[Elecraft] XV432 Oscillates with Amp connected

[Edward R. Cole]

Stu,

OK so you are keying the XV432 Tx control line and not applying RF 
drive (not sure how you accomplish this, but not important in this 
discussion).  It oscillated unless there is a 50-ohm RF load on the output.

I think its time to contact Elecraft for help.  I assume you did not 
build the XV432 and it was assembled by Elecraft (what I am saying 
this is not sold as a kit), and should not do this.  Maybe a 
candidate for warranty service.

I had a 1296 transverter (from another mfr) that did a similar thing 
though it took drive to get self-osc. started.  I could remove drive 
and still had a couple watts of RF out, until I unkeyed it from 
Tx.  Unkeying biased the amps off in the transverter.  I measured the 
self-osc. frequency and it was osc. at operating freq.  The mfr had 
it twice to fix and finally we got it tamed at a lower power out (and 
I sold it).

But circuits can self-osc. anywhere from audio to mw.  There are 
design techniques to prevent this: usually RF chokes and bypass 
caps.  In tough cases interstage shielding and parasitic chokes.  But 
I have not heard of this problem being common with the XV432 so time 
to call Elecraft.

Ed - KL7UW
------------------------------

Message: 17
Date: Tue, 10 May 2011 19:19:50 -0400
From: Stu2 <stu at stu2.net>
Subject: Re: [Elecraft] XV432 Oscillates with Amp connected
To: elecraft at mailman.qth.net
Message-ID: <4DC9C816.6020900 at stu2.net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Thanks guys!

It's quite weird. When I say key up, I mean activate the XV432 PTT with
the rig (exciter) in CW mode without pressing the CW key.  That is, no
RF goes into the transverter. Just the simple fact of keying the
transverter while it is connected to something other than a 50 Ohm load
seems to set up an oscillation.

Several people have suggested a pad. The output of the XV432 is a low
pass pi circuit. Since I don't want to reduce the RX gain, it would seem
this either needs to be in the amp's TX chain or the XV432's TX chain.
(i.e. before the TR relay)

After I wrote my first message, I installed an SWR meter between the
XV432 and amp so I could adjust the Henry for the best SWR.
Unfortunately, this didn't cure the problem. The XV432 still oscillated.
I have another amp I can try. I also have a TS2000 to try to help set
the amp up.

Thanks again for the ideas.

Stu2





73, Ed - KL7UW, WD2XSH/45
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