[Elecraft] [K2] K2 Always full power out

[Don Wilhelm]

Dave,

The KPA100 diodes D16 and D17 in the wattmeter have likely been damaged.
Those diodes are right on the output of the KPA100 and are intended to 
measure power output - which is not happening with your KPA100 (because 
they are damaged).  There are other components in the KPA100 that can 
cause the same symptoms -- U5, U6, and Q10, Q11, but the most common 
failure is the diodes.

Static from the antenna is the most common cause for failure of those 
diodes.  You should assure that there is a DC path across all your 
antenna feedlines to bleed off static.  A 5k to 50k resistor will do 
that job as will a 100 uHy choke.  The best place to install that static 
bleed is in your antenna switch.  In addition, when not in use, switch 
the K2/100 to a dummy load.  Antenna static can be caused by wind, rain 
or nearby lightning surges.

As for power control of the K2, the K2 system is more like commercial 
transceivers than most other amateur transceivers -- it is a closed 
control loop system.  The Power knob is not a direct power drive 
control, but is input of the requested power to the MCU.  The actual 
power output is monitored by the wattmeter at the output and sent back 
to the base K2 MCU where the two are compared. The BFO drive to the 
Transmit Mixer is adjusted to try to make those two equal.  If the 
KPA100 wattmeter is damaged, it will not indicate any power output and 
the K2 will ramp up the drive to maximum in an attempt to produce "more 
power output".  Fix the reading of the actual power output and things 
will be restored to normal.

One note of caution -- do not operate the K2 in that condition for very 
long intervals.  The fact that the K2 is producing maximum drive can 
stress the PA transistors in the base K2, and if that condition is 
present for very long, those PA transistors will "give up the ghost" and 
will have to be replaced.

The fact that you still get maximum power output indicates that the base 
PA transistors are still OK, but do not push that situation too long or 
you will have to replace those PA transistors in addition to the KPA100 
wattmeter diodes.

73,
Don W3FPR



On 5/12/2014 9:01 PM, Dave Mueller wrote:
> 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.