[Elecraft] KPA500 RFI Problem

[Randy Farmer]

Howdy, all. I'm attempting to cure a weird RFI problem that is affecting 
a K3/KPA500 combination. I've done some work to troubleshoot the 
problem, but before I get too much deeper into detective mode, I'm 
looking to see if anyone else may have experienced this problem and 
maybe even cured it.

It's obviously a common mode RF problem. It appears only on 20 meters 
when I point the antenna in one particular direction. With the help of a 
MFJ RF current probe I've determined that for some unknown reason the B 
port control cable on my microHAM Double Ten switch is picking up lots 
of common mode RF. I've put a bunch of ferrites on the cable and this 
has reduced the current on the cable but not eliminated the RFI problem. 
It doesn't seem to be coupling from one of the other antennas, as I've 
disconnected them all and the problem persists. Also, the RFI is at its 
worst when the antenna is pointed at right angles to all the other 
antennas. I'm stuck with the physical environment and antenna geometries 
I have now, so I'd prefer to just find the point of RF ingress that 
causes the problem and attack it there.

The problem shows up as a creep in output power when the transmitter is 
keyed. At antenna headings away from the trouble zone it takes about 31W 
to produce 500W output from the amp. In the trouble zone at key down the 
output power quickly climbs to 600W+ and I have to back down the drive 
power from the K3 to keep the amp from faulting. I can't find a stable 
drive power that will maintain rated output -- I have to drop the drive 
power to about 15 or 16 Watts to hold the output to the 300W range 
before the output stabilizes.

Accompanying this effect is a modulation of the signal that is reported 
to give it a "buzzy" sound, and I also get a pronounced buzz in the 
headphone monitor audio from my MK2R+ SO2R box under these conditions. 
Even with the drive down to where the amp output is about 100W the 
signal still has noticeable modulation on it. This leads me to believe 
the problem is in the KPA500. My first guess was that the RFI is getting 
in via the serial bus in the ACC cable. I have tried winding varying 
numbers of turns of the ACC cable around a 2.4" Mix 31 ferrite core with 
no noticeable result.

Does anyone have an idea of where the RF could be getting in to cause 
this kind of problem? I'd greatly appreciate any reports from those who 
may have encountered this situation and would especially welcome 
suggestions on how to find the cause and fix it. In the interest of 
brevity, I've probably left out many pertinent details that may give 
further clues, but for now let's see what we can do with the information 
I've given so far.

Thanks & 73...
Randy, W8FN