Don't forget the power supply. Although it *appears* the noise is
coming from one of the com ports, it may not be originating there. My
experience has been the two most common sources are the power supply and
I currently have 4, state-of-the-art, quad core CPUs with the core
clocks running 3 Gig or faster, plus an older, "slightly smaller unit".
The 4 are in steel full size towers, while the smaller one is in a steel
mid size tower. Some of the antennas run within about 20' of the
computers, while there are places that computer cables run with coax,
rotator, and control cables. So far I've heard nary a peep, *except* if
I place an HT right on the cases it will just break the squelch on
147.00. I'm running "Ham Radio Deluxe" to the control for the Icom 756
Pro and Tokyo Hy-Power HL1.5Kfx. (serial cable to the interface)
Were it me (and it's not) I'd start out by disconnecting everything
including removing all possible accessory cards.
Finding a problem is much easier when adding components than removing
You didn't say which OS you are running, so depending on that this might
require some reconfiguring. Turn it into a *basic* PC with keyboard ,
mouse, and monitor. Then start adding functions, one-at-a-time. The
logical place to start would be the com port. If it's persistent and
your motherboard has on-board video and you are using a PCI or AGP card,
make sure the on-board video is disabled. If the problem persists,
remove the video card and try it with the on-board video.
Except for high end, or gaming systems, most PCs have very cheap and
marginal power supplies with terrible wave forms on the output.
Conversely they really don't care much about what they are fed. The
nice thing about substituting power supplies is they normally can just
be hung on the leads without going to the trouble of removing the one
mounted in the case. Just make sure *ALL* of the leads from it are
disconnected before hooking up the spare.
Some one mentioned capacitors. I had one go out and burn a hole right
through a top end motherboard.
Also the pressing on the caps? As was asked, does the noise change when
touching the caps or when actually pressing down. I'd definitely to
that with an insulator and not my hand to eliminate stray noise pickup.
This raises the question of proper grounding of the mobo to the metal
chassis and at the proper places. Normally all mounting screws are
either on a grounding pad, or completely insulated.
One point that is a sometimes problem...Is the radio and computer on the
same electrical circuit?
That's about all I can think of at present.
So, 73 and good luck,
Dale Svetanoff wrote:
> A few questions:
> 1. How close is your antenna system (or any part of it) to the offending
> 2. Does the 746 Pro go quiet with the antenna removed? If should, but if
> not, then there may be multiple coupling paths between the PC and rig.
> 3. Is the PC a stock commercial item with FCCand CE mark labels, or have
> you (or someone) done "custom mods"?
> In any event, remember FCC Part 15 does not require ZERO emissions, but
> emissions limited to a given level and measured at 3m from the unit.
> Add-ons and custom mods will often increase the emission levels to well
> above the legal limit. Your data shows some of this effect. BTW: When
> you opened up the case of the PC, does the case have internal metal
> shields? If so, are these intact and making contact as designed? Keep in
> mind that reducing emissions is only one way to attack the problem; the
> other way is to increase the separation distance between the PC and your
> Keep us posted, and best wishes on the project.
> 73, Dale
>> [Original Message]
>> From: Peter Laws <firstname.lastname@example.org>
>> To: <email@example.com>
>> Date: 1/2/2010 11:41:28
>> 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 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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