COSSOR 927

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My problem is a very dim picture. It has gradually deteriorated over 12 months or so. Until now it is necessary to have the mains voltage selector set for 210 volts A.C. (the supply here
is 230 v.) in order to get a decent picture. The
tube is rather old and I think possibly the emission has fallen off. But before I go to the
expense of a new or rebuilt tube, I would like
to be sure that the tube is at fault. Would you tell me, please, any other valve or component that would be most likely to cause a fault of this
kind or a method of testing the tube’s emission ?
Another fault which occurs intermittently is a bright line across the screen horizontally. It may last for a minute or so, varying in its length of
stay, then disappearing quickly ; this may happen
two or three times in an evening’s viewing.
Yet another fault is a small bright spot the size
of sixpence at the top of the screen, which is
there constantly. -C. Lamb (Rotherham).
We would say that your tube is low emission.
but that you may be able to extend its life by using a 6.3 volt boost transformer on its heater.
The general symptoms of low emission are a patterning of the white parts of the picture on increasing either brightness or contrast, and a readiness for the picture to turn negative. Try the setting of the ion trap, which should be set
for maximum light on the screen

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