[RFI] Computer Noise

John Brosnahan John Brosnahan <broz@csn.net>
Thu, 7 May 1998 06:15:26 -0600 (MDT)

This is worth cross posting so that Kelly Boswell gets a message
and rfi@contesting.com is also the appropriate forum for this

                         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

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


K6NA adds:
All the other suggestions offered may be helpful here and there, but until
you follow John's advice above, you are wasting your time chasing this kind
of noise around the shack.  He offered this advice on here about a year ago.
I've built three computers since then, and EVERY TIME his fix was the answer!

In San Diego we can buy mini-cases for as cheap as $25.00. But the switching
PS is a noisy dog, and there is no filtering.  Replacement of the a.c. input
connector with a filtered type is an easy one-hour job, start to finish and
it will probably get rid of 95% of your problem.  For the remainder you
might want to play with snap-on chokes, but after I ran coax braid from the
computer chassis to the station ground buss to get everything at the same
potential, I needed only one or two chokes to finish the job.

I also noted reduction of video noise when I powered the monitor from the
accessory a.c. socket on the back of the computer p.s., instead of just a
random socket in the station. When inside the p.s. box I also had added some
ferrites to the leads going to this output socket, as a precaution. I
recommend powering your monitor the same way.

It would be a HUGE JOB to successfully shield a monitor completely, for
example, and may not be necessary at all if you take care of business in the
computer p.s. first.

Thank you John!

73, Glenn K6NA

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