K2/HF Packer amp incompatibility?

[Pat Byers]

A friend recently dropped off his K2 and HF Packer amp for me to
troubleshoot a problem -- he was getting poor quality SSB reports on
the air when he ran his K2 through his HF Packer amp. I had built
both kits and had tested them separately so I was a bit skeptical
that there was anything wrong. :-D However, when I ran the K2/HF
Packer combo into a dummy load and monitored the transmitted signal
with my Icom IC-703 I confirmed that the transmitted signal was
significantly distorted. Hmmm . . . What's wrong?

Just to narrow down the problem I tried the amp with my IC-703 and
the transmitted signal sounded great with no discernible distortion.
The only discernable difference with the amp switched in or out was
the signal strength. I then monitored the K2 barefoot and it sounded
fine as well. As a further check I had a friend bring over his K2
which had been aligned by Elecraft. His radio sounded fine barefoot
but when it was used with the amp there was significant distortion in
the monitored signal. Just in case of an impedance mismatch between
the K2s and the amp I ran the experiments with both K2s using the
built-in ATUs and with the ATUs out of the signal path. The results
were distorted audio in all cases.

I also checked to ensure that the amp wasn't being overdriven. The
amp was set up for a transmitter input of 2 Watts so I used an
accurate QRP Watt meter -- yes, I've checked its accuracy --  to
ensure that the K2s were transmitting just under 2 Watts.

I was powering the amp and the radios with a reliable and
well-filtered 10 amp power supply. Just for fun, I powered the K2
with a 7 A/h gel cell and the amp with the power supply in case there
was some interaction through the power cables when the radios and the
amp were run from the same supply. Still no joy.

During all of this testing I noticed that occasionally the rf sensing
T-R switch in the amp would turn off even though the K2's power
output was supposed to be just under 2 Watts which should be enough
to reliably keep the T-R switch engaged. I didn't observe this
behaviour when I used the IC-703 with the amp. Hmmm . . . Why is this
happening?

My guess is that there's something going on with the K2's ALC that's
affecting the transmitter output. That would account for the T-R
switch dropouts but would ALC issues create distortion? Has anyone
else experienced this problem? If so, is there a solution?

The K2 in question is Serial Number 1120 but it has been brought up
to Revision B.

We're in the midst of some house renovations so the shack is full of
boxes and I'm too busy to get back to testing but after the dust
settles I intend to inject an audio signal into the K2's mic input
and monitor the output of the amp with an oscilloscope. I've also
considered placing a resistive pad between the K2 and the amp to
further isolate one device from the other. Unfortunately, I can't see
getting around to those things for a few weeks. In the meantime, I'm
open to suggestions.

TIA and 73,

Pat VE3EUR

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[Don Wilhelm-4]

  Pat,

Are you using a common power supply on the K2 and the Amp?
If so, try using separate supplies and see what happens.  If the problem
does not present itself, you can conclude that SSB modulation of the V-
power supply line is the problem.  See the discussion that appeared on
this reflector over the last 2 days, subject K3 "RFI in transmitted audio".

73,
Don W3FPR

On 9/13/2010 10:08 PM, Pat Byers wrote:

> A friend recently dropped off his K2 and HF Packer amp for me to
> troubleshoot a problem -- he was getting poor quality SSB reports on
> the air when he ran his K2 through his HF Packer amp. I had built
> both kits and had tested them separately so I was a bit skeptical
> that there was anything wrong. :-D However, when I ran the K2/HF
> Packer combo into a dummy load and monitored the transmitted signal
> with my Icom IC-703 I confirmed that the transmitted signal was
> significantly distorted. Hmmm . . . What's wrong?
>
> Just to narrow down the problem I tried the amp with my IC-703 and
> the transmitted signal sounded great with no discernible distortion.
> The only discernable difference with the amp switched in or out was
> the signal strength. I then monitored the K2 barefoot and it sounded
> fine as well. As a further check I had a friend bring over his K2
> which had been aligned by Elecraft. His radio sounded fine barefoot
> but when it was used with the amp there was significant distortion in
> the monitored signal. Just in case of an impedance mismatch between
> the K2s and the amp I ran the experiments with both K2s using the
> built-in ATUs and with the ATUs out of the signal path. The results
> were distorted audio in all cases.
>

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[Geoffrey Mackenzie-Kennedy]

Pat,

I cannot find the schematic of the HF Packer amp to verify this, but my
suspicion is that the input impedance of the amp might be changing with the
level of drive signal. This change will occur rapidly of course in the case
of SSB.

If this is the case then distortion could result either because the driver
(the K2) cannot handle a rapidly changing load impedance, or there is
something amiss in the amp's feedback network which causes the amp's input
impedance to change with drive level - but my inclination is to suspect the
former.

One clue is the occasional turning off of the amp's rf sensing T-R switch
when the driver is a K2, but not when the IC-703 is the driver. This
suggests that the K2 is not happy with a rapidly changing load impedance,
which in turn suggests that the distortion is generated by the K2 not the
amp. You could try reducing the drive level from the K2 to see if the
distortion disappears at some point.

Your idea to place a resistive pad between the K2 and the amp could well
cure the problem.  The reason why the K2's ATU did not cure the problem is
most likely that the ATU cannot switch fast enough to keep up with the
rapidly changing load presented by the amp.

Good luck.

73,

Geoff
GM4ESD


Pat Byers wrote on Tuesday, September 14, 2010 at 03:08 +0100

>A friend recently dropped off his K2 and HF Packer amp for me to
> troubleshoot a problem -- he was getting poor quality SSB reports on
> the air when he ran his K2 through his HF Packer amp. I had built
> both kits and had tested them separately so I was a bit skeptical
> that there was anything wrong. :-D However, when I ran the K2/HF
> Packer combo into a dummy load and monitored the transmitted signal
> with my Icom IC-703 I confirmed that the transmitted signal was
> significantly distorted. Hmmm . . . What's wrong?

<snip>


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[John Fritze]

Did you contact Virgil about this?  He has always been very helpful
with the Packer amp and related problems.

John
K2QY
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