[Elecraft] K2 -- loss of output power control

[Brian Lloyd]

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?

Brian Lloyd
Granite Bay Montessori School          9330 Sierra College Bl
brian AT gbmontessori DOT com          Roseville, CA 95661
+1.916.367.2131 (voice)                +1.791.912.8170 (fax)

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