Jim - W0EB wrote
> I have to echo W3FPR's advice. I had a Viewsonic monitor myself, and I DID
> shield the inside, put ferrites on the VGA input cable, put chokes,
ferrites
> & capacitors on the AC input as well, and didn't even manage to reduce the
> radiated noise. I suspect it's coming from the CRT itself. You will most
> likely spend more money trying to fix that monitor than you would buying
an
And it depends what kind of noise it is. Swiching power supply, flyback,
or video.
The last is characterized by very strong, stable carriers whose frequency
changes with display mode and whose strength changes with brigtness and
contrast. This makes sense, the electron beam is turned on and off fast (<
1ns) at pixel rates. About 15 years ago, when I was working for a large
electronics retailer and (at the time) computer maker, I occasion to fix a
couple of such monitors just to show the manufacturer it could be done.
Their problem was associated with the CRT cathode current, and the radiated
field was strong enough that nearby equipment *that wasn't even turned on*
exceeded the FCC conducted emissions limit.
In these cases I was able to reduce emissions by adding capacitive coupling
between the CRT-mounted video amplifier and the tube. This was really just
a piece of thin sheet metal soldered to the teh cathode driver board's
ground plane and extended it forward, providing a return path for HF
current that would otherwise be radiated. I don't know if any manufacturer
actually did this, mind, but it's fairly easy to do.
The usual caveats apply about working on CRT's. They're high voltage
devices and can deliver a dangerous shock some for time after being turned
off.
Cortland
KA5S
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