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
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