SERVICING THE
PROJECTION TELEVISION SYSTEM

THE OPTICAL SYSTEM

In order to obtain the required magnification, the viewing screen must be situated at some considerable distance from the optical system. This distance, known as the 'Throw', ranges from 25½ins. to 104ins. In order to accommodate the 'throw distance' in a cabinet of reasonable size, the optical path is 'folded' by a system of mirrors, much in the same way that the effect of a long telescope is obtained in a compact pair of binoculars by means of reflecting prisms.

FRAME NO. 1—THE OPTICAL PRINCIPLE OF THE MULLARD PROJECTION TELEVISION SYSTEM.

X represents a single picture element situated at the top of the raster on the face of the picture tube A. Light radiated forward from picture element X is collected by the spherical mirror B and is reflected as a convergent beam on to the plane mirror C mounted at an angle of 45° to the axis of the tube. It should be noted that there is a central aperture in mirror C through which the cathode ray tube protrudes slightly. The light reflected from the spherical mirror B on to plane mirror C is again reflected, still as a convergent beam, through the corrector lens D on to a second plane mirror E, also arranged at an angle of 45° to the tube axis. From mirror E the light is redirected to the viewing screen F which is located at such a distance that the magnified image of spot X is correctly focused on the screen at Y.The throw distance is the sum of the distances GH and HJ. The function of the corrector lens D is to correct an inherent lack of sharpness of focus associated with all spherical mirrors. It also determines the throw dis- tance at which the whole of the picture is in focus on the viewing screen. The difference between the various models of the projection unit thus lies merely in the corrector lens.

FRAME NO. 2—THE COMPONENTS OF THE MULLARD PROJECTION TELEVISION SYSTEM.

Here can be seen the essential components of the Mullard Projection Television System. They comprise:

1. The 2½ins. diameter picture tube MW6-2
2. The optical unit embodying the tube mounting, focus coil, deflection coils, spherical mirror, the first plane mirror, and the corrector lens.
3. An oil filled E.H.T. supply unit.

FRAME NO. 3—MULLARD MW6-2 CATHODE RAY TUBE.

The MW6-2 projection, picture tube has a 2½ins. diameter screen and is designed to operate at an anode voltage of 25kV. It has the normal heater rating of 6-3V, 0-3A. The E.H.T. terminal is located in a glass shield to obviate risk of flash-over or leakage between the E.H.T. connection and the deflector coils or the external graphite coating. This coating, which must be earthed, forms, with the glass envelope and the internal conductive coating, a capacitance of approximately 450pF which, with a 1 megohm resistor embodied in the 25kV lead, serves as the final smoothing for the E.H.T. supply.

FRAME NO. 4—SECTIONAL DRAWING
OF MW6-2 CATHODE RAY TUBE.

The gun is of the triode type, that is to say there is only one anode. The only unusual feature of the gun is the spark trap - a ring-shaped electrode located between the anode and the grid. This is connected to one of the base contacts, and must be earthed. Any discharge which might result, say, from the release of a small amount of gas due to unintentional overload, will take place between the anode and spark trap, thus avoiding damage to the cathode. The luminescent screen is backed by a thin layer of aluminium which not only minimises the risk of ion burn, but, by reflecting outward much of the emitted light which would otherwise be directed to the rear of the tube, greatly im- proves the optical efficiency.

X-RAY PROTECTION

All cathode ray tubes produce a small amount of 'soft' X-rays at the screen. Because of the high E.H.T. potential employed in projection television, the soft X-ray emission from the MW6-2 is somewhat greater than from a direct viewing tube. This matter has been very fully investigated by, among others, the National Physical Laboratory, in order to assess the risk of danger to health from X-ray emission. These investigations showed that it is only when the MW6-2 tube is operated outside the projection unit and at maximum brightness that the 'maximum permissible dosage rate' is exceeded. The only precaution necessary, therefore, is to take care that if the tube is operated outside the unit, e.g. during servicing, it should be adjusted for the minimum brightness consistent with examination of the picture. When the tube is operating in the unit there is no risk whatsoever, and no special precautions need be taken.

TUBE PROTECTION

In order to prevent the tube from being ruined by burning of the screen in the event of failure of either one or both the timebases, means are provided in all projection television receivers to suppress the beam if a timebase failure should occur. The operation of the protection circuit, and its implications in servicing are dealt with in greater detail later.