[Elecraft] K3 RF in SSB audio on 6M

[Jim Brown]

On 10/8/2011 5:26 AM, Howard Sherer wrote:
> I have installed Fer chokes on the audio input
> lines with no improvement. Any suggestions.

What sort of ferrite choke(s) have you installed?  A single turn through 
a #43 or #31 ferrite core puts the resonant peak in the 150 MHz range, 
while a single turn through #61 puts it in the 500-800 MHz range.  If 
you want a choke to be effective at 50 MHz, you must wind at least two 
turns through the core, and you will likely need multiple two-turn 
chokes in series on whatever cable is picking up the RF.

While I completely agree with Don that it's critical to keep RF off the 
coax, a 6M antenna that is close to the shack could easily radiate 
enough signal to excite a Pin One problem, and you won't solve it by 
choking the coax, you must choke whatever outboard cables are receiving 
the RF.  Also, VERY important -- the nature of Pin One problems is that 
it is not ONLY audio cables that can have the problem. ANY cable whose 
shield (or common) goes to the circuit board rather than to the chassis 
can be a Pin One problem, including key lines, PTT lines, headphone 
lines, speaker lines, serial cables, and the AUX cable.

A completely different mechanism that SOUNDS like RFI is NOT RFI at 
all.  That mechanism is modulation of the V- line from the power supply 
by the IR drop that results from modulation of the DC that powers the 
radio.  That modulation can be as much as a volt, depending on the 
current, the line length, and the wire size.  If some piece of 
unbalanced audio equipment that feeds the transmitter is powered from 
the power supply end of the cable, and if V- is bonded to the chassis in 
the power supply, that modulation will appear in the unbalanced signal 
return, and will be added to the audio. It SOUNDS very much like SSB 
audio detected by an AM detector.  Why might it appear on 6M and not on 
other bands? It might if that the power supply is also feeding a 6M 
brick amp, or if whatever outboard gear is powered from the power supply 
end of the cable.

If you have NO outboard boxes connected, then it's almost certainly an 
RFI problem, and almost certainly a Pin One problem (which means it can 
be killed with the right choke(s) in the cable(s) that are acting as the 
RX antennas.

73, Jim K9YC