Intrinsicality: The Mullard philosophy of product design and service; The building into a Mullard product from its inception those qualities and characteristics which are essential to the efficient, reliable and economical performance of the equipment into which the product is fitted
The television picture tube of today is a far cry from that of twenty years ago. The tube of the immediate post-war years was about fourteen inches long, although its circular face had a diameter of only nine inches. The face was convex and one could therefore only view with comfort by sitting directly in front of it. The brightness of the picture was so poor that it could only be viewed in a darkened room.
Sand, the main constituent of glass, arrives at the Mullard glass factory at Simonstone. After this stage the glass-making process is completely enclosed and fully automatic.
The "Panorama" picture tube of today, with its nearly flat rectangular faceplate, its short neck, its 110 degree scanning angle, its aluminised screen and its implosion-safe construction is the product of twenty years of Mullard research and development.
In the Mullard picture tube factory at Simonstone it is evident that the care taken at the research and development stages is backed up by an equal amount of care during manufacture. This is Intrinsicality in action—care and attention to detail at every stage of research, development and production. Because every part of the picture tube is made within the Mullard organisation—not only the electron gun components and the phosphors, but even the glass—Mullard quality can be built in at every stage of manufacture.
The glass parts, which are made in Mullard's own glass factory, are inspected before despatch to the tube production line. The faceplates are inspected for flaws, air bubbles or foreign bodies embedded in the glass as well as for surface imperfections. Any faulty faceplates are broken up and the glass is melted down and used again. When inspecting the cones and necks, surface imperfections are not so important, but parts containing flaws which might affect the strength of the finished tubes are rejected. The glass parts now pass to the machine where the neck and cone are joined together. On this machine the neck and cone are slowly brought up to the required fusing temperature by a battery of gas jets, and a smooth joint made. A hole is cut in the side of the cone and the metal e.h.t. connection inserted. After this the cone and neck assembly is passed through an annealing oven to remove all the stresses and strains set up within the glass during the joining processes. When it emerges from the annealing oven it is ready to have its faceplate welded on. The faceplate welding process, which is completely automatic, is again followed by annealing. The complete bulb is given four washes in hydrofluoric acid and distilled water alternately to remove all traces of contamination. Distilled water is used because even the small quantity of impurity to be found in tap water might impair the eventual quality of the fluorescent screen. The hydrofluoric acid has the additional function of etching the inside of the faceplate to facilitate adhesion of the phosphors.
The bulb is now extremely clean and ready to receive the phosphor deposits which turn the faceplate into a fluorescent screen. This process is known as settling. The bulb is placed on the settling machine with its neck uppermost and a buffer solution is added which acts as a medium through which to deposit the phosphors. The supply of the buffer solution is auto- matically cut off immediately the correct level is reached because this level is vitally important for the even distribution of phosphors over the faceplate. When the right depth of buffer solution has been pumped in, the phosphors are introduced in the form of powder in sus- pension. A mixture of two different phosphors is used, each phosphor emitting light of a different colour, and these phosphors are blended in precisely controlled proportions to give a white picture. It is very important that the particle size distribution should be uniform, because if great care is not taken to ensure this, there is a tendency for the particles of one of the phosphors—the one which emits a yellowish light—to be bigger than the other. The bigger particles settle in greater quantities in the middle of the screen and if this were not controlled the picture would not be of the same colour all over but would have a yellowish centre. This would obviously be undesirable but the accuracy with which the phosphors are prepared ensures that it cannot occur in Mullard tubes. Particle size is not, however, the only factor which makes a good phosphor. Excessive contamination of the phosphor could result in gases being given off after evacuation which might poison the cathode and thereby shorten the life of the tube. It is therefore essential that the proportion of contaminants should be kept to a minimum and in Mullard tubes it is as low as 0.01 per cent. The phosphors in suspension are introduced into the buffer solution through a device specially designed to distribute the phosphors evenly throughout the solution, because for the high-quality screens built into Mullard tubes it is essential that the screen phosphors should be evenly laid. The buffer solution and the liquid in which the phosphors are suspended react to form a silica gel which is deposited on the faceplate and acts as an adhesive to hold the phosphors on the screen. When the correct thickness of phosphors has been deposited, the remainder of the liquid is decanted. The layer of phosphors is dried before the bulb proceeds to the next stage.
In the aluminising process it is essential that a flat aluminium layer of even thickness be deposited on the back of the screen. To help achieve this, the back of the phosphor-coated screen, which is rather uneven, is first given a coat of clear lacquer. Great care is then taken to ensure that all traces of the lacquer are removed from the neck and cone of the bulb and the bulb is thoroughly dried by means of a jet of air. A conducting strip of graphite is now painted on the inside of the bulb from the e.h.t. connection in the side of the cone to the neck where it will eventually make contact with the electron gun. The bulb is now evacuated and a small billet of aluminium is inserted and heated until it evaporates. The aluminium condenses when it comes into contact with the sides of the bulb. It is important to the quality of the screen that the aluminium layer should be of a precisely controlled thickness and Mullard engineers have designed an ingenious way of achieving this. A device is incorporated in the aluminising machine which measures the thickness of the aluminium layer electronically. When the aluminium is of the correct thickness a signal from the measuring device is used to switch off the power to the heater, stopping any further evaporation. The whole aluminising process takes only a few seconds, and the rapid action of the electronic switch enables the thickness of the aluminium to be controlled very accurately.The bulb is now baked, the baking process having the effect of evaporating the lacquer from under the aluminium. The lacquer diffuses through the aluminium and is then flushed out leaving nothing between the screen phosphors and the aluminium layer. The neck is now given a further thorough wash and is then ready to have the electron gun inserted.
In the department where the electron guns are made everything is kept scrupulously clean because any grease or foreign matter around the electrodes can impair the insulation, shortening the life. Nobody is allowed into the workroom unless they are wearing a lint-free nylon overall. The room is air-conditioned and smoking is strictly banned. Jigs are used whenever possible to give greater accuracy to the assemblies. The design and manufacture of these jigs are very specialised and are executed by Mullard's own mechanical engineers. The female assemblers are given dexterity tests before being chosen and are carefully trained so that their natural skills can be trained so -mat me used to best advantage. Quality control is applied to all stages of assembly. The extreme accuracy of every gun assembly is ensured by the use of optical devices, which magnify 70 times at the final inspection. All the gun components are thoroughly cleaned and de-greased before assembly and the finished guns are thoroughly was hed once again before being mounted in the tubes.
The gun is inserted into the bulb and the base sealed to the neck. The tube is pumped to a very high degree of vacuum and the getter fired. A hard vacuum is essential for long life and in Mullard tubes the residual gas pressure is measured in millionths of a millimetre of mercury. The tube now undergoes two processes known as ageing and spot-knocking. Ageing consists of energising the heater until the cathode reaches a stage of steady emission, and spot-knocking consists of applying high voltage pulses to remove any slight traces of foreign matter which may have crept in and could cause arcing in spite of all the precautions taken to prevent it.
The almost complete tube is now given several tests including display of a test card, and it then only remains for a layer of graphite to be deposited on the outside and, in the case of a "Panorama" tube, for the reinforcing ring to be fitted around the faceplate. The tube is then ready for packing. The quality laboratory at Simonstone takes random samples of complete tubes and subjects them to stringent tests. Every stage of the tube's assembly is mechanised and at every stage inspection is carried out by experts of long experience. These are some of the factors which ensure the long life and consistent performance of Mullard TV picture tubes. Each factor contributes in its own way to the grand pattern of Mullard Intrinsicality.
NEXT : Part 2 of the artical
|CLICK HERE TO RETURN TO
THE VALVE TECHIE PAGE
2nd February 2003