[TowerTalk] computer noises on low bands

[John Brosnahan]

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


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