[Elecraft] Odd audio feedback problem

[Jim Brown]

On Tue, 7 Sep 2010 10:01:26 -0700, George A. Thornton wrote:

>I also tried silencing
>the computer microphone, presuming that the microphone was picking up
>audio from the spoken Tx and feeding it back somehow through one of the
>sound cards.  

Look at this again. This is a very common problem, usually caused by a 
piece of software switching the Windows Mixer in ways that it thinks you 
want. Remember that there are TWO Windows Mixers -- one for record, and 
the second for playback. 

There is another mechanism that sounds just like RF feedback but is 
actually modulation of the DC voltage as a result of IR drop when 
transmitting SSB. If the V- bus in the power supply is bonded to the 
chassis, that modulation, which sounds just like SSB on an AM RX, gets 
added to the TX audio. W8JI was the first to alert us to this problem, and 
I have done testing to duplicate it in my shack. This problem can also 
happen if you get power for accessories direct from the power supply and 
then connect them to the rig. 

73, Jim K9YC