US3723857A

US3723857A - Method of detecting cathode poisoning contaminants in a cathode ray tube

Info

Publication number: US3723857A
Application number: US00227167A
Authority: US
Inventors: A Kloba; R Maskell
Original assignee: Zenith Radio Corp
Current assignee: Zenith Electronics LLC
Priority date: 1972-02-17
Filing date: 1972-02-17
Publication date: 1973-03-27
Anticipated expiration: 1990-03-27
Also published as: CA970036A

Abstract

A method of detecting cathode poisoning contaminants in a cathode-ray tube of a given tube type while allowing the tube to remain intact comprises the steps of first measuring the cathode emission of the cathode-ray tube to establish a first reading. Suspected contaminated areas of the cathode-ray tube are then subjected to electromagnetic radiation for a predetermined time, after which, the cathode emission of the cathode-ray tube is remeasured to establish a second reading. Thereafter, the percentage change of the first and second readings is compared to a predetermined standard. The cathode-ray tube is considered to be contaminated if the comparison represents a percentage decrease in cathode emission which exceeds the predetermined standard.

Description

nited States Patent 1191 Kloba et a1.

1 1 Mar. 27, 1973 [54] I METHOD OF DETECTING CATI-IODE POISONING CONTAMINANTS IN A CATHODE RAY TUBE [75] Inventors: Anthony Kloba, Bensenville; Roy

Maskell, Oak Park, both of I11.

[73] Assignee: Zenith Radio Corporation, Chicago,

[22] Filed: Feb. 17, 1972 [21] Appl. No.2 227,167

[56] References Cited UNITED STATES PATENTS 3/1953 Reid ..324/23 Shoup ..324/20R 3,361,958 1/1968 Martin ..324/20 R Primary ExaminerRobert J. Corcoran Attorney-Nicholas A. Camasto 57 ABSTRACT A method of detecting cathode poisoning contaminants in a cathode-ray tube of a given tube type while allowing the tube to remain intact comprises the steps of first measuring the cathode emission of the cathode-ray tube to establish a first reading. Suspected contaminated areas of the cathode-ray tube are then subjected to electromagnetic radiation for a predetermined time, after which, the cathode emission of the cathode-ray tube is re-measured to establish a second reading. Thereafter, the percentage change of the first and second readings is compared to a predetermined standard. The cathode-ray tube is considered to be contaminated if the comparison represents a percentage decrease in cathode emission which exceeds the predetermined standard.

5 Claims, 2 Drawing Figures METHOD OF DETECTING CATHODE POISONING CONTAMINANTS IN A CATHODE RAY TUBE BACKGROUND OF THE INVENTION The present invention is directed to a method of detecting cathode poisoning contaminants in a cathoderay tube but findsparticular application in the detection of cathode poisoning contaminants in a color reproducing cathode-ray tube.

In the manufacture of the screen for the panel portion of a color tube the red, green and blue phosphors which form the screen, are individually applied to the panel in the form of slurries each comprising a polyvinyl alcohol carrier, an appropriate phosphor and a dichromate sensitizer. In known fashion each slurry mixture is sequentially exposed (with, of course, intervening washes) to actinic energy through an apertured shadow mask to form a dot pattern to the end that upon completion of three exposures, a screen comprising a plurality of color phosphor dot triads is laid down upon the face panel. Thereafter, a thin film of laquer is deposited over the phosphor dot pattern to provide a smooth base for a light reflecting backing layer of aluminum.

The panel is now ready for the bake-out process, a manufacturing step which consists of transportingthe panel through an oven that subjects the panel to a temperature approximately 400 C in order to, inter alia, remove undesired or unnecessary constituents such as the polyvinyl alcohol and the lacquer. Since polyvinyl alcohol and the filming lacquer have thermo decomposition temperatures well below 400 C, they are readily vaporized and therefore, for the most part, removed during the bake-out process.

The panel is then joined to a funnel andfrit sealed thereto after which an electron gun structure and a getter device are inserted in the neck portion of the funnel. Finally, the completed tube is evacuated to remove as much gaseous residue as possible and then the envelope is sealed off. However, since it is impossible to remove all the gas residue by evacuation, the remainder is removed by gettering, a gas pumping process which entails firing the getter, a device formed of a metal such as barium, magnesium or sodium. When the getter is tired, vaporized metal condenses upon the inner surfaces of the funnel portion envelope wall as well as upon the rear face of the shadow mask where it serves to absorb substantially all the residual gas remaining in the envelope. In fact, this vaporized metal continues to pump," or absorb, gas for the life of the tube. Insofar as undesired constituents remaining or trapped within the evacuated tube are concerned it has been found that polytetrafluoroethylene, more known by its trade name, Teflon, has a very deleterious effect upon cathode mission should it be introduced, even in minute quantities, into the envelope during the manufacturing process. This obtains because Teflon has a thorough decomposition temperature much higher than any temperature encountered by the face panel or the envelope during bake-out or frit sealing. In fact, Teflon is thermally stable up to about 530C. The problem,'however, arises when the Teflon particles are subjected to electron impingement during normal operation of the tube. This impingement reduces the Teflon to its reaction products, one of which, fluorine gas, is highly detrimental in that it contaminates the cathode. Specifically, the fluorine combines with barium (the electron emitting agent of a cathode) to form barium fluoride, a substance that does not emit electrons at normal cathode operating temperatures.

A peculiar problem arises when a color cathode-ray tube is contaminated by a cathode poisoning constituent, such as Teflon in that the cathode poisoning does not proceed to such an extent, under normal operating conditions of the tube, to allow the contamination to be detected during the normal manufacturing process. In particular, the cathode usually does not show any deleterious effect on the operation of the finished tube until after the tube has operated for many hours under normal conditions in the consumers home. Obviously, to prevent such an occurrence, it is most advantageous to sort out the contaminated cathode-ray tubes before they are incorporated into a finished I V receiver for consumer use. However, prior techniques for detecting cathode poisoning contaminants within such cathode-ray tubes have proven to be ineffective or prohibitively expensive.

One method of course, would be to life test certain ones of the suspected tubes to see if under normal operating conditions and after many hours of operation, the tested tubes fail. Unfortunately however, such a life test takes many hours to perform, making it prohibitively expensive to test each suspected tube individually.

A more recent approach has been to use a mass spectrograph apparatus for determinating the constituents inthe evacuated atmosphere of a cathode-ray tube. This procedure involves forcing a probe through the cathode-ray tube envelope wall while maintaining the vacuum within the tube and then to use the mass spectrograph to determine if contaminating constituents are within the tube. However, this procedure is not very desirable from the standpoint that if the vacuum within the tube were lost the tube would have to be once again evacuated and sealed. Also, a tube subjected to this procedure because of the break in the envelope wall would most likely be inappropriate for future commercial sale which would result in the loss of the time and expense of manufacturing the tube.

Prior methods therefore have obviously not solved the problem. Either the tube is damaged, or the excessive time involved to test each suspected tube is prohibitively expensive.

It is therefore a principal object of the invention to provide a new and improved method for detecting cathode poisoning contaminants in a cathode-ray tube.

It is a further object of the invention to provide a method of detecting cathode poisoning contaminants in a cathode-ray tube which allows each suspected tube to be tested.

It is a specific object of the invention to provide a method of detecting cathode poisoning contaminants within a cathode-ray tube which assures that only uncontaminated tubes reach the ultimate consumer.

SUMMARY OF THE INVENTION The invention provides a method of detecting cathode poisoning contaminants within a cathode-ray tube ofa given tube type while allowing the tube to remain intact which comprises the initial step of measuring the cathode emission of the cathode-ray tube to establish a first reading. Suspected contaminated areas of the cathode-ray tube are then subjected to electromagnetic radiation for a predetermined time period followed by a re-measuring of the cathode emission of the cathode-ray tube to establish a second reading. Thereafter, the percentage change of the first and second readings is compared to a predetermined standard, the cathode-ray tube being considered to be contaminated if the comparison represents a percentage decrease in cathode emission which exceeds the standard.

BRIEF DESCRIPTION OF THE DRAWINGS The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawing and in the several figures of which like reference numerals indicate identical elements and in which:

FIG. 1 is a side view of a cathode-ray tube, partly in section, of a type amenable to practicing the invention;

and FIG. 2 is a cross-sectional view, partially cut away, taken on line 2-2 of FIG. 1.

Prior to discussing the detecting process, that is the process employed to detect cathode poisoning contaminants within a cathode-ray tube, attention is directed to the cathode-ray tube depicted in FIG. 1. Tube 10 comprises a face panel 1 1, a funnel section 12, which is frit sealed to panel 11 and which terminates in a neck portion 13. A phosphor dot screen 14 is deposited upon the inside surface of panel 11 and a shadow mask 15 is suspended within the panel adjacent screen 14 in conventional fashion. A high voltage anode button 16, fitted upon one surface of funnel 12, is connected to a source of accelerating potential, HV. An electron gun structure 17, extends into the funnel portion of the tube. Device 19, which is activated after the tube has been evacuated and sealed off, serves to deposit a thin layer of getter material upon the inner wall of the funnel 12, as well as upon the surface of the shadow mask 15 which confronts the electron gun structure of the tube. This layer of getter material then functions to absorb or pump gaseous residue remaining within the evacuated envelope. Finally, a controllable deflection apparatus is associated with tube 10. This apparatus which comprises a pair of

deflection windings

21, 22, differs from the conventional deflection yoke in that a pair of

adjustable sweep generators

23, 24, respectively are individually associated with an assigned one of the deflection winding so that any desired raster configuration can be generated.

During normal operation of the tube 10, the impingement of high energy electrons upon any cathode poisoning contaminant such as Teflon particles trapped in the tube serves-to break these particles down into a number of reaction products. Specifically, when Teflon is bombarded, it decomposes into CF 4, carbon tetrafluoride, a gas, which, in turn, under bombardment is reduced to F fluorine gas. The cathodes employed in picture tubes are mainly formed of barium oxide and it is the barium that is principally the emitter of the electrons. However, in the presence of fluorine the barium combines with that gas to form barium fluoride, a substance that does not emit electrons at normal cathode operating temperatures. Thus in ii cathode-ray tube containing even traces of Teflon, the periodic bombardment of the Teflon by the electron beams eventually break the Teflon down into the contaminating fluorine gas.

The present invention is predicated upon the discovery that by establishing the proper conditions within the suspected contaminated tube to accelerate cathode poisoning, and by establishing a standard of cathode activity before total cathode destruction takes place, suspected tubes may be tested and uncontaminated ones sorted out from contaminated ones in a relatively short time. Since no tube cathode is completely destroyed, the contaminated tubes may even be subsequently reclaimed.

A specific manner in which the cathode poisoning contaminants in a cathode-ray tube may be detected, and as a preferred embodiment, may best be understood by once again referring to FIG. 1. The cathode activity such as the cathode emission current of the cathodes of gun structure 17 is first measured. This may be done under a low filament voltage condition with the electron gun structure cut off. Filaments 18 are supplied with a filament voltage of 5.7 volts as compared to their normal operating voltage of 6.3 volts.

Grids

30 and 31 of electron gun structure 17 are electrically connected together. A volt dc is applied to grid 32 and the voltage applied to

grids

30 and 31 is adjusted so that the electron gun structure 17 is in a cut off condition. The parallel arrangement of resistor 33 and volt meter 34 is connected in series with cathode 35 and grid 31. Volt meter 34 is of the type which is suitable for measuring current in the range of 0-10 milliamperes, when measuring the voltage drop across resistor 33. Once the elements of gun structure 17 have been provided with the appropriate voltages, the cathode current flowing from cathode 35 to grid 31 is measured by volt meter 34 to establish a first reading.

After the first reading has been established suspected contaminated areas of cathode-ray tube 10 are subjected to electromagnetic radiation for a predetermined time period. This breaks down the contaminants which may be in the tube into the cathode poisoning constituents such as fluorine gas. The suspected areas of contamination are usually those areas which are of a metallic material such as shadow mask 15, shadow mask support frame 40 and electron shield 41.

The source of electromagnetic energy used may be the most readily available source, that is the electron beams of the tube itself. The filaments 18 of the cathodes are heated to a lower than normal operating temperature by maintaining the filament voltage at 5.7 volts, this in contrast to normal filament voltage of 6.3 volts. This filament voltage is used, because it has been found that cathodes are most sensitive to cathode poisoning if they are heated by filaments which are at a voltage of approximately 10 percent below normal operating voltage. This is done to establish the cathodes in the worst condition for poisoning to make the amount of time necessary to perform the test short enough to satisfy manufacturing economics, and still effect the cathode sufficiently to detect a change in cathode activity. The final voltage at anode button 16 is adjusted to be in the range of 22 kilovolts to 26 kilovolts which is used to accelerate the electron beams onto the suspected contaminated areas of the tube.

The suspected areas of contamination, namely shadow mask 15, mask frame 40, and electron shield 41 may more clearly be seen in FIG. 2. As explained earlier, adjustable sweep generators 23 and 2-4'are individually associated with an assigned one of the

deflection windings

21, 22 so that any desired raster configuration can be generated. This enables only those areas suspected of contamination to be scanned such as the peripheral area of face panel ll.which contains mask 15,mask frame 40 and electron shield 41 to contribute to the establishment of an environment within the tube which accelerates the cathode poisoning. It has been found by subjecting only those areas suspected of contamination to the electromagnetic radiation and by maintaining the cathodes of the gun structure at lower than normal operating temperature, a time period of only one hour is required.

After the suspected contaminated areas of the tube are subjected to the electromagnetic radiation of the electron beam, the cathode emission current of electron gun structure 17 is re-measured under the same conditions used to establish the first reading. This is performed after the electron gun structure is allowed to cool for approximately minutes and establishes a second reading.

Thereafter, the percentage change between the first and second readings is compared to a predetermined standard which is arrived at by, for example, experimentation. The experimentation-may take the form of performing the test procedure previously described on a relatively few of the cathode-ray tubes suspected of cathode poisoning contamination and determining the percentage change between the first and second readings for each tube and then life testing the tubes to determine what the predetermined standard should be. In other words,.the percentage decrease in cathode emission which can be tolerated and still have a tube which will not subsequently fail due to cathode contamination is determined. Once this standard is determined, the percentage change of the first and second readings of each tube tested can then be compared to this predetermined standard and if the comparison represents a percentage decrease in cathode emission which exceeds-the standard, the tube is considered to be contaminated.

The standard deduced on the experimental basis may be different for various tube types processed under different processing conditions. However, once the standard is established for that tube type andfor that production process, one can be assured that a valid test for each tube has been made.

The present invention provides a method of detecting cathode poisoning contaminants within a cathodetaminated is described and claimed in co-pending [1.5. application Ser. No. 205,180 in the name of Leonard Dietch and Martin Lerner and is also assigned to the assignee of the present invention.

While a particular embodiment of the invention has been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

We claim:

1. A method of detecting cathode poisoning contarninants in a cathode-ray tube of a given tube type while allowing the tube to remain intact, which method comprises:

measuring the cathode emission of said cathode-ray tube to establish a first reading;

- subjecting suspected contaminated areas of said cathode-ray tube to electromagnetic radiation for a predetermined time period;

re-measuring the cathode emission of said cathoderay tube to establish a second reading;

and thereafter comparing the percentage change of said first and second readings to a predetermined standard, said cathode-ray tube being considered to be contaminated if said comparison represents a percentage decrease in cathode emission which exceeds said standard.

2. A method in accordance with claim 1 where said cathode emission is measured to establish said first and second readings while said cathode-ray tube is cut off.

3. A method in accordance with claim 1 where the cathode of said cathode-ray tube is established in a sensitive cathode poisoning condition while said suspected contaminated areas are subjected to said electromagnetic radiation.

4. A method in accordance with claim 1 where said suspected contaminated areas of said cathode-ray tube are subjected to said electromagnetic radiation for a time period of 1 hour.

5. A method in accordance with claim 3 where said cathode is established in a sensitive cathode poisoning condition by maintaining said cathode at a temperature which is lower than its normal operating temperature.

Claims

1. A method of detecting cathode poisoning contaminants in a cathode-ray tube of a given tube type while allowing the tube to remain intact, which method comprises: measuring the cathode emission of said cathode-ray tube to establish a first reading; subjecting suspected contaminated areas of said cathode-ray tube to electromagnetic radiation for a predetermined time period; re-measuring the cathode emission of said cathode-ray tube to establish a second reading; and thereafter comparing the percentage change of said first and second readings to a predetermined standard, said cathode-ray tube being considered to be contaminated if said comparison represents a percentage decrease in cathode emission which exceeds said standard.