[Elecraft] K3 power loss on 10 meter USB

[Don Nelson]

Hi,

I am not out of the woods with respect to the K3 reducing power while
transmitting as I experienced during the 2008 December 10m contest. But I am
standing on the edge of the forest.

The K3 has built-in protection for the 100w power amplifier and
automatically reduces power when some sensor is triggered. I had been
operating the K3 on a “long” extension cord from my Astron RS-35A power
supply. The voltage drop at the end of this extension cord was over a volt
when transmitting 100w as seen by the built-in volt meter in the K3. The
power supply voltage observed when in receive mode by the K3 is 13.6 volts.
On the extension cord, the K3 reports a power supply voltage of 12.5 volts
when transmitting 100w. This change in voltage sometimes causes my K3 to
reduce the transmit power out to something much less than 100w. The
transient power supply voltage as seen by the K3 may well have been less
than what voltage the K3 display indicated. This behavior was very frequent
while operating on 10m and much less frequent while operating on lower
bands.

I now have the K3 connected to the power supply using only the cable that
came with the K3. The K3 now observes 13.4 volts when transmitting 100w. The
percentage of transmissions that I now experience loss in power has dropped
by more than one order of magnitude. The K3 firmware was the 2.58/1.94
vintage. 

There still are times when my K3 fails to transmit 100w. Today for example,
I was on a QSO on 17m and transmitting 100w where I observed loss in
transmitted power maybe once in every 20 transmissions. i.e. out of 20
transmissions I would transmit 19 times and and observe 100w output and once
I would experience much less than 100w output. I have tested the K3 on 10m,
in particular, with 100w output on every transmission.

Bottom line: keep the supply voltage to the K3 as constant as possible. To
accomplish this, keep the IR drop in the power cord to a minimum and use a
power supply that can handle the current demand of the K3.

There is something else going on to cause an occasional loss of full power.
Any suggestions?

Don, N0YE


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