[Elecraft] [K2] K2 Always full power out

[Dave Mueller]

Thanks, Don.  I've never had a problem with static before, but always 
felt I was pushing my luck.
I also use the KAT100 external tuner, and leave both antennas connected 
all the time.  Does the Vf and Vr inside the KAT100 get sent back to the 
K2, and what do you think the chances are of the 1n5711's being bad in 
that too?

All of my testing has been without the tuner, but it was inline when I 
first noticed the problem.

Dave

On 5/12/2014 9:25 PM, Don Wilhelm wrote:
> 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.
>
>