode situated on the side of the display screen has an axis which is situated eccentrically with respect to the axis of the relevant electron gun in a plane through the main axis of the tube and the axis of the electron gun and at a larger distance from the main axis of the tube than the axis of the electron gun. This last electrode also has a larger diameter than the other electrodes of the electron gun. As a result of the eccentrically placed last electrodes, convergence of the electron beams is obtained in a simple manner and at the same time the electron beams are each focused separately.
U.S. Pat. No. 3,772,554 discloses an integrated system of electron guns operating in an analogous manner. A system of electron guns operating in an analogous manner and in which the focusing lenses of the guns not situated on the tube axis are asymmetrical is known from German Patent Application 2,406,443 laid open to public inspection. All these constructions are less attractive because they exhibit a very important disadvantage. A variation of the strength of the focusing lens in such guns at the same time has a direct influence on the convergence of the electron beams, which is not desired.
It is therefore the object of the invention to provide a simple construction for focusing and converging electron beams independently of each other by means of electron guns the axes of which are parallel so that a simple, rapid and accurate manufacture and assembly are possible.
According to the invention, a colour display tube of the kind mentioned in the opening paragraph is characterized in that the axes of the electrodes of all electrode means are parallel to the axes axes and that of the second electrode means which are eccentric with respect to the main axis of the tube, the last electrodes (76, 96, 106) situated on the side of the display screen have axes (54) which are eccentric with respect to the axes (55) of the associated preceding electrodes (75,95, 105) and to the axes (62) of the associated first electrode means, the axes (55) of those preceding electrodes (75, 95, 105) having a smaller distance to the main axis of the tube than the axes (54) of the associated last electrodes (76, 96, 106) situated on the side of the display screen, the last-electrode axes (54) in turn having a smaller distance to the main axis of the tube than the axes (62) of the associated first electrode means (71, 72, 73, 91, 92, 93).
The invention is based on the recognition that, when an electron beam is incident in such a mechanically non-symmetric electrode system at a given angle with the gun axis, a symmetric focusing of the electron beam can nevertheless be obtained so that a variation of the strength of the focusing lens has no influence on the convergence. This given angle which depends on the gun dimensions can be determined experimentally on an optical bench.
A preferred embodiment of such a colour display tube embodying the invention is characterized in that all these axes are situated in one plane and the axes of one of the first electrode means and the associated second electrode means coincide with the main axis of the tube and the axis of two other first and second electrode means are situated symmetrically with respect to the main axis of the tube.
The invention will now be described in greater detail with reference to a drawing, in which:
FIG. 1 is a cross-sectional view of a colour display tube embodying the invention,
FIGS. 2 and 3 are cross-sectional views of prior-art electron guns, and
FIGS. 4 to 6 are cross-sectional views of a number of embodiments of electron guns used in colour display tubes embodying the invention.
FIG. 1 is a cross-sectional view of a colour display tube embodying the invention. In a neck 4 of a glass envelope 1 further composed of a display window 2 and a conical part 3, three electron guns 5, 6 a
nd 7 are provided which generate the electron beams 8, 9 and 10. The axes of these electron guns are situated in one plane, the plane of the drawing. The axis of the central electron gun 6 coincides with the main axis 11 of the envelope. The three electron guns consist of a number of cylindrical electrodes placed along an axis. As is known, it is possible to construct one or more of the juxtaposed electrodes of the guns as one assembly. A large number of triplets of phosphor lines are provided on the inside of the display window. Each triplet comprises a line consisting of a green luminescing phosphor, a line consisting of a blue luminescing phosphor and a line consisting of a red luminescing phosphor. All triplets together constitute the display screen 12. The phosphor lines extend perpendicularly to the plane of the drawing. A shadow mask 13 having a large number of elongate apertures 14 parallel to the phosphor lines, through which apertures the electron beams 8, 9 and 10 pass, is placed before the display screen. Since the electron beams enclose a small angle with each other and converge on the display screen, each beam is incident only on phosphor lines of one colour via the elongate apertures. As is known, it is alternatively possible to provide the electron guns in a triangular arrangement in the tube, each gun being situated at the corner of an equilateral triangle. In that case the shadow mask has circular apertures and the display screen is composed of triplets of phosphor dots.
FIG. 2 is a cross-sectional view of a prior-art electron gun (U.S. Pat. No. 3,957,106). The means to generate the electron beams each consist of a cathode 15, a grid electrode 16 and an accelerating electrode 17. The convex portion 19 of electrode 18 is provided with apertures 20 and 21. As a result of the convex portion 19 of electrode 18 a non-symmetrical electrostatic field is formed between the electrodes 17 and 18 so that the electrode beams 22 and 23 are bent towards the axis 24 in such manner that these beams converge on the display screen 12. The apertures 25 and 26 in electrode 27 and the apertures 28 and 29 in electrode 30 are provided so that they are placed in the path of the electron beams. The curvature of the convex portions of the electrodes 27 and 30 in which said apertures are provided is such that their surfaces always extend perpendicularly to the paths of the electron beams. As a result of this and by applying a sufficiently large potential difference between the electrodes 27 and 30 a symmetrical lens field is obtained between the electrodes which has a symmetric focusing effect on the electron beams. As a rsult of this, variations in strength of the lens field have no influence on the convergence. The manufacture of electrodes having such accurately curved surfaces is very difficult and the assembly is inaccurate because assembly pins have to be used which enclose an angle with each other. FIG. 3 shows a system of electron guns (U.S. Pat. No. 3,906,279) in which all the axes 31, 32 and 33 of the electron guns 34, 35 and 36 extend parallel to each other and are situated in one plane. The gun 34 has a cathode 37 and a grid 38 and an anode 39 and grids 40 and 41. The corresponding electrodes of gun 35 are referenced 47 to 51. The corresponding electrodes of gun 36 are referenced 57 to 61.
As is shown in this Figure, the grids 41 and 61 have a larger diameter than the associated grids 40 and 60 and the axes 42 and 43 are situated farther away from the axes 32 than the gun axes 31 and 33. The lens fields between the electrodes 40 and 41 and between the electrodes 60 and 61 are hence not symmetrical and deflect the beams 44 and 45 towards the central beam 46. These lens fields and the lens field between the grids 50 and 51 also serve to focus the electron beams. A small variation in the voltage difference between the electrodes 40 and 41 and between the electrodes 60 and 61 hence has an influence on the convergence and also on the focusing of the electron beams. It will be obvious that this is undesired since it should be possible to provide variations in the focusing and convergence preferably independently of each other.
FIG. 4 shows a first embodiment of an electron gun system in which no curved parts are necessary, all the axes of the electrodes extend parallel to each other and nevertheless a convergence is poss
ible which is independent of the focusing voltage (the voltage difference between the last two electrodes in an electron path). It consists of three guns 70, 80 and 90 having the cathodes 71, 81 and 91 in grids 72, 82 and 92 and opposite to the electrodes 73, 83 and 93. By means of these electrode means, three electron beams 74, 84 and 94 are generated which initially extend parallel to each other. By providing the grids 75 and 95 with apertures 52 and 53 which are situated so as to be not symmetrical with respect to the beams 74 and 94, the electron beams 74 and 94 are deflected towards the central electron beam 84 in a manner analogous to that of U.S. Pat. No. 2,957,106. The focusing is done by the lens fields between the electrodes 75 and 76, 85 and 86 and 95 and 96. In contrast with the construction disclosed in U.S. Pat. No. 3,906,279, any variation of the focusing lens fields between the electrodes 75 and 76 and between the electrodes 95 and 96 of the outermost electron guns has no influence at all on the convergence because the electron beams 74 and 94 are incident through said lens fields at a given angle with the gun axes. As a result of this, a focusing lens acting symmetrically on the beam is obtained by means of a few electrodes which are situated non-symmetrically.
An example of the electric voltages (in Volts) applied to the various electrodes is shown in FIG. 4 for gun 70. A number of dimensions of electrodes and their mutual distances are recorded in the table below:
| ______________________________________ |
| electrode length diameter mutual dis- diameter open- no. (mm) (mm) tance (mm) ing (mm) |
| ______________________________________ |
| 76 8 7.6 76-75 1 75 16.2 7.4 1.5 75-73 1.4 73 5.4 0.75 73-72 0.35 72 0.75 72-71 0.12 71 |
| ______________________________________ |
The distance from axis 54 of electrode 76 to the gun axis 62 is 0.3 mm. The distance from axis 55 to axis 62 is 0.4 mm and the distance from axis 56 to axis 62 is 0.2 mm. For other gun dimensions, other mutual axial distances are necessary. These can be determined experimentally on an optical bench or can be calculated. The thickness of the material (Cr-Ni-steel) from which the varous electrodes are manufactured is in this embodiment 0.13 to 0.2 mm. The distance between two gun axes is 10 mm. FIG. 5 is a cross-sectional view of a second embodiment of an electron gun system according to the invention.
For clarity, the same reference numerals are used as in FIG. 4. The convergence of the electron beams 74, 84 and 94 is obtained in this embodiment by causing the ends of the electrodes 75 and 95 situated oppositely to the electrodes 73 and 93 to enclose an angle of approximately 87° with the gun axis. This convergence method is also disclosed already in U.S. Pat. No. 2,957,106. The various dimensions correspond approximately to the dimensions indicated with reference to FIG. 4. The electron beams 74, 84 and 94 also converge on the display screen 12. The convergence is independent of the strength of the focusing lens. The convergence of the electron beams can alternatively be obtained by shifting and/or tilting the electrodes 73 and 93 as a result of which the non-symmetrical deflecting lenses are obtained in cooperation with the electrodes 75 and 95. This will not be further described.
FIG. 6 is a cr
oss-sectional view of a third embodiment of an electron gun system embodying the invention. The electron gun system comprises a number of electrodes 102, 103, 105 and 106 which are constructed so as to be common for the three electron beams. The Figure is drawn approximately to the same scale as FIGS. 4 and 5. For clarity, the same reference numerals are used as much as possible as in FIGS. 4 and 5. It will be obvious that one of the electrodes may be divided into two sub-electrodes or that an extra electrode may be added without this influencing the essence of the invention.
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