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[RFI] Computer Interference

To: <rfi@contesting.com>
Subject: [RFI] Computer Interference
From: John Brosnahan <broz@csn.net> (John Brosnahan)
Date: Sat, 13 May 2000 23:13:47 -0600
Here is a note I wrote in late 1996.  It seems even more
relevant today as more and more computer manufacturers cut
out more and more parts (Muntz TV strategy) in misguided
attempts to lower costs.  Thought it might be worth reposting.

--John  W0UN


Many of the newer computers are omitting the RFI line 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
rectangular 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 I made the
modification by bending a capacitor out of the way in order to clear
the filter.

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 existing 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 commercial 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

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, which I consider to be 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.


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