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[CQ-Contest] best pc power supply to eliminate rfi?

Subject: [CQ-Contest] best pc power supply to eliminate rfi?
From: shr@ricc.net (W0UN--John Brosnahan)
Date: Wed Sep 4 14:55:51 2002
Here is a note from 1997 that I posted to the cq-contest reflector for
Carlos - PY1CAS and others.  All the more applicable today, although
as the manufacturers continue to shrink the components it may be
more difficult to retrofit a commercial filter.  In some power supplies
today the pads for the filter have been eliminated as well as the

--John   W0UN
Carlos, 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 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 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 

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