XV432 Oscillates with Amp connected

[Stu2-3]

Good day,

I'm working with the XV432, Flex 1500 and Henry  2004A Amp. The rig,
xvrtr and amp PTTs are sequenced. Hoping somebody might have been down
this path before and has a solution.

If I connect the transverter output to the amp (or different pieces of
open coax) and key the transverter, the power meter goes full scale and
I see an oscillation on the spectrum analyzer. The frequency of the
oscillation varies with the load. (i.e. amp connected or different
lengths of coax) If I connect the XV432 to a 50 ohm load, no
oscillations and all works well.

I ran through the alignment procedure again with the XV432 connected to
a 50 Ohm load. All the voltages look right and the tuning is smooth.
(e.g. 20mV Quiescent Current Adjustment, TP3/TP4) It's obvious the XV432
is happy into 50 Ohms, but not into impedances that wander from 50 Ohms.

BTW, I have added all the mods as per the Elecraft website.

Any ideas?

73's
Stu2
W7IY
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[Steve Kavanagh]

Stu

A couple of thoughts.....

You didn't mention if the oscillation was occurring with the amp driving an antenna or a dummy load.  The Elecraft transverters have lots of nice features but they are quite poorly shielded.  If you were using an antenna you might be getting an RF feedback loop via the various rear-panel connectors which rely on the PC board for grounding rather than the case.  Clip on ferrite beads on the cables can help.  If you were running the amp into a dummy load then the problem is more likely impedance related as you suggested.

When I built my XV222 the manual failed to suggest that the paint be scraped off the case where the output coax connector is attached.  If you haven't already done this, it might be worth a try just to ensure proper grounding.

73,
Steve VE3SMA
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[w2bvh]

  On 5/9/2011 6:32 AM, Steve Kavanagh wrote:
> Stu
>
> A couple of thoughts.....
>
> You didn't mention if the oscillation was occurring with the amp driving an antenna or a dummy load.  The Elecraft transverters have lots of nice features but they are quite poorly shielded.  If you were using an antenna you might be getting an RF feedback loop via the various rear-panel connectors which rely on the PC board for grounding rather than the case.  Clip on ferrite beads on the cables can help.  If you were running the amp into a dummy load then the problem is more likely impedance related as you suggested.
>
> When I built my XV222 the manual failed to suggest that the paint be scraped off the case where the output coax connector is attached.  If you haven't already done this, it might be worth a try just to ensure proper grounding.
*Likewise, try scraping paint off the four 2-D attachment points on both
the side panels. The paint is very tough and hard to remove but its
worth the work. --Lenny W2BVH*

> 73,
> Steve VE3SMA
> ______________________________________________________________
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>

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[John Huggins, kx4o]

All the suggestions about trying different cable lengths are good thoughts
to help find the "sweet spot" of compatibility between the XV432 output
circuit impedance and the Henry input impedance.

Folks on the repeater-builder group talk about this very issue often; Of
course in their case they are concerned about the transmitter feeding a
reactive duplexer assembly.  Oscillations don't seem to be the issue
presented, rather, the tendency of the transmitter output stage to produce
"other" by-products when forced to feed a reactive load, such as a narrow
band-pass duplexer, causes unhappiness in the transmitter.  Spurious
signals are the result.

The oft touted "simple" cure proposed is "try different lengths of cable
between TX and Combiner."

The more professional solution is to place an Isolator between the TX and
the Amp during transmit.  One example is shown here...

  http://www.rflambda.com/pdf/isolator/RFLI101M40M50.pdf

This way, the TX only ever sees a nice infinite coax assuming, of course,
the third port is properly terminated into 50 ohms.

Of course you need to receive as well and the isolator won't help with
that by itself.  So all I have done here is to propose a workable, but
unwieldy and expensive solution if, in fact, the cause of the oscillations
is impedance issues between the XV432 output stage and the Henry input
stage.

The point is impedance issues between a Source and Load can cause the
source to not play nice.

Let's hope your problem is just a feedback path solved with chokes.

John

> Good day,
>
> I'm working with the XV432, Flex 1500 and Henry  2004A Amp. The rig,
xvrtr and amp PTTs are sequenced. Hoping somebody might have been down
this path before and has a solution.
>
> If I connect the transverter output to the amp (or different pieces of
open coax) and key the transverter, the power meter goes full scale and I
see an oscillation on the spectrum analyzer. The frequency of the
oscillation varies with the load. (i.e. amp connected or different lengths
of coax) If I connect the XV432 to a 50 ohm load, no
> oscillations and all works well.
>
> I ran through the alignment procedure again with the XV432 connected to
a 50 Ohm load. All the voltages look right and the tuning is smooth. (e.g.
20mV Quiescent Current Adjustment, TP3/TP4) It's obvious the XV432 is
happy into 50 Ohms, but not into impedances that wander from 50 Ohms.

>
> BTW, I have added all the mods as per the Elecraft website.
>
> Any ideas?
>
> 73's
> Stu2
> W7IY
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
>

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