[Elecraft] KPA100 going into oscillation...help!

[Douglas Maxwell]

Hi Don,
My KPA100 was shipped in October 07 and the value of C31 is .22uF("u22J63")
63v, Part no . E530295. I'm not sure about the oscillation, but my symtoms
sound very similar to the old oscillation problems documented on the web
i.e. 40m/SSB/KPA100 combination for the problem to manifest.

The KPA100 works 100% in CW on all bands. I can vary the power up and down
and it behaves accordingly. This leads me to believe the KPA100 is
constructed correctly. When the KPA100 is bypassed at <=10w there are no
problems. When the KPA100 is in line i.e. 11w on 80,20,15 & 10m all is well
and I can play quite happily. When I switch bands to 40m SSB @11w and speak
into microphone I get 100w out on voice peaks together with my voice audible
through the speaker. It will stay in this conditon until I turn the power
control all the way up and then back down to 11w whilst talking into the
microphone. It then works 100% until I either touch the mode button (by
cycling back round to L) or power the radio off then on, whereupon in both
cases it reverts to the 100w voice peak condition.

My KSB2 board  was bought together with my K2 in June 05 and no
modifications have been carried out on it since then. I didn't think it had
anything to do with the ALC circuit as the radio performs 100% on all other
bands and when the KPA100 is bypassed on 40m.

Thanks for the help. Hope this info helps a bit.

Doug GM0ELP



----- Original Message ----- 
From: "Don Wilhelm" <w3fpr at embarqmail.com>
To: "Douglas Maxwell" <douglas.maxwell at virgin.net>
Cc: <elecraft at mailman.qth.net>
Sent: Saturday, February 23, 2008 2:24 PM
Subject: Re: [Elecraft] KPA100 going into oscillation...help!


> Douglas,
>
> If your 'latest spec' KPA100 was shipped from Aptos after January 2007,
> then indeed you do have the latest - check all your construction work.
>
> If it was shipped before January 2007, check the value of C31.  The new
> design requires a 0.22 uF capacitor.
>
> Are you certain it is an oscillation?  If so, can you determine the
> frequency?  There is a difference between an oscillation and simply
> going to the maximum power output, and the direction for attacking your
> problem depends on the correct answer to that question.
> Do you have any mods to the KSB2 board - in particular the KI6WX
> increased RF gain mod.  If that is present with the KPA100, the SSB ALC
> can be overdriven (but there is an easy fix to the SSB ALC circuit).
>
> 73,
> Don W3FPR
>
> Douglas Maxwell wrote:
> > Hi All,
> >     My K2 is serial no. 4688. I have just purchased and built the latest
spec KPA100 unit for it. The manual for the KPA100 suggests modifications
are required for K2's with serial nos 4059 or lower. As my serial number is
higher, I assumed all would be well. Firstly the KPA100 performs well on all
bands using CW mode. Recently I tried SSB and found that the PA is going
into oscillation when on 40m.
> >
> > Setting the power out to 20W and speaking into the microphone produces
100W output peaks accompanied with a low buzzing sound in the speaker on
audio peaks. Varying the power control brings it back into line only to
start misbehaving again after a few more words are spoken or if the mode is
cycled back round to LSB or you switch off then on again. This happens when
connected to a dummy load with no other connections other than the mike and
power supply. I changed out the power supply, but the same problem exists.
> >
> > I have done an extensive search of the internet to try and find
solutions:
> > 1) I have ensured gnd continuity from the KPA100 to every part of the K2
chassis.
> > 2) I covered the hole in the shield near the pl259 with copper tape and
ensured gnd continuity.
> > 3) I saw the article about installing the 270 ohm metal film resistor to
increase Zin, but my K2 circuit is newer than the authors and I haven't seen
an updated version.
> > 4) I have ensured the 2 lugs from the antenna socket to the shield are
good.
> > 5) I have removed the KPA100 from the K2 entirely and the same problem
exists.
> >
> > Does anyone know the solution or recognise these symptoms?
> >
> > Douglas Maxwell
> > GM0ELP
> >
> >
>
>
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