Topband: 160m noise

[Bill Tippett <btippett@alum.mit.edu>]

K8IA wrote:
>This all happened this past weekend, so my next task is to research some of
the EMI products that can attenuate this headache. I see that Corcom and 
others
have slick "entry modules", essentially an AC receptacle with a filter
enclosed, to replace the one on the power supply. These are generally cost 
effective
from what I quickly saw in a Google search.

        I also did some investigating before solving my problem and found
the following post from May 1998 by W0UN who I respect greatly:
http://dayton.akorn.net/pipermail/rfi/1998-May/000195.html


------------------------
                         COMPUTER NOISE SOLUTION

W0UN writes:
Many of the new computers are omitting the RFI filter in the
power supply that keeps the garbage from the switching mode supply
from entering the AC mains.  I have had excellent results (as have others
who have taken my suggestion) in virtually eliminating the interference
by replacing the AC connector on the back of the power supply with an
integrated AC connector and RFI filter such as the Corcom 6EF1.  The
mounting hole for the connector must be widened a few millimeters
on each side--something that can be done with a file in a minute or two.
Just make sure there is enough room behind the connector position
to clear the somewhat longer RFI filter.  In one case that I made the
modification I needed to bend a capacitor out of the way.

Some power supplies have a place on the circuit board for a filter but have
eliminated the components to save some money, and have just placed
jumpers in the positions where the components had been.  It would
be possible to make a new filter and add it to the exisiting location--or to
add it between the circuit board and the existing connector----but the
commercial Corcom filter is probably the best and easiest way to go.

The commerical filter has 1.0 mH coils (bifilar rated at 6 amps in series
with each side of the line.  On the computer side each side of the AC line
has 2800 pF to circuit ground.  On the AC mains side of the filter there
is a 9000 pF capacitor across the mains (not to ground).  Just make sure
that you use capacitors that are rated to be installed across the AC
mains!

The filters are available from most US electronics suppliers for around
$10 or so, but can often be found in the surplus market for $1-$2.  I used
the 3 amp version (3EF1, a little marginal) because I found some for $1.

There is also a 3EF2 and a 6EF2 that will work and may be even easier
to fit inside the power supply--their terminals come out the top/bottom
rather than the end.

If adding and AC mains RFI filter doesn't completely cure the problem then
additional RFI suppression will be needed--but in my cases (3 computers)
it eliminated the problem.

gl es 73

John  W0UN

---------------------------

        Here is a document describing Corcom's offerings:

http://www.onlinecomponents.com/pdfs/COREF.PDF

...and I also noticed many Corcom products are available at
reasonable prices on eBay.

                                                        73,  Bill  W4ZV




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