[Elecraft] K3 Hum on Audio...

[Jim Brown]

On 2/3/2012 12:43 PM, Guy Olinger K2AV wrote:
> This is an endemic "uncurable" problem.

Maybe not.  One mechanism I suspect, based on the symptom that it 
happens with the Line Input turned up, is magnetic coupling into the 
unshielded audio transformer on the Line Input. That coupling is nearly 
always pure 60 Hz, without harmonics, and is a well known problem.. In 
data modes, the Line Input is rolled off sharply at some frequency like 
200 Hz or so, which knocks the hum down by something like 18dB.. I 
recommend doing that for SSB too, because the lower octaves carry no 
intelligibility but can waste a lot of TX power.

The separation that matters this this case is the generator of the 
magnetic field, usually the power transformer for a big power amp, but 
it can also be caused by wiring errors in the power wiring in a home. It 
can help a lot to rotate the amp to put its field at right angles to the 
magnetic receiver (the audio transformer in the K3), and also to move it 
further away from the K3.

As to RFI with lines running past a computer -- I had that issue in 
Chicago, where I used a long wire antenna on 80 and 160, and it locked 
up the serial connection.  The culprit there was the K2 serial cable, 
which used PARALLEL wires inside a shield. I replaced it with TWISTED 
PAIRS without a shield, with one pair for each circuit used between the 
computer and the rig, with the return for each pair going to the chassis 
on each end, and the problem went away.  With the original cable, it 
locked up at 12 watts. With the twisted pair, I could run full power 
from my Titan 425.

73, Jim K9YC