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Much of the diagnosis and correction of faults in projection television receivers conforms to standard practice in servicing conventional direct-viewing receivers, since the circuits of the two types of equip- ment are basically similar. On the other hand, there are a number of operations in projection television servicing which are either peculiar to projection equip- ment or necessitate special precautions.
In a direct-viewing receiver, where the picture is viewed on the end of the cathode ray tube itself, focusing is a simple matter of adjusting the current through the focusing coil or, in the case of permanent magnet focusing, adjusting the permanent magnet on the neck of the tube.
|FRAME NO. 19—CORRECTAND
INCORRECT MECHANICAL FOCUSING.
In projection television, however, in addition to accurate focusing of the
picture on the face of the cathode ray tube, it is neccessary to adjust the
position of the tube face with respect to the spherical mirror. The need for
this adjustment will be clear from the diagram. In a projection televisor,
the picture will be in focus on the viewing screen only when two conditions
are ful- filled: (a) The picture is correctly focused on the face of the cathode
ray tube. (b) The face of the cathode ray tube is concentric with the surface
of the spherical mirror, and at the correct distance from it. In the diagram,
the correct position of the face of the cathode ray tube with respect to the
spherical mirror A is shown by the solid line B. If, however, the axis of
the cathode ray tube is tilted so that it no longer coincides with the axis
of the mirror, the face of the tube will occupy a position such as C, indicated
by dotted line. It will be seen that in this sectional view, only one spot
on the tube surface C coincides with the correct surface B, namely at point
D. Only the point D, therefore, will be in focus.
|FRAME NO. 20—EFFECT OF
INCORRECT MECHANICAL FOCUS.
This means that, taking the whole surface of the tube face into consideration, only a narrow strip of picture will be in correct focus on the viewing screen. The exact position and width of the strip of picture in focus will depend upon the amount by which the axis of the tube is tilted away from the axis of the mirror, and the direction of tilt. Incorrect focus due to this cause can be remedied by adjusting the axis of the cathode ray tube. In the Mullard projection unit means are provided for tilting the axis in both the vertical and horizontal directions. Focusing a projection television receiver is, in reality, a very simple operation provided it is approached in the right way. To make it easier still, it has been reduced to a simple sequence. The focusing adjustment should preferably be made while a test pattern is being transmitted. It can, however, be performed when no signal is available, the image then consisting only of the scanning lines. The important point to note is that it is useless to attempt to focus the picture on the viewing screen or to centre it on the screen until it has first been accurately focused and centred on the tube face.
Stage 1—Adjustment of the picture on the face of the cathode ray tube.
|FRAME NO. 21—COIL ASSEMBLY
COMPLETE WITH CATHODE RAY TUBE.
|FRAME NO. 22—ADJUSTMENT FOR CENTRING
THE PICTURE ON THE TUBE FACE.
To centre the raster, first loosen the locknut of locking screw 6, and unscrew several turns. The focus coil now has a. fixed location with respect to the cathode ray tube only at stud 30, its location on stud 29 being against the load of spring 33. Now adjust distance stops 41 and 42 until the raster is correctly centred on the tube face. Adjustment of stops 41 and 42 causes the focus coil to swivel about its location on stud 30, thus altering the position of the raster. When doing this it is important to ensure that the focus coil does not press against the neck of the tube. Having centred the raster on the tube face, lock the adjustment by screwing in locking screw 6 until the tip of the screw just touches the end of pot 36, and tighten the locknut of screw 6. The coil assembly, complete with cathode ray tube can now be replaced in the optical unit.
Stage 2—Mechanical focusing of the tube face in the optical unit.
Having ascertained that the image is correctly focused and centred on the face of the cathode ray tube, it only remains to focus the picture accurately on the viewing screen. As already explained, this is achieved by adjusting the axis of the cathode ray tube. The cathode ray tube mounting in the optical unit is secured to yoke 4, and the yoke is capable of a certain amount of movement against spring pressure.
Thus adjustment of screw 61 tilts the axis of the tube vertically, i.e. upwards or downwards against the load of springs 59 and 60. Similarly, adjustment of screws 62 and 63 moves the cathode ray tube towards or away from the spherical mirror, against the load of springs 64 and 65. If both screws 62 and 63 are turned simultaneously in the same direction the tube face moves horizontally forward or backward. If only one of these screws is adjusted, or if both are adjusted simultaneously in opposite direc- tions the axis of the tube is tilted slightly to the left or right, i.e. in the horizontal plane. By appropriate adjustment of the three screws 61, 62 and 63, therefore, the position of the tube face with respect to the surface of the spherical mirror can be corrected.
This final operation may require slight readjustment of the three screws 61, 62 and 63 to obtain best results. When correct focus has been achieved, tighten the knurled locknuts of screws, 61, 62 and 63. The red locking plate should not be replaced, as this may throw the focus out of adjustment.
Provision is made in all projection television receivers for shifting the position of the optical unit slightly in order to centre the picture accurately on the viewing screen. In most cases all that is required is to slacken the screws which secure the optical unit to the cabinet and then to move the unit until the picture is correctly centred, when the screws should be tightened down again.
Once more, however, it should be emphasised that it is quite useless to attempt to centre the picture on the viewing screen until the raster has first been centred on the tube face as already described. It is essential that the optical unit is so located that both the corrector lens and the viewing screen are per pendicular to the optical path. This is ensured at the factory by the mounting of the optical unit and the screen.
When adjusting the centring of the picture in the screen, therefore, the optical unit must never be tilted, for example by placing packing underneath it. This is clearly indicated by the diagram shown.
If the optical unit is tilted, the only effect will be to distort the shape of the picture.
With a cathode ray tube operating at very high anode voltage, as in the case of the MW6-2, failure of one timebase will cause the tube to trace a single line on the fluorescent screen, and the screen will be destroyed along the line. If both timebases should fail, the spot will remain stationary at the centre of the tube face and the screen surface will again be destroyed, and this destruction takes place practically instantaneously. It is essential, therefore, that a projection television receiver should incorporate a protective circuit which will automatically cut off the beam in the event of either or both of the scanning voltages failing. Different set-makers use different forms of protective circuit but they have one point in common, namely, that should one or both timebases fail, the picture tube is biased well beyond cut-off, i.e. the beam is suppressed. This means that a receiver which has developed a timebase fault will exhibit a blank screen. The same symptom will also result should a fault develop in the protective circuit itself, so that the fact . that the picture tube has become inoperative does not in itself provide an indication of the location of the fault. The following procedure will, however, enable the circuit in which the fault has occurred to be identified.
Interference patterns, similar to the familiar 'bars' due to low frequency interference, but due to interference between the E.H.T. unit and the vision channel, may appear on the screen. The pattern due to Ikc/s interference is the more likely to occur, and takes the form of twenty horizontal bars across the picture. These bars will not necessarily appear stationary. The frequency of Ikc/s is the repetition frequency of the blocking oscillator in the E.H.T. unit, and the interference is usually traceable either to radiation from the supply leads of the E.H.T. unit or to common coupling via the power pack. To prevent the latter, an L.C. filter is sometimes fitted in the H.T. lead which supplies the E.H.T. unit. If Ikc/s interference patterns are experienced, it is probably due to a fault in this filter. The pattern due to 25kc/s interference takes the form of from two to three diagonal bars across the picture. This frequency is the 'ringing' frequency of the transformer from which the E.H.T. supply is derived. This form of interference is less likely to occur than Ikc/s interference, but may in some cases occur if the earth- ing of the external graphite coating of the cathode ray tube or the earthing of the optical unit is defective.
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10th June 2003