[Lowfer] Sheilding power cords with aluminum foil ??

[C. Turner]

Hi Andy (and the group),

I had a similar problem with a PC - that is, the noises from the 
computer's internal circuity causing problems (screen updates, etc.) and 
in that case, simply tightening the screws in side the power supply 
fixed the majority of it.

Typically, the phenolic board they use is held down at each corner by a 
screw and the solder-tinned pad underneath is supposed to make the 
ground connection to the power supply's case.  Well, they don't always 
get those very tight, they work loose somehow, or oxide builds up, etc.

I first ran across this when my boss (also a ham) noticed one day that 
he was getting tons of QRM on his AM radio and tracked it down to the 
computer - which he'd had for a while and had never caused the problem 
before.  He did exactly as you did (disconnect all cables but the power 
cord) and it didn't go away until he shut it of AND unplugged it.  (The 
low-current "always-on" mode used to power the "wake up on LAN" was 
still causing QRM even though it was turned "off.")

Since he had another power supply, he swapped it out and solved the 
problem - but brought it in for me to look at.  It took only a few 
minutes to discover the loose screws.

After that, he soon brought in another power supply that was causing 
*some* QRM - not much, but enough to be annoying.  It's screws weren't 
loose, but adding the extra bifilar power supply choke and more 
capacitors completely quieted that one, too.

I try to collect the dead power supplies and I harvest the ferrites, 
heat sinks (mostly for the "thermo pads" and transistor mounting 
hardware), input chokes (usually bifilar), the output chokes, the 
high-voltage input capacitors - including the high-voltage electrolytics 
(which, unlike the output electrolytics, almost never go bad) and the 
fistful of useful, multi-colored wires to go in my "short wire" box and 
I've noticed that some cheaper supplies - usually ones suspiciously 
devoid of "FCC" and other regulatory markings such as "UL" - have 
jumpers in place of the input chokes.  I have no idea where these came 
from, but I'm glad to see them out of circulation!

As for that PC from which I was hearing "mouse noises" and screen-update 
garbage, my bonding the rear panel of the motherboard to the rear panel 
of the case fixed that as I mentioned in a previous email.  I might add 
to that that often, motherboard cases have a very thin "snap-in" rear 
panel that you choose to match the connector layout of the motherboard 
and sometimes it's necessary to solder these to the case in a few places 
to hold them in as well and to guarantee that they make electrical 
connection to the case.

***

Just a few days ago I DFed and located a QRM source that several local 
hams were complaining about:  It was audible every 60.95 kHz or so from 
HF through 2 meters (it probably went much lower) and was audible at a 
park about a mile away from their QTHs (the two reporting this lived 
only a block apart.).  As we drove around we realized that it was coming 
from the house of one of the hams that had been complaining.  Since he 
was home, we wandered around with a 2-meter Yagi until we zeroed in on 
his cable modem in his garage.  Powering it down made the garbage go 
away - as did simply disconnecting the LAN connection, so cramming the 
about 4-5 turns of the LAN cable through 4 stacked ferrites (which was 
as many as we could manage with the connector still on it) knocked it 
down 10dB or more at HF and VHF:  We figure that since his HF rig (a 
fancy one that may have had its own LAN connect) was also connected to 
his computer which had a LAN cable, the grunge was being conducted on 
his coax to his antenna.  Unfortunately, a listen on these same 
frequencies as we were driving around revealed that he wasn't the only 
one with the same type of cable modem - but the QRM of these others 
seemed to radiate only a few houses away at most, not being coupled to 
HF antennas...

***

In a *really* stubborn case, a friend of mine had a telephone switch in 
his house.  As you might guess, this being a "Class A" device not 
intended for residential use it threw all sorts of QRM across the HF 
bands and phones would ring at random when he transmitted, depending on 
frequency.  In addition to adding the extra chokes/caps to the switching 
supply we had to do something else:  EVERY telephone and interface wire 
was put together in a large bunch and we ran as many turns of that 
bundle as we could through several "stacked" ferrite cores from TV yokes 
assembled as if they were large, awkwardly-shaped toroids.  This 
required that we extend every cable to allow that extra length, but 
since things were connected by RJ connectors on the switch to a punch 
block (and he was a "phone guy" anyway) it wasn't too painful, taking 
only an hour or so to do.  This reduced the garbage from "way over S9" 
on HF to "Still there, but I can live with that."  The addition of a few 
more bypass capacitors on a few select lines (determined by unplugging 
the RJ connectors and noting the QRM level) knocked things down a bit 
further, but unless we were willing to put the entire phone switch in a 
shielded box and choke/bypass every line with feedthrough caps, we knew 
that we were already at the point of diminishing returns.

I'm just glad that he's not an LF listener!

73,

Clint


Andy - KU4XR wrote:
> Clint:
>
> This is some very good, detailed information. Thank you for 
> taking the time to explain it like you did. The power supply
> in mu PC dosen't seem to be generating very much hash. The
> problem I am having appears to be coming from the video
> generating section of the computer.IE: move the mouse across 
> the screen, and hear rapid fire sounds, open the browser, or
> a file on the hard drive, and hear all kinds of noise as the
> screen changes. When nothing is changing , the noise is steady,
> but refresh a page, or a grabber page refreshes, and noise
> galore.  Here's the kicker ::: unplug the monitor, and 
> dis-connect the SVGA cable, NO MONITOR connected, the noise
> is still there !! I have dis-connected every cable to the PC
> while it's on, mouse, keyboard, sound cables, ethernet, etc.
> this frazzlin noise just won't go away.  I can't help but think
> it's something to do with the motherboard video. 
> I can " Beef Up " the powwer supply as one fella posted, and as
> you mentioned by adding a dual choke with 3 MilliHenries Imp.
> on each side, but I'm just a little leary of the " KABOOOMM "
> that can happen with these SMPS. I've had numerous PC's but
> have never had what I would describe as a " Quiet one "
> I may try the extra Inductance, and hope it don't go Kaboom.
>
> 73 ti you and thanks for your info.   :::
>
>
> Andy - KU4XR - EM75xr - Friendsville, TN.
> LOWfer Beacon " XR " @ 185.29866 KHz ( QRSS-60 )
> Coordinates:  N:  35* 43' 54" - W:  84* 3' 16"
> http://www.myspace.com/beaconxr
> http://webpages.charter.net/ku4xr/
>
>