[Elecraft] RF in Microphone

[Kenneth Christiansen]

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
w0cz at i29 dot net

Sent from my iPad

> On Mar 26, 2017, at 4:38 PM, Jim Kutsch <Jim.Kutsch at ky2d.com> wrote:
> 
> First, thanks to all who answered personally and on the list reflector. To
> provide more info as several have requested, here's the setup:
> 
> The K3S is connected to a KPA500 then to a KAT500 then 25 feet of coax out
> to the back porch. There, the coax ends in a  common mode choke, which  in
> turn is connected by an 18 inch coax to a DX Engineering 4:1 BALUN. That
> feeds a Cobra UltraLite Senior via its 81 feet of 450 Ohm ladder line. The
> mic distortion happens with and without the KPA500 but reports say it's
> worse at 500 Watts.
> 
> The power supply is an Astron RS35 providing 13.8V idle and 13.5V CW key
> down at 100 Watts.
> 
> The mic gain is set to 45 and compression to 10. Those seemed to give the
> correct indication in the ALC meter but the distortion still occurs with 0
> compression. I cannot read the ALC indicator myself due to no vision so had
> another local ham assist with that part of the K3S setup.
> 
> Jim
> 
> 
> -----Original Message-----
> From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Jim
> Kutsch
> Sent: Sunday, March 26, 2017 12:31 PM
> To: Elecraft at mailman.qth.net
> Subject: [Elecraft] RF in Microphone
> 
> I am a new K3S owner and very pleased. However, on SSB, I am getting reports
> that my audio is distorted sounding like RF is getting into the microphone.
> I'm using a Heil PR-781 with the XLR3 to 8-pin microphone adapter cable. I
> had some RF microphone issues earlier using the PR-781 with a KX3 but a
> better shielded mic cable and a common mode choke resolved it. Any
> suggestions before I go in search of a different mic? Also, any
> recommendations for a boom mount microphone that would be a better choice
> for the K3S than this PR-781?
> 
> 
> 
> Thanks and 73, Jim, KY2D
> 
> 
> 
> 
> 
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