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From: "g3pjt" <g3pjt at whsmithnet.co.uk>
To: <elecraft at mailman.qth.net>
Date: Mon, 12 Aug 2002 09:50:13 +0100
Subject: [Elecraft] Instability in KPA100 - more

Hi

Further to my earlier post I had to disassemble the K2 in order to retighten
the PA devece screws. I removed the extra shield I had put in to kill the
oscillation I reported earlier and low and behold without the screen it
oscilaated again. However when I re assembled the PA with the screen back in
place it still oscillated !
Bearing in mind that the choke RFC10 was suspected as being involved with
the feedback causing this problem I have now removed RFC10. I have looked at
the circuit diagram and I don't understand RFC10 function ( is this a left
over from the days when we used high voltage PAs and this choke was a safety
feature against failure of the anode blocking capacitor ?  :<)).
Anyway it doesn't oscillate anymore.
However I am wondering if I have a component failure coming up ( visual
inspection looks OK)

I will probably dissassemble the KPA and inspect the bottom of the board as
well next time I have the lid off.

73 and thanks for the help,

Bob G3PJT

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From: "Lyle Johnson" <wa7gxd at fidalgo.net>
To: <elecraft at mailman.qth.net>
Subject: RE: [Elecraft] Instability in KPA100 - more
Date: Mon, 12 Aug 2002 06:22:42 -0700

> ... I have now removed RFC10. I have looked at
> the circuit diagram and I don't understand RFC10 function...

It is to provide a DC path to ground so static buildup on your antenna
system doesn't blow any output capacitors or other devices in the radio.  I
have seen this happen with whip antennas as short as a few inches - it is a
real problem, not merely a theoretical one.

Removing RFC10 is IMHO a Bad Idea.

73,

Lyle KK7P

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To: g3pjt at whsmithnet.co.uk, elecraft at mailman.qth.net
Date: Mon, 12 Aug 2002 07:34:02 -0700
Subject: Re: [Elecraft] Instability in KPA100 - more
From: k6se at juno.com

Bob, G3PJT wrote:

"I don't understand RFC10 function ( is this a left over from the days
when we used high voltage PAs and this choke was a safety feature against
failure of the anode blocking capacitor ?"
==========
The primary purpose for the RF choke in series between the anode and HV
supply in a tube amplifier is to isolate the RF output from the HV
supply.

RFC10 in the KPA100 is in parallel with the RF output (across the antenna
connector) and presumably is there to provide a discharge path to ground
for any static buildup which might occur on the antenna.

73, de Earl, K6SE

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