[Elecraft] KPA500 RFI Problem

[Randy Farmer]

Thanks to all the folks who replied with advice, both on and off the 
list. I spent the better part of the weekend working the problem and 
thought I'd update y'all on what I found.

It turns out there were two major problem areas. The first was RF 
getting into the K3 via the ACC jack from the connections between the 
radio and the KPA500, the XV144 and the microHAM MK2R+ box. The KPA500 
is NOT affected by the common mode RF on the ACC cable. The power 
gyrations were clearly due to RFI effects within the K3. I *mostly* 
fixed this by building a short extension cable for the ACC jack and 
wrapping six turns of it around a 2.4" mix 31 toroid. This lets me use 
up to 500W output at the problem antenna heading without making the 
radio go nuts. If I increase power slightly it still begins to act up, 
so I haven't completely cured the problem. It's easier to just back the 
power down a few watts than it is to figure out how to get another few 
dB suppression on this path. I plan to live with it.I still have a 
little RF getting into the receive audio line at the worst case antenna 
heading and causing a bit of hum on the audio, but this is not really a 
big deal.

The second problem turned out to be the 13.8V power supply I use for the 
antenna switching relays and powering the Station Masters. This thing is 
a cheap knockoff of an Astron supply that uses a 723 voltage regulator. 
RF was apparently getting into the supply and modulating the DC output. 
This caused the antenna relays in the Station Master to buzz. This was 
undoubtedly the source of the "buzz" on my signal reported by listeners. 
I took several turns of the output DC cable around another mix 31 toroid 
near the supply output terminals and this seems to have cured the problem.

Thanks again to those who weighed in with suggestions. You gave me a lot 
of troubleshooting approaches that let me home in on the problem pretty 
quickly once I got started.

73...
Randy, W8FN