[RFI] Video-card related severe RFI

[Pete Smith]

OK, here's a weird one - This weekend, I suddenly had severe RFI from 4 
MHz to at least 21 MHz, which took the form of a buzz-saw noise with 
narrow, sharp peaks every 48 Hz.  It was audible regardless of antenna 
or receiver, AC or battery-powered, and was very clearly visible on the 
panoramic display of my QS1R.

After a little floundering around, I discovered that my 7-year-old Dell 
Dimension 2400, which has always been RF-quiet before this, had 
developed a problem.  At first, I suspected a failure in the line filter 
in the power supply, because the noise went away when I shut down the 
computer.  Then I discovered that it was directly related to my video 
card, an NVidia 5200.  I use this card to drive two monitors, in place 
of the on-board video, which was disabled.  When I booted up the PC, the 
noise peaks began at about 32 KHz apart, then switched to 48 KHz, and 
when the displays blanked because of monitor timeout, the interference 
went away.

To test the hypothesis, I switched back to the on-board video, moved one 
monitor over to it, and reset the Device Manager.  Noise gone.  So then 
I returned things to their normal places - both monitors on the nVidia 
card - and the RFI went away!  I can only surmise that I had a loose 
monitor cable that somehow allowed sync signals to leak.  Very strange, 
anyway, but I thought it might be worth noting here, in case someone 
else runs into it.

-- 
73, Pete N4ZR

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