K3 power loss on 10 meter USB

[Don Nelson]

During the 2008 10 meter contest this past weekend I experienced some power loss during USB transmissions. The K3 was set to 100 watts, more or less. The K3 firmware is the 2.58/1.94  vintage. In responding to another party calling CQ I would send my call sign, N0YE. The N0 would be transmitted at the full 100 watts. The YE would be transmitted at about 10 watts. The power level observations were made on the built in power indicator in the K3. This behavior was highly repeatable. The K3 would transmit about 4 syllables of speech at full power and drop to a low power there after. This behavior occurred with whatever I spoke while transmitting.

I used more than one antenna during the contest. Each antenna had it's own tuner and so the K3 antenna tuner was bypassed. I can not say for certain that the SWR to the rig was always better than 2:1.

I do not know what I can do to get the K3 into this abnormal operational state. And I am not totally sure how I got the K3 into a “normal” operating state. I think changing power levels is one trigger to get into this abnormal state. I think going into the TUNE mode for a while is a way to get in the normal state. I also think changing bands say from 10m to 12m and back to 10m without transmitting is another way to get to the normal state. Power cycling the K3 may be another way to get to the normal state. Once in the normal mode, the K3 would function normally for a long time.

After I built the K3 kit, I experienced power fluctuations while transmitting. These fluctuations were cured by grounding pins 7 and 8 onto the MIC connector shell. The K3 has functioned normally on 40m, 20m, 17m and 15m bands since that fix. The modified MIC connector was used during this contest. This other problem and fix are mentioned in the off chance there may be a connection to this 10m transmit power problem.

So my questions to the group are:  Is this a problem observed by others? Is this a solved problem? If this is not a solved problem, are there things I could do to mitigate the problem?

Don, N0YE

[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

[Ignacy]

On all bands except 10m my K3 transmits down to 9.5V with pretty reasonable supply current. This is commendable. That K3 requires 13.5V to shut down not as often on 10m indicates a problem. I tried new KPA3 and it was more less the same . Our problem could be in  a bad capacitor in LPF for 10m or something similar. Gary from Elecraft wants my K3 in his shop.
Ignacy