The following is article appeared in the very first edition
Practical Television magazine way back in April 1950.
THE Mullard projection television system was shown at the Radio Component Manufacturers' ederation exhibition and at the Physical and Optical Society's exhibition last April, and was demontrated in a range of prototype receivers at Radiolympia.
The principle employed is to produce a miniature picture of very great intrinsic brightness on the screen of a 2½ in. diameter picture tube operated at an anode voltage of 25 kV., and to project an enlarged image of this picture by means of a system of mirrors and lenses on to a flat screen.
The essential equipment for the system, which Mullard Electronic Products, Ltd., are now making available to television manufacturers, comprises three units : the 2½in. screen cathode ray tube itself (Mullard Type MW6-2) ; an optical unit incorporating also the tube mounting, and focus and deflecting coils ; and a compact E.H.T. supply unit. This apparatus relieves the set designer of all the special problems associated with projection television as such, leaving him only those more familiar problems connected with circuit design.
Fig. 1 is a simplified diagram of the optical system, which is a " folded " version of the mirror-lens system devised for use in astronomical telescopes. Its operation can best be understood by considering a single picture element A situated at the top of the raster in the pictufc tube C.
Light radiated forward from picture element A is collected by spherical mirror D and reflected as a convergent beam on to plane mirror E mounted at an angle of 45 deg. to the axis of the tube. It will be observed that there is a central aperture in mirror E, through which the end of the cathode ray tube protrudes slightly.
The light reflected from spherical mirror D on to plane mirror E is again reflected, still as a convergent beam, vertically upward through corrector plate F, on to a second plane mirror G, arranged at right angles to E ; and front mirror G the light is re-directed to screen H, which is located at such a distance that the magnified spot is accurately focused at J. Similarly, light from a picture element B at the bottom of the raster is brought to a focus at point K on the viewing screen.
The corrector plate F is a lens of special contour which compensates for spherical aberration, i.e., the property of a spherical mirror whereby the reflected light does not all come to focus at the same point. By introducing the corrector plate accurate focus at the screen surface is assured. The production of this component presented a major technical and economic problem. If the lens had been made in glass by the normal grinding technique, or even if a plastic lens were used, the cost would have been prohibitive. The solution was found in moulding the lens in gelatine on to a glass plate.
Fig. 2 is a photograph of the complete optical unit with the picture tube in position. The optical components are factory-adjusted and need no further attention. Simple mechanical adjustments are, however, provided for centering the picture and for final focus.
The MW6-2 picture tube is illustrated in Fig. 3. It is a 2½in. screen triode tube designed for operation at an anode voltage of 25 kV. The heater is rated at 6.3 v., 0.3 A. The luminescent screen is backed by a very thin layer of aluminium which, by reflecting outward much of the light which would otherwise be directed to the rear of the tube, increases the output of forward-going light by some 75 to 80 per cent. It also eliminates internal reflections and thus improves picture contrast. Furthermore, the metallising s also serves as an effective ion trap.
Special features of the tube include a spark trap—a ring-shaped electrode located between anode and grid, and connected to earth through one of the „ base contacts. It thus safeguards the cathode in the event of a discharge due, for example, to the release of a small amount of gas as the result of unintentional overload.
Another feature is a glass shield surrounding the anode terminal to eliminate risk of flash-over or leakage from the E.H.T. connection.
The external surface of the neck and cone is coated with a graphite preparation. This coating forms, with the glass envelope and the internal metallisation of the tube, a capacitor of about 450 µµF which, with a 1 megohm resistor in the 25 kV. lead, serves as the final smoothing of the E.H.T. supply.
Because it would be neither economic nor safe to produce the 25 kV. E.H.T. supply from the 50 cycle mains in the conventional manner, an E.H.T. unit operating on the "ringing choke" principle lias been adopted. A blocking oscillator (the triode section of an EBC33 double diode triode) generates a voltage of , approximately sawtooth form. This voltage is applied to the control grid of an EL38 pentode which is biased well below cut-off so that anode current flows during only part of the sawtooth cycle. Each time anode current ceases, the energy stored in the inductive component of the anode circuit is released as a damped train of high-frequency oscillations. The peaks of these oscillations are rectified and smoothed to produce the E.H.T. supply.
These peaks are of approximately 8 to 9 kV., and a voltage tripler rectifying circuit, employing three EY51 rectifiers, is used to produce a rectified output at 25 kV. Good regulation of the E.H.T. voltage is ensured by using the diode sections of. the EBC33 to produce an automatic variation of the grid bias on the EL38 driver valve. As will be seen from Fig. 5, the E.H.T. unit is very compact, and all parts carrying E.H.T. potentials are oil-immersed in a sealed can.
It has only been possible in this article briefly to outline the principles of the optical projection system, but in later issues articles will explain in greater detail this important development in television reception.
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11th January 2003