[Elecraft] [K2] K2 Always full power out

[Dave Mueller]

My K2/100 has been working fine for a few years. Last weekend, I was 
greeted by a HI CUR alarm when I first keyed down.

After some troubleshooting, here's what I've found:
If I remove the KPA100, the power control changes output power as 
expected, and the power bar graph works to full scale.
With the KPA100 installed, and the power control below 10 W, the power 
is 17 Watts regardless of the setting (0 to 10 W).  If the power control 
is set anywhere above 10 W, my meter is pegged at over 100 W out.  I get 
no indication on the bar graph at any setting.

I measured the V RFDET line at the DB9 on the KPA100 pin 7: in low power 
mode (stuck at 17 W), the voltage is .310.  In high power mode (pegged 
over 100 W), the voltage is .410 V.

At first I suspected the DAC U8, but it works without the KPA100.
I'm guessing an input to the MCU is bad, causing it to drive the DAC U8 
to full output.

A few questions:
For the base K2, it appears that the forward power calibration is fixed 
in firmware (my settings don't match my calibrated power meter).
Where does the actual power control occur?  I found "V PWR" coming out 
of DAC U8 pin 2, but can't find it anywhere else on the schematic.
What are the inputs the MCU uses to drive the DAC?

All tests were done on 40 meters into a high quality 50 Ohm load. Other 
bands have the same symptoms but I didn't bother to correct for the 
coupling value for my CP1 coupler.

Since I can't vary power, I'm not sure what to expect for the Vforward 
and Vreflected voltages in the KPA100 coupler circuit.

Thanks
Dave AA3EE
K2 s/n 5929