[Elecraft] 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