[Elecraft] RF in Microphone

[john at kk9a.com]

Interesting, how bad is the audio when this occurred? I thought that the
K3S had a protection circuit to protect the finals from damage. The finals
certainly see high SWR when setting up the KAT500. I am not sure what any
of this has to do with the KPA500 as it has it's own finals and it will
quickly shut down due to high SWR.

John KK9A


From: Kenneth Christiansen W0CZ
Sun Mar 26 20:22:57 EDT 2017

Hi Jim

The answer to your problem is you have a bad FET in the final of your K3S.
I had the same problem and my K3S is at Elecraft right now being repaired.
The distortion above the 12 watt level is the clue.

I also have the KPA500 and the KAT500 and I had a problem with my antenna
that it would sometimes short out at 500 watts and the KPA500 was not set
up for the KPA500. I had figured since I bought both units together they
would just work. I had even read the missing instructions but they did not
make sense to me so I did not do them. Now that I found out what was
involved they make sense but I think that part of the instructions should
be more clear or a control in the MENU should allow the settings to be
compatible with the KAP500.

To make this long story short if the KAT500 is not set up to be comparable
with the KPA500 than if there is a high SWR it does not shut down the
KPA500 and when the KPA500 does shut down all those high SWR voltages get
back to the K3S and blow one of the FETs. There are two FETs so the other
one tries to make up the power and of course the bad one causes distortion
similar to a diode being placed across the final.

I hope this information helps and for what it is worth I managed to
destroy my finals twice before I found out what was wrong.

Good luck and 73

Ken
W0CZ