CRT begins to display a "fuzzy" image ...

You might find a focus adjustment. But, since the focus is non-uniform, the high voltage supply might be out of tolerance (no adj.).

"Charles Schuler" a écrit dans le message de news: snipped-for-privacy@comcast.com...

Do you talk about feeding circuit of the high-voltage sol ?

As far as I know, there is normally a high-power diode in such a circuit; I suspect that a defect of this diode could produce a kind of "noise" in drive signal for the high-voltage sol. On the other hand, I believe that a trouble with the high-power transistor which commands the oscillating circuit for the HV sol could also produce a 'noise' in that signal.

(( BUT, I am not a specialist, and I probably say stupidities...))

First you talked about focus and now you talk about noise. I am confused!

Focus in a CRT is accomplished via an electrostatic "lens" which is established by several voltages applied to the CRT. The second anode voltage is thousands of volts (20 or so) and the focus voltage is less. Both must be correct for the electrons to converge at the correct point on the screen.

The focus voltage is often adjustable, the second anode voltage is seldom adjustable. Find the focus adjustment (it might not exist) and try it.

The second anode voltage (and the focus voltage also) is derived from the horizontal output circuit. This circuit is failure prone. It has diodes and transistors and other parts that can become weak or just fail.

Sorry, but you are stretching here ... there is usually no simple fix for problems of this type.

It could be the CRT, but it's possible it's just a problem with the focus circuit. I've seen a bad focus divider in the flyback cause fuzzy edges with a sharp middle.

I want to thank everyone who answered to my message on this newgroup. Thanks to your advices, my monitor came back to the life!

The funny aspect of the experience: I had to use a wood Chinese chopstick that I cut to the right shape for adjusting the focus potentiometers :)

Thanks again ! Dominique