K2/100 output power fluctuation

[Ken, KA0W]

Hello!

I'm noticing a power output fluctuation of about 10w on an external Array Solutions digital wattmeter and on an analog wattmeter out of my K2/100 (sn 6384).

The load isn't changing, tried different dummies, coax, and antennas too. Past recent history of station - no load problems thru 1.5kw.
 
It is more of an output jump, up and down - continuously back and forth. A fast jump, near instant. Doesn't matter much about power setting, whether 30w or 100w.
 
At the 10w level (PA not switched in) out fairly stable, no jump up and down jumping.
 
When using the 20w limited tune initially starts at about 27w than drops to about 18.5w and remains there - no jumping.
 
When bypassing the 20w limited tune (holding Display and Tune)  I get the jumping.

RF getting into a  K2 sensing circuit that controls output?
 
Another possibly related problem is thumbing noise in computer speakers. I have the audio running into the computer. At power levels 10w and under no thump. 11w and above (PA switched in) there is a thump. Computer is otherwise RF proof from QRP thru 1.5kw, no problems with many other rigs and setups.
 
Book measurements and performance outside of problem listed above are excellent. Receive is outstanding, amazingly quiet on 40m and 80m.
 
Anyone with similar K2/100 history ? Suggestions?
 
Thank you for any and all help.
 
Ken, KA0W
 
 
 
 
 
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[Don Wilhelm-4]

Ken,

This is not a 'problem', but is a condition that does occur in some K2s.
The situation is that the K2 power control mechanism is a loop, and if
the gain in the RF stages is high (the gain increases when the KPA100
switches in), then the power control loop can oscillate with a period
from a half second to 2 seconds.

The cure is to add damping to the control loop which will cause the loop
oscillation to diminish more quickly.  The element to change is RF board
R98.  Remove the KPA100 and operate only the base K2, then temporarily
replace R98 with a variable resistor (about 2K should work fine).  
Adjust the pot to higher and higher resistances watching the power
output on 10 meters.  When the 10 meter power begins to drop, that is
about as far as you can go without impacting the power output on the
higher bands - measure the resistance of the pot (after removing it) and
use the next lower standard value resistor in place of the original R98.
This information can be found on the Elecraft website under Application
Notes as "Optional K2 Power Control" or look at the direct link
http://www.elecraft.com/Apps/Power_Control_Mod.html.

Your 'thumping' in the computer speakers indicate that you should get
better shielded speakers (or shield the wires into and out of the computer).

73,
Don W3FPR

Ken, KA0W wrote:

> Hello!
>
> I'm noticing a power output fluctuation of about 10w on an external Array Solutions digital wattmeter and on an analog wattmeter out of my K2/100 (sn 6384).
>
> The load isn't changing, tried different dummies, coax, and antennas too. Past recent history of station - no load problems thru 1.5kw.
>  
> It is more of an output jump, up and down - continuously back and forth. A fast jump, near instant. Doesn't matter much about power setting, whether 30w or 100w.
>  
> At the 10w level (PA not switched in) out fairly stable, no jump up and down jumping.
>
>  

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