Years ago I experienced severe (over S9 ) noise on 160, 80, some on 40m
after replacing a defective PC power supply. The supply was a cheap ebay
special that worked well except for the rf noise. After analysis, I found
that 2 other PCs in other rooms were contributing a low level amount of
noise also. This is what I found and the solution:
First, none of the supplies were FCC Class B certified (low level EMI for
residential use). Class A allows a higher level of noise (for commercial
use). No label (like mine) probably means "no filtering".
So, I made reuseable filter boxes for each of the computers in the house and
removed ALL detectable noise from the computers.
I simply purchased 10 Corcom filters on ebay ( I think they were $2 each +
shipping). I cut a power supply cord (universal plug type) about 6 inches
from the computer and wired the filter into a plastic electrical box with
the other end of the cord out the other end of the box. I used standard
electrical cable clamps to secure the cable to the box and added the
cheapest electrical plastic cover. The whole thing cost me about $6 each and
I built 5 of them in a couple of hours. The boxes live behind the computer
so the blue color appearance doesn't matter. When I purchased newer
computers, I just moved the filtered cord to the new computers.
I used one of these on a switching power supply (which had the universal
type plug) and it fixed it's noise problem also.
I was tempted to build one of these into a box with outlet on it but I
figured that the long plug in cord from the noise generator (power supply)
would allow too much noise radiation, so I didn't try it.
Tom
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of ZR
Sent: Sunday, July 21, 2013 8:25 PM
To: Tom W8JI; jim@audiosystemsgroup.com; topband@contesting.com
Subject: Re: Topband: Desktop Power Supply Brand/Noise Question
>> On 7/21/2013 8:49 AM, Tom W8JI wrote:
>>>
>>> I can't understand why throwing some ferrite beads at a problem, or
>>> changing the supply, are the only two solutions.
>>>
>>> Many times, if not most times, a few .01 uF line voltage rated
>>> bypass capacitors are significantly better than a sting of cores, or
>>> a winding through cores.
>>
>> Yes, IF the problem is differential-mode coupling to the power line.
>> But often it is NOT --
>
> Bypass capacitors will cure common mode also, because common mode
> cannot be generated without differential mode someplace in the system
> between two points.
>
> In virtually every situation, the power supply is in a metal box. The
> trash is between the power line leads, and also between those leads
> and the case. This is the nature of switching supplies, because they
> tie a "chopper" with squarewaves across the power mains, and the
> system is not well balanced.
>
> As a general rule the stuff exiting the dc side is much less problematic.
> If it is problematic, it needs cleaned up. Bypassing to the cabinet,
> groundplane, or case fixes or greatly assists in fixing it, too.
>
> If a line is properly bypassed to the case, the system can't produce
> much common mode or differential mode on that particular line.
> In the rarer case, where two or more lines are involved, they all must
> be suitably bypassed.
>
> The same things that work for lightning protection work for RFI
> issues, and vice versa. Payback for doing things right, rather than
> just tossing beads at wires, is much wider than the initial target.
>
> 73 Tom
I certainly agree about not throwing ferrite at problem without
understanding why.
My introduction to serious line and computer noise was after moving here in
89 and I remembred a QST article by DeMaw a few years earlier where he
introduced a differential mode noise filter using ferrite rods and caps in a
pi filter. The line was noisy and I had just put my Commodore 64 out to
pasture to be replaced on the new fangled DX Packet Cluster by a 386-33 PC
with 12" monochrome monitor. It was soon also used for various DOS RF and
other programs, Windoze 3.1 and a long learning curve.
I built DeMaws circuit in a couple of outlet strips and used them for the
PC and the TS-940's which were all on seperate circuits.
Most of the noise was gone and using toroids on the keyboard and monitor
cables took care of the rest. This was before ferrites were included from
the factory.
About 2-3 years ago when I took an interest in the LF frequencies I realized
that DeMaws circuit wasnt effective down there and rebuilt the filters into
2 stage affairs with a 100KHz cutoff.
There are now 6 PC's in the house and each got its own filter as well as the
modern and boatanchor operating desks. A new level of quiet appeared as if
by magic by attacking the differential noise first.
Any remaining annoyances were locally radiated coming in via various
antennas (including portable radio loopsticks and whips) which was
eliminated by a lot of large 43 and then 31 toroids on power cords, router
cables and some feedlines.
With a couple of Beverages moved another 600-700' away back in the woods and
feeding a new switch box and its own 1/2" CATV hardline run back to the
house, even neighbors noise is mostly tolerable with none....so far....on
any good DX frequencies.
Carl
KM1H
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