K3 Slow power change

[k1htv]

K3 Slow power change

I've been using WSPR for the past week with my 100 Watt K3 at QRP (5W) and QRPp (100mw to 500mw) power levels. I noticed that if I move the K3 'PWR' power level control either up or down one increment (i.e from 5W to 4W or from 0.5 to 0.6W) the power drops way down and slowly rises. Here is how long it takes the power, as read on the K3 and on a W2 watt meter, to recover after the PWR control is changed slightly.

Elapsed time - Power output
0 seconds    -  5 Watts
1 seconds    -  2 Watts
7 seconds    -  3 Watts
19 seconds   -  4 Watts
30 seconds   -  5 Watts

Making a small incremental change in the power output using the PWR control will cause the slow recovery if done before or during a WSPR transmission.

Is this normal?

73,
Rich - K1HTV
K3 #3657
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[Julian, G4ILO]

Yes, if you don't have sufficient audio drive. Long-press DISP so that the ALC scale is visible then adjust the input level for a steady 4 - 5 bars reading. (This assumes you are using the DATA mode not USB.)

Rich - K1HTV wrote

K3 Slow power change

Making a small incremental change in the power output using the PWR control will cause the slow recovery if done before or during a WSPR transmission.

Is this normal?

[Kok Chen]

On Jan 27, 2011, at 1/27    7:46 AM, Rich - K1HTV wrote:

> Here is how long it takes the power, as read on the K3 and on a W2  
> watt meter, to recover after the PWR control is changed slightly...  
> Is this normal?


No, it is not "normal."  It is not hard to make it go away, however.

The power creep is due to insufficient audio drive.  My guess of what  
is happening is that the DSP stage is trying to compensate by applying  
more gain to counter the low audio, but cannot do it rapidly since it  
will cause other problems, possibly increased transmit IMD.

Take a look at this to find a "good" audio range that works for  
different RF power levels:

http://homepage.mac.com/chen/Technical/K3/Digital/digital.html

What I had found was that a line input audio from the sound card in  
the region of 250 mV and 350 mV RMS, and a Mic Gain of 6 works for  
most RF power output levels.

In your case, take a look at the "5 Watts" table under the "PSK31 -  
Data-A Mode" section.

It also produces the drive that generates the best transmit IMD.  The  
transmit IMD of the K3 is not that great, and you need all the tricks  
that you can muster to keep it in check.  While your RF Power is  
creeping, the transmit IMD is probably also quite high (in the  
measurements above, it shows up as IMD also creeping).

You can also use a lower audio drive, but correspondingly more mic  
gain.  However, I like to keep the audio level as high as I can (short  
of saturating the line input preamp) to keep problems such as ground  
loops in check.

I don't use SSB, but I suspect the same thing can happen there also.

With most other rigs, the non-ALC RF output power for "AFSK" modes is  
determined by the applied audio drive -- set one control and you have  
the RF output that you need.  In the K3, the RF output is determined  
by the RF power knob, but you also need to make sure also that there  
is sufficient audio drive, or you will see the RF power creep problem.

73
Chen, W7AY



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