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[RFI] Motherboard RFI

To: rfi@contesting.com
Subject: [RFI] Motherboard RFI
From: Peter Laws <plaws@plaws.net>
Date: Sat, 2 Jan 2010 11:40:55 -0600
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
Took a little time before the RTTY Roundup to try to track down some
noise on my IC746Pro.  PC was definitely the cause noise is there even
if I stop the system at the GRUB screen so likely not disk-related
since they were pretty inactive (but not a sure thing).

Pulled the cables off one by one.  Got to the motherboard's COM1 and
all noise vanished ... well, at least where I was listening on 17 m
(around 18130.0).  OK, a couple snap-on chokes got rid of some but not
all.  Decided to try it on a different serial port (I have a Siig
4-port that had two free on the octopus).  No noise.  Cool!

Not so much.  As I started to plug in other things to the motherboard
(not the PCI cards), I got the same noise there.  OK, took off the
USB-PS/2 adapter from the mouse and ran right into USB.  A little
noise there that a choke didn't really make go away - I'll try moving
that to the USB hub, but that hub still plugs into the MoBo.

Keyboard is also a problem and it's NOT USB.  Put a few snap-ons on
the cable and the AT-PS/2 adapter (yes, you read right). and again,
couldn't get rid of it all.

Opened up the case with the system running and poked around.  I noted
an odd thing.  With COM1 -- the noisiest -- connected, I could reduce
the noise by pushing on the top of some of the caps.  OK, odd.  Am I
adding capacitance?  I pushed on other things, potentially conductive
and not ... and got the same.

So what could be "loose" that would generate (I assume) common mode
noise?  It's only stuff plugged into the MoBo - nothing plugged into
the sound, video, or PCI serial card.

Any ideas on a real fix?  The noise is only in specific places in the
spectrum, so I'll play in RTTY RU today/tomorrow, but now that I know
that it's actually fixable, I'd like to!


Peter Laws | N5UWY | plaws plaws net | Travel by Train!
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