[Elecraft] K2/100 unstable power...

[Don Wilhelm]

Paul,

That is the correct fix for the problem.

The K2 controls power using a control loop that includes the entire 
transmit chain.  The control loop is stable with the base K2, but when 
the gain of the KPA100 is added, the control voltages can overshoot - 
just how much depends on the gain of a particular K2/100 when operated 
at the high power setting.

Since the transmit gain is usually greatest on 40 meters, you will 
likely notice the problem more there, and it will be at its minimum on 
10 meters where the gain is less.  The exact gain will vary depending on 
the particular component tolerance mix and the amount of careful 
construction used when building the K2.

Increasing R98 will dampen the loop response, but be careful not to go 
too high or th epower output on 10 meters may never get up as high as it 
should.  Decide how much you are willing to impact the 10 meter power 
output and use the highest value resistor at R98 that still allows that 
power on 10 meters.

Each K2 is different (because of slight differences in gain).  Some will 
adequately dampen the loop  overshoot with the normal 270 ohms while 
others can use a value as high as 1500 ohms at R98.   Determine the best 
value for your K2 by testing with a temporary pot  as indicated in the 
Power Control Mod instructions.

73,
Don W3FPR

IK1QHB wrote:
> Hi folks
>
> I have end to build the KPA100 few days ago but there's a problem to fix. 
> When drive my Titan 425 gold series with my K2/100 ( nø 5985) have unstable
> power control ( 20, 21, 23, 24, 19 watts), the grid current and output power
> jump up and down (tipically 800 to 1100w with 20w input) on every band,
> specially on 40 and 80m.
> When KPA100 is OFF i can see very little flutter on simple k2 at 5w... 0,1
> 0,2 watts of flutter on internal k2 wattmeter
> Seems not Feedback problem or similar, my antenna is a GP and Ros is
> perfect...
> Have read the mods by elecraft:
>
> http://www.elecraft.com/Apps/Power_Control_Mod.html
>
> Let me know if somebody have fix the problem with this or other mods. 
>
> Many thanks in any case, 73 de 
>
> Paul IK1QHB  
>   
>