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


"Dominique MOREAU" <dominique.moreau@xxxxxxxxx> wrote in message
news:43c6ce87$0$1167$ba620e4c@xxxxxxxxxxxxxxxxx
>
> "Charles Schuler" <charleschuler@xxxxxxxxxxx> a écrit dans le message de
> news: MfydnYy2ebUVI1veRVn-hA@xxxxxxxxxxxxxx
>>
>> "Dominique MOREAU" <dominique.moreau@xxxxxxxxx> wrote in message
>> news:43c6ac4c$0$4766$ba620e4c@xxxxxxxxxxxxxxxxx
>>>
>>>
>>>
>>> Hi everybody,
>>>
>>>
>>> Is it possible to adjust a computer monitor when the CRT begins to
>>> display fuzzy image that seems to be a kind of bad focus ?
>>> (principally in the peripheral zones - center zone is still ok)
>>
>> You might find a focus adjustment. But, since the focus is non-uniform,
>> the high voltage supply might be out of tolerance (no adj.).
>>
>
> 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.

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.



.

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