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Re: [RFI] Motherboard RFI

To: "'Peter Laws'" <plaws@plaws.net>, <rfi@contesting.com>
Subject: Re: [RFI] Motherboard RFI
From: "Dan Violette" <danki6x@earthlink.net>
Date: Sat, 2 Jan 2010 10:13:41 -0800
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
Maybe you have bad caps on the MoBo.  Was a problem in 5-10 year old MoBos.
See:  http://en.wikipedia.org/wiki/Capacitor_plague  for some info with
references.  Suppose bad caps could be allowing unintended radiation.  /Dan

-----Original Message-----
From: rfi-bounces@contesting.com [mailto:rfi-bounces@contesting.com] On
Behalf Of Peter Laws
Sent: Saturday, January 02, 2010 9:41 AM
To: rfi@contesting.com
Subject: [RFI] Motherboard RFI

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

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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