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TELEVISION PROJECTOR OPTICS SYSTEM, 1940's

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TELEVISION PROJECTOR OPTICS SYSTEM, 1940's

Used with Philips projector Television, Schmidt Optics system. At the time only the UK and France had television services. This item could produce a picture 4ft x 3ft (1.22m x 0.91m).

Your comments:

  • I own one of the televisions complete and also a spare 2" CRT.
    I recall that one other problem with the system was the generation of soft X rays caused by the 25KV eht and the tube size.
    .......... Dennis Cowdey, Reading Berks, 27th of August 2013

  • The Mullard Optical Bos used the Mullard MW6-2 CRT to produce around a 17 inch Diagonal Picture in Rear Projection Sets in the 1950's.

    The earliest sets were single channel BBC only chassis and it's true that the same unit could be used for Front Projection as in the Decca Projection TV but because of the limited light output the Screen which hung on the wall was made with a reflective aluminium surface and had a gain of around ten times.

    Projection sets using the Folded Schmidt Mullard Optical Box were only really usable in condition where the light falling on the screen could be controlled otherwise the picture tended to be washed out.

    Although the 2inch Cathode Ray Tube was very bright it suffered from obvious limitation - not the least being the Ringing Choke Power Supply that was developed for it. This consisted of a resonant ferrite transformer and a 3 valve EY51 rectifier assembly mounted inside a Tin-Can which was filled with Transformer Oil. This EHT unit
    was driven with Pulses from a 1kHz Oscillator which was very audible and the Damped Train of High Frequency oscillations which occurred between each input pulse was rectified and Tripled to produce the 25kV required by the MW6-2 CRT.

    This level of EHT produced X-Rays and so the CRT was shielded by the Alloy Casting of the Optical Unit as well as the 6inch Pyrex spherical Mirror. The Optical Path of the Mullard Box was folded to allow the Scan and Focus coils to be out of the way of the Light Path and the Centre of the Spherical mirror was blacked out to avoid light from reflecting back directly onto the CRT Faceplate improving contrast.

    The two main effects which could be seen and which could detract from viewing pleasure were due to the composition of the Projection Tube Phosphor which was comprised a mix of Blue and Yellow and the way the EHT PSU responded to sudden changes in load.

    The brightest picture was obtained with the Blue Phosphor saturated and so the Picture Highlights tended to go slightly Yellow although not a great problem in its self. However if a sudden change from light to dark occurred in the broadcast image the EHT supply could be caught out and being under damped it would oscillate for a moment or two before settling down again - unfortunately when this happened the CRT would defocus a little so that previously yellow Highlight became bright white blobs until normality was restored.

    Manufactures such as Nera and White-Ibbotson developed more Commercial Projectors using the Mullard Optics and some quite involved systems were developed to improve the EHT performance to stop these effects. One such system fed the EHT Generator from the Audio Amplifier in such a way the EHT was stabilised by controlling the current in the Audio Amplifier output stage.

    For those with old copies of the Wireless World - the success of these manufactures may be judged from a photograph of a Billy Graham meeting where either side of the stage are Large Screen displays using the Mullard Optical Box and MW6-2 combination.

    Philips and other manufactures went on to make Large and Medium screen Projectors typically using 5inch CRT's and going onto triple optical systems Projectors for Colour. One maker Rank Cintel developed a 4inch
    CRT colour projector with 10.5 inch mirrors and a 30kv EHT supply. It produce a 6 feet x 4 feet picture and was used on TV for Rear projection during News Programs and for Demonstrations of Surgery as well as for Flight Simulator displays.

    It's interesting to note that although almost every later CRT projector used normal Projection Lenses instead of Mirror Optics the light output of the Mirror System is much superior having a typical speed of f0.7 which is very useful with CRT's having limited light output. The drawback of the Mirror Optics is the need of an aspheric
    Corrector Lens to overcome the Spherical Aberration of the main Mirror. These lenses are made for a specific Throw to the Screen and so the Mullard Optical Box used in a 17inch Rear Projection Set could be used as a Front Projector and would give a picture up to 5 feet diagonal - it would not focus as well as the same unit fitted with the proper Corrector Lens.
    .......... John Paine, London UK, 13th of December 2011

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