[Elecraft] K2 -- loss of output power control

[Don Wilhelm]

Brian,

You have some problem with the wattmeter in the KPA100 - T4, D16, D17, 
R26, R27, U5, U6 or on the VRFDET signal pathway back to the K2 control 
board.

You seem to have a somewhat normal voltage variation at U5 pin 7, which 
(for the moment) says everything is working through U5, but check the U6 
pin 6 output under the same conditions and also check for the same 
voltage at K2 control board P4 pin 10.

Since this happened only by removing and re-installing the KPA100, be 
suspicious of a solder joint that is not well soldered - those may work 
for a while and then when touched can fail.

73,
Don W3FPR
 

Brian Lloyd wrote:
> Our K2 was working perfectly until I opened it up, tightened the power 
> transistor mounting screws, realigned the receiver, and put it back 
> together. Now power output no longer follows the power setting. When 
> set to a power output less than 10W, I hear the bypass relay but I get 
> the High Cur display (momentarily) and the display then shows a power 
> output of 0.3W with an SWR of 3.0. Output power as measured with a W1 
> is 20W. If I advance the power control, as soon as I pass the 10W 
> point the PA comes on and the power jumps to 150W output. Turning the 
> power level control fully CCW (0.1W setting) produces 6W of output.
>
> Looking at pin 7 of U5 on the KPA100 I can see good analog voltage 
> that varies with power. I can see the value of VFWD change when the 
> range changes (SCALE changes) also. I can also see varying VFWD at pin 
> 2 of U1 so this does not appear to be an analog problem.
>
> FWIW, voltages at pin 2 of U1 for various power outputs are:
>
>   6W -- 0.15V
>
>  20W -- 0.30V
>
> 150W -- 0.29V
>
> At pin 7 of U5 (U5b):
>
>   6W -- 0.15V
>
>  20W -- 0.30V
>
> 150W -- 0.77V
>
> On the t-r menu item I have "8r hold" set.
>
> I do have output on VRFDET but the value only varies from about 
> 0.3V-0.7V so that appears to be working also.
>
> I am planning to fly to Dominica on Friday and operate from there but 
> I can't take the K2 unless I solve this problem.
>
> Suggestions?
>