NotTaR of Television Sets : Bad focus and adjustment changes brightn..  
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Bad focus and adjustment changes brightness

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.

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.