Bush CTV25: Focus problems

Problem

The problem with this set is poor focus. It originally started with an intermittent focus electrode spark gap discharge on switching on, two or three discharges during the first 5 mins. Attempts to improve matters by resetting the focus control and by changing over course focus selector connections  have failed to improve matters. The picture brightness and size are normal. By unplugging the focus lead at the tube base with the set operating, the picture comes into focus and then drifts completely out of focus.

Solution

In some cases this fault can be cured by removing the focus spark gap, thoroughly cleaning the area around it with methylated spirits and then fitting a new one. The focus and e.h.t. potentials are critical however:ensure that the voltage selector is correctly set and carry out line drive and e.h.t. adjustments as given in the manual. Check the high value resistors in the focus circuit, particularly those on the earthy side of the focus control. ( MK.3 version timebase).