----- Original Message ----- 
From: "John Coppens" <john@jcoppens.com>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Sunday, July 20, 2008 12:19 PM
Subject: Re: [EE] Reparing my CRT monitor


> On Sun, 20 Jul 2008 10:02:56 -0300
> "Dennis Crawley" <dennis.crawley@usa.net> wrote:
>
>> There is a noise of about 15Khz. When I drop down the resolution to
>> 800x600 the noise stops.
>> As well as 1 hour after switched on.
>
> I don't know if it has ever been done, but a good first step is clean the
> monitor inside (use pressured air if you have a compressor). Sometimes,
> dust provides a discharge path for the high voltage (though mostly it
> manifests itself on the screen then).
>
**
High voltage discharge will not manifest itself as a 15kHz noise. Instead, 
it will be an arcing, snap, or sizzle.

Using compressed air from an air compressor is normally NOT a good idea.
Unless you regulate the pressure down to a rather low level, you take the 
chance of causing damage to some of the very flimsy boards/connectors used 
in many monitors.
More importantly, some compressors discharge a small amout of oil with the 
air. This will create an environment where the buildup of dust will increase 
drastically causing, among other problems, excessive heat buildup.
If you must blow the dust out (seldom an issue unless used in a dirty 
environment), use a can of clean compressed air or an attachment on your 
vacuum cleaner hose, with the hose attached the discharge side of the vacuum 
cleaner (DO NOT try to suck the dust out!). If you must try brushing extreme 
buildup, do so VERY gently with a soft brush (like a makeup brush), to 
avaoid damage.
**

> If the noise really comes from the high voltage circuit, it could be an
> indication that the flyback transformer is about to give up. And near
> that transformer, there are a couple of capacitors that I've known to
> start generating noise.
>
**
>From your stement that changing resolution makes the sound come and go, it 
makes sense that the problem is in the horizontal frequency 
generation/horizontal deflection section. Afer all, one of the things that 
changes with different resolutions is the horizontal scanning frequency.

Check around the horizontal output section for one or more small iron-core 
coils. These are notorious for having loose cores. Pressing on the coil with 
a non-conductive tool may make the offending sound go away. If so, 
replacement may not be an option (monitor parts are sometimes difficult to 
come by, as well as generally expensive), but you can desolder it and apply 
a bit of RTV or hot glue (use a type that cools to a soft consistency) to 
the base of the coil (where the core is usually inserted). Sometimes, this 
will do the trick.
**

Good luck,
Richard

> Judging from the age, I don't think the power supply is switched, so that
> shouldn't be the source.
**
Even if this unit uses an SMPS (and they were in wide use in 1996), there 
would have to be a short, or near short, to cause the frequency to get low 
enough to be audible by humans, so, that is not likely to be the source of 
the noise.
**
>
> John
> -- 


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