This is the classic symptom of a short between the focus and screen
supplies - probably in focus/screen divider which is part of the flyback
or tripler. If you have a high voltage meter, measuring the focus voltage
will show that (1) it is low and (2) it is affected by the SCREEN control
Similarly, the SCREEN voltage will be affected by the FOCUS control (which
is what is changing the brightness.
|NotTaR of Television Sets : Bad focus and adjustment changes brightn..
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There is a slight possibility that this may be in the CRT as well. Measure
the FOCUS and SCREEN voltage with a high voltage meter. If they are identical
pull the plug on the CRT. If they are now their normal values, then a
shorted CRT is a distinct possibility - see the section:
Rescuing a shorted CRT.