PHONOLA (PHILIPS) 16CT2216/20S LUINI CHASSIS CTX SAM CRT TUBE PHILIPS A42-570X.

Strengthening means for a CRT in-line electrode component:

 The invention relates to improved strengthening means for a substantially planar one-piece electrode component utilized in an in-line multi-beam CRT electron gun assembly. Advantageous ruggedizing properties are achieved by discretely formed longitudinal channels oriented along the sides of the component with defined ledge portions extending outward therefrom; beneficially dimensioned supporting projections are formed as integral extensions thereof. A plurality of cooperating mini-channels are included as transversals in the component to provide a markedly improved structure.

 1. Improved strengthening means for a substantially planar one-piece electrode component in a plural electrode in-line multi-beam cathode ray tube electron gun assembly integrated by a plurality of longitudinal insulative support members, said electrode component evidencing alpha and beta surfaces and having opposed side and end regions with defining L--L' and W--W' axes thereacross; said component having a center and two side-related spatially positioned apertures therethrough located in an in-line relationship substantially coinciding with said L--L' axis, the center aperture being located at the intersection of said L and W axes, with said side-related apertures oriented equidistantly therefrom along said L--L' axis on either side of said W--W' axis; said strengthening means comprising: at least one longitudinal channel located in each of the side regions of said component in parallel relationship with said L--L' axis, each of said channels being indented inward from said beta surface of said component for the full length of said region to form a longitudinal rib projecting from said alpha surface, each of said channels being an open-ended trough formation having width and depth dimensions formed by three adjoining longitudinal surfaces defining an outer wall portion, an inner wall portion and a bottom portion therebetween, each of said channels displaced inwardly from either side of said component to form ledges in the beta surface between the channels and the side edges of the component, the ledge edges being substantially parallel with said L--L' axis and extending along the full length of said side region; each of the channel-related ledges having an outstanding dimension at least substantially equal to the thickness of the component material and supporting projections extending from either side of said component; the supporting projections being integral planar extensions of said ledge formations.


2. An improved electrode component of the in-line CRT electron gun assembly according to claim 1 wherein each of said channels evidences two longitudinal and parallel strengthening bends therein, said bends being separated to define the bottom width dimension of said channel.

3. An improved electrode component of the in-line CRT electron gun assembly according to claim 1 wherein each channel evidences a substantially uniform depth dimension that is at least substantially equal to the thickness of said component material.

4. An improved electrode component of the in-line CRT electron gun assembly according to claim 1 wherein each channel has a substantially uniform width dimension that is at least substantially equal to the thickness of said component material.

5. An improved electrode component of the in-line CRT electron gun assembly according to claim 1 wherein each of said channel-related ledges extends in a substantially right-angle relationship with the outer wall of said channel.

6. An improved electrode component of the in-line CRT electron gun assembly according to claim 1 wherein at least two mini-channels are formed as elongated lateral indentations in the beta surface of said electrode component, said mini-indentations transversing the area between said side longitudinal channels and being located substantially midway between said apertures in parallel relationship with said W--W' axis.

7. An improved electrode component of the in-line CRT electron gun assembly according to claim 6 wherein each mini-indentation forms an elongated protrusion from the alpha surface of said electrode component, the height of said protrusion being less than the thickness of said component material.

8. An improved electrode component of the in-line CRT electron gun assembly according to claim 6 wherein each of said apertures is oriented in an individual spaced-apart dish-like depression formed in said alpha surface to project as a separate protuberance from said beta surface, and wherein said mini-channel indentations are located in the spacings between said aperture protuberances.

Description:

TECHNICAL FIELD
This invention relates to a substantially planar one-piece electrode component in a multi-beam in-line cathode ray tube electron gun assembly, and more particularly to improved strengthening means incorporated into the structure of a substantially planar electrode member.

BACKGROUND OF THE INVENTION
Cathode ray tubes (CRT's) commonly used in color television and related display applications conventionally utilize unitized electron gun assemblies which direct a plurality of controlled electron beams to the display screen of the tube. In certain gun assembly constructions, the first and second grid electrode components, such being normally control and screen grid electrodes, are often formed as substantially planar members oriented in substantially parallel planes in spaced apart superposed relationship. In multi-beam guns each of these first and/or second planar electrodes contains several spatially related apertures to accommodate the respective electron beams generated within the assembly. It is very important that these several apertures be accurately and consistently spaced relative to the related apertures in the adjacent electrode components, and, in the case of the first electrode component, with the respective cathode surfaces from which the specific electron beams emanate. These and associated electrodes are conventionally affixed to at least two longitudinal insulative support members of the integrated gun assembly by supporting projections extending from the respective electrode components.
Fabrication of the gun assembly involves embedment of the supporting projections of the related electrode components into the temporarily heat-softened longitudinal insulative support members. In this operation, which is commonly referred to as "beading", the softened support members on opposed sides of the assembly are pressured inward toward the several electrode components thereby forcing the supporting projections thereof into the support members. The opposing compressive pressures tend to exert a distorting force upon the electrode components, this being especially critical to the planar components wherein a bowing or arcuate bending effect sometimes results. Such bowing, however slight, changes the aperture locations relative to those in the adjacent electrode components, thereby producing deleterious inter-electrode spacing relationships within the gun structure. These uncontrollable changes in the related aperture spacings are particularly troublesome in in-line gun constructions wherein the first and the second grid electrodes usually have related apertures of small diameter and close spacings.
Two serious manufacturing control problems are caused by the bowing or warping of the first (G1) and/or second (G2) electrode components. The first of these is variation of cutoff and associated cutoff ratio. Cutoff is defined as the positive cathode (K) voltage at which the electrons cease to flow through the G1 aperture. Cutoff ratio is the ratio of the highest cutoff voltage to the lowest cutoff voltage of the three guns in a given tube. Cathode cutoff ratio is now commonly specified at 1.25, a condition which requires precise G1, G2, and K-G1 spacing control. This has proven to be one of the more difficult manufacturing control problems.
The second control problem relating to bowed G1 and G2 electrodes is variation of focus quality. This is largely determined by gun design, but for the gun construction to be successful, three factors are essential: (a) high quality parts must be used, (b) parts alignment must be accurately maintained, and (c) K-G1 and G1-G2 spacings must be precisely controlled at or near design center for optimum focus performance. This factor is directly related to bow-free electrodes. The most difficult production control parameter is the endeavor to achieve consistent K-G1 spacings for the three associated beams.
There are disclosures in the prior art to ruggedize in-line planar type electrodes by incorporating strengthening ribs such as those taught by Floyd K. Collins in U.S. Pat. Nos. 4,049,990 and 4,049,991.
A second grid electrode having channels therein is also shown in the gun structure disclosed by Allen P. Blacker and James W. Schwartz in U.S. Pat. No. 4,058,753.
While teachings of incorporating strengthening ribs fulfilled the existing needs at the time of disclosure, the state of the CRT art has advanced to stages of greater constructional sophistication wherein gun assemblies are made smaller and more compact, and tube operating requirements more stringent and exacting. In view thereof, improved strengthening of planar type electrodes, to prevent bowing during tube fabrication, is essential to achieving the desired tube performance characteristics required in the present state of the art.

DISCLOSURE OF THE INVENTION
It is therefore an object of the invention to provide a substantially planar CRT in-line electrode component having improved ruggedizing structural means incorporated therein to counteract the distorting forces encountered during the electron gun assembly fabrication procedure.
Another object of the invention is to provide an improved in-line ruggedized electrode component that is formed in a manner to optimize the maintenance of initial shaping when incorporated in a plural beam gun assembly thereby providing the desired subsequent inter-electrode spacings within the gun structure.
These and other objects and advantages are achieved in one aspect of the invention wherein improved strengthening means are provided for a substantially planar one-piece electrode component in a plural electrode in-line multiple beam CRT gun assembly integrated by a plurality of longitudinal insulative support members. The substantially planar component evidences alpha and beta surfaces wherein there are opposed side and end regions having L--L' and W--W' axes thereacross. The component contains a center and two side-related spatially positioned apertures located in an in-line relationship substantially coinciding with the L--L' axis. The center aperture is positioned at the intersection of the L and W axes, while the side-related apertures are located equidistantly therefrom along the L--L' axis on either side of the W--W' axis.
The invention relates to electrode component strengthening means in the form of at least one longitudinal channel located in each of the side regions thereof in parallel relationship with the L--L' axis. Each of these channels is indented inward from the beta surface to extend the full length of the respective side region to form a longitudinal rib projecting from the alpha surface. Extending outward from each channel, for the full length thereof, in the plane of the side region, is a defined ledge having a leading edge substantially parallel with the L--L' axis. Additionally, at least a pair of spatially-related supporting projections are extended outward equally from either side of the component as integral planar extensions of the respective ledge formations. The facing edges of each pair of projections are beneficially spaced from the W--W' axis by dimensions in the order of substantially half the separation distance between apertures.
Each of the ruggedizing longitudinal channels is further defined as an open-ended trough formation having width and depth dimensions formed by three adjoining longitudinal surfaces comprising an outer wall, an inner wall and a substantially planar bottom therebetween. As such, each channel evidences two separated longitudinal and parallel strengthening bends therein, the distance therebetween defining the bottom width dimension of the channel.
Each channel evidences a substantially uniform width dimension being in the order of at least twice the thickness of the component material. In like manner, a substantially uniform depth dimension is also evidenced, such being at least substantially equal to the thickness of the component material. Each of the channel-related ledges, which extends in a substantially right-angle relationship with the outer wall of each channel, has an outstanding dimension that is also at least substantially equal to the thickness of the component material.

The electrode component preferably also evidences at least two mini-channels formed as elongated lateral indentations in a surface thereof, preferably the beta surface, being located substantially midway between the center and side-related apertures in parallel relationship with the W--W' axis thereby transersing the area between the longitudinal side channels. Each of these mini-indentations forms an elongated protrusion from the opposite surface of the component, the height of this protrusion being less than the thickness of the component material.
The electrode component may be further defined as being fabricated in a manner wherein each of the apertures is oriented in an individual spaced-apart dish-like depression formed in the alpha surface in a manner to project as a separate protuberance from the beta surface. In keeping therewith, the aforedescribed mini-channel indentations are located in the spacings between the aperture protuberances.
The aforedescribed strengthening features incorporated in the structural configuration of a substantially planar electrode component effects the beneficial desired ruggedization thereof in a manner not heretofore achieved. 

 

 

 CRT TUBE  PHILIPS  A42-570X

Hi-Bri COLOUR  PICTURE TUBE
@ 90° deflection
@ In-line gun, thermally stable; electrostatic hi-bi potential focus
@ 29,1 mm neck diameter
© Hi-Bri screen with pigmented phosphor featuring high brightness and increased contrast performance
@ Soft-Flash technology offering improved set reliability
@ Slotted shadow mask optimized for minimum moiré
@ Fine pitch over entire screen
@ Phosphor lines follow glass contour
@ Quick-heating cathodes
@ Internal magnetic shield
@ Reinforced envelope for push-through mounting
@ Anti-crackle coating.

FLASHOVER PROTECTION
With the high voltage used with this tube (max. 27,5 kV) internal flashovers may occur. 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. 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 heater pins. No other connections 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 11 kV (1,5 x Vg3 max. at Va gq = 25 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 kohm) 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 one magnetic coil winding mounted on the cone of the picture tube.


Symbols denoting electrodes/elements and electrode/element connections
f Heater
k Cathode
g Grid: Grids are distinguished by means of an additional numeral:
the electrode nearest to the cathode having the lowest number.
a Anode
m External conductive coating
m’ Rimband
Q Fluorescent screen
ic. Tube pin which must not be connected externally
nc Tube pin which may be connected externally

Symbols denoting voltages
Unless otherwise stated, the reference point for electrode voltages is the cathode.
Vv Symbol for voltage, followed by a subscript denoting the relevant electrode/element
Ve Heater voltage
Vp-p Peak-to-peak value of a voltage
Vp Peak value of a voltage
VGR Grid 1 voltage for visual extinction of focused raster (grid drive service)
VKR Cathode voltage for visual extinction of focused raster (cathode drive service)


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 necessar y, 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 as No qualified spread figures, as maximum is given may for differ or minimum values according of characteristics are to nominal the number ones. in settings of It is tubes evident substantially of a that certain average differing type or that nominal from are those being values, checked. specified as well guaranteein 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 negligibl e 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. value The equipment for the intended manufacturer service is should exceeded design with so that, any device initially under and the throughout worst probable life, no absolute operating maximum conditions with respect to supply voltage variation, equipment compone nts spread and variation, equipment control adjustment, load variations, signal variation, environ mental 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 characteristics 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 exceeded 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. The expressions ‘long-term’ and ‘short-term’ are used to denote either the maximum time-averaged beam current for one gun to limit the cathode loading, or the maximum time-averaged anode current for three guns to limit the screen loading. ‘Short-term’ is not related to a specific period of time, but can be interpreted as the condition where the content and intensity of the displayed image vary continuously, as during live television pictures. ‘Long-term’ means that the image is stationary for an indefinite period of time, as during the display of test pictures, computer images, teletext data or stationary television scenes lasting longer than 30 seconds. 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. In 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.

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 DC, AC or a combination of both. Unless otherwise stated, the maximum values quoted indicate the maximum permissible DC voltage. If a combination of DC and AC 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 Vk¢ max. holds for both polarities of the voltage; however, a positive cathode is usually the most favourable in view of insulation during life. A DC connection should always be present between heater and cathode. Unless otherwise specified the maximum resistance should not exceed 1 MQ; the maximum impedance at mains frequency should less than 100 kohm.

INTERMEDIATE ELECTRODES
 (between cathode and anode) in no circumstances should the tube be operated without a DC connection between each electrode and the cathode. The total effective impedance between each electrode and the cathode should 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 1kohm.

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 V as compared with that of a focused
raster.

TUBE OPERATING PRECAUTIONS
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 EHT 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 EHT capacitance before deflection has ceased after equipment has been switched off.

To prevent stray emissions:
— the anode voltage should be less than 12 kV within 5 seconds of switch-off. To prevent permanent damage to the screen:

— it is strongly advised to provide the video drive circuitry with a facility which blanks the tube automatically in the event of a deflection failure. This is particularly important in applications where the deflection coil is DC coupled to the vertical output stage, as a short-circuit fault in this stage may otherwise lead to immediate de-evacuation of the tube (pierced neck).

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 EHT 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 DC 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 MQ resistor capable of handling the peak voltages be inserted between the metal band and the point of contact with the external conductive 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 MQ resistor be bypassed by a 4.7 nF 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 nF capacitor also serves to improve EHT smoothing by adding the 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. 1mm. 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 EHT 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-F lash 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. 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 should be worn, particularly eye shielding. When suspending the tube assembly from the mounting lugs ensure that a minimum of 2 are used; UNDER NO CIRCUMSTANCES HANG THE TUBE ASSEMBLY FROM ONE LUG.


SOAK TESTING
To ensure that the operating conditions of the tube are optimized for the long term, a short stabilization period is required, afer which, the cut-off adjustment should be made and the performance assessed. It is recommended that the tube should be soak-tested for a minimum period of 2 hours running time, before it is adjusted to its final operating conditions. After soak testing, if the tube is switched off for a period of 90 seconds or more, a reheat time of 15 minutes is required before making final cut-off adjustments and picture assessment. Where the tube is switched off for less than 90 seconds, the reheat time required is 10 times the switched-off period. 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 rimbnad 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. The tube should under no circumstances be subjected to accelerations greater than 350 m/s?. Observe any instructions given on the packing and handle accordingly.

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.

DEGAUSSING
Colour picture tubes employ internal magnetic shielding. However, for individual tube types, optimal degaussing coils and circuitry are advised. Strong magnetic fields possibly existing during transportation of the tubes, and the manufacturing process of the television sets, may induce magnetic remanence.-This remanence cannot always be removed by the automatic degaussing circuitry of the set. It is therefore strongly recommended to apply an external degaussing field of sufficient magnitude and uniformity on the assembly line. This should be followed by activation of the internal set degaussing, with the set positioned in the same terrestrial orientation as for testing and performance judgement.

LOCAL MAGNETIC FIELDS
Care should be taken to avoid local AC or DC magnetic fields such as loudspeakers and transformers. The influence to beam shift may not exceed 5 microns anywhere on the screen surface.