NotTaR of Television Sets : Bad focus (fuzzy picture)               
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Bad focus (fuzzy picture)

Focus voltage on the CRT is usually in the range of 2-8 kV DC and should be controllable over a fairly wide range by the focus pot - usually located on the flyback or a little panel in its vicinity:

Also see the sections: "Focus adjustment" and "Focus drifts with warmup".

The focus wire usually comes from the flyback or if the general area or from a terminal on a voltage multiplier module in some cases. It is usually a wire by itself going to the little board on the neck of the CRT.

If a sparkgap (a little 2 terminal device with a 1/8" gap in the middle) is arcing with power on, then the resistive divider has shorted inside the flyback, focus board, or HV multiplier - whatever you TV has - and the this unit will need to be replaced. Ditto if the SCREEN control affects focus and/or vice-versa.

Using a suitable high voltage meter (range at least 10 kVDC, 1000 M ohm or greater input impedance), you should be able to measure it connected and disconnected. The ground return will be the outside coating of the CRT which may or may not be the same as the metal chassis parts. If the voltage is very low (less than 2 kV) or too high and the pot has little effect:

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