[Elecraft] [K2] Update: K2/100 loss of power control (long post)

Sat Jun 11 20:04:45 EDT 2011

  David,

Actually using a switch in the antenna line to switch in the dummy load 
is best.  If you consider that with a PL-259, the center conductor is 
connected before the shell, so if there is a charge on the antenna due 
to wind, rain or lightning static, it can zap the diodes.

You can get the diodes most anywhere.  I usually get mine from Diz - 
www.partsandkits.com, inexpensive and he ships quickly.

Thank you for your kind words.

73,
Don

On 6/11/2011 7:48 PM, david wrote:
> Don,
>
> Thanks for confirming my diagnosis.  I *usually* remember to disconnect
> the antenna when I am not operating.
>
> I assume that the standard 1N5711 will work, Elecraft doesn't screen
> them for any special characteristics do they?
>
> As always, I'm sure I not the only one that appreciates the knowledge
> you bring to the Elecraft community.
>
> David AJ4TF
>
>
> On Sat, 2011-06-11 at 18:13 -0400, Don Wilhelm wrote:
>> David,
>>
>> Yes, it looks like U5, U6, Q10, Q11 and the associated components are
>> behaving as they should.
>>
>> It certainly could be that D16 and D17 are not either open or shorted,
>> but are not fully functioning as they should.  The fact that you could
>> only get the resistance measured at the wiper of R26 as high as 30k does
>> confirm that -- replace the diodes.
>>
>> It also seems from your description that things are a bit intermittent
>> and erratic, so while you are in there to replace the diodes, give the
>> soldering in the wattmeter area (particularly T4) a good inspection and
>> perhaps re-flow the solder with a hot iron (800 degF for the KPA100
>> board), and perhaps add a tiny bit of new solder (actually you are
>> adding a bit of flux).
>>
>> Yes, those diodes are sensitive to lightning surges.  Switch the K2
>> antenna to a dummy load when not in use for best results.
>>
>> 73,
>> Don W3FPR
>>
>> On 6/11/2011 4:42 PM, david wrote:
>>> K2/100 s/n 7006
>>> Initial symptom:  No control of output power at any knob setting.  At
>>> 1-9W setting, output was>10W as measured by external wattmeter, SWR
>>> read back>9.9 (external meter shows around 1:1).  At 10-100W setting,
>>> output>100W, SWR>9.9.  HI REFL message displayed.
>>>
>>> Unit was previously working fine for several months.
>>>
>>> Troubleshooting procedure I used:
>>>
>>> 1) Disassembled KPA100 from radio (but still connected).
>>> Repeated test using 50 ohm dummy load, same results.
>>> SWR reading>9.9
>>>
>>> 2) Set to 40M, 7200 kHz, set power to 5W, and made the following KPA100
>>> measurements during TUNE cycle:
>>> U5-5: 0.2V  (VFWD)
>>> U5-7: 0.2V
>>> U5-3: 2.01V (VREF)
>>> U5-1: 2.01V
>>>
>>> So, I assume the microcontroller is seeing this and measuring a high
>>> SWR.  HI REFL message was displayed.
>>>
>>> 3)  Using diode check function on Fluke 8021B, measured D16 and D17 in
>>> circuit,  each showed approx. 0.4 V forward bias, and open reversed.
>>> However I am not yet convinced that the diodes are not damaged in some
>>> way (see below).
>>>
>>> 4) I went through the SWR Bridge Null adjustment procedure in the KPA100
>>> manual.  However, I was unable to get a measurement of 43 kohms on the
>>> wiper on R26 to ground,  I was only able to get a maximum of 30 kohms,
>>> and with the wiper centered, it read about 23 k ohms.  I left both R26
>>> and R27 centered.
>>>
>>> 5) At 5W power, I was able to adjust C1 to obtain a 1.0 SWR reading,
>>> noting that C1 appeared to be in almost the same position as before.
>>>
>>> 6) Set to 40M, 7200 kHz, set power to 5W, and made the following KPA100
>>> measurements during TUNE cycle.  Indicated power 6.0W,  indicated 1.0
>>> SWR reading from display.  In each case, I allowed about 5 seconds for
>>> the output to stabilize before taking each measurement.
>>> U5-5: 1.4V
>>> U5-7: 1.4V
>>> U5-3: 0.02V
>>> U5-1: 0.02V
>>> U6-6: 2.8V
>>> U6-3: 1.4V
>>> At this point I am reasonably certain that U5 and U6 are working.
>>>
>>> 7) set power to 15W, (heard relay switch to KPA100) and made the
>>> following KPA100 measurements during TUNE cycle.  Indicated power 15W,
>>> indicated 1.2 SWR reading from display, external wattmeter showed>20W
>>> and 1:1 SWR.
>>> U5-5: 2.25V
>>> U5-7: 2.25V
>>> U5-3: 0.3V
>>> U5-1: 0.3V
>>> U6-6: 1.41V
>>> U6-3: 0.71V
>>>
>>> 8) set power to 20W, and made the following KPA100 measurements during
>>> TUNE cycle.  Indicated power 20W,  indicated 1.6 SWR reading from
>>> display, external wattmeter showed ~40W and 1:1 SWR.
>>> U5-7: 2.55V
>>> U5-3: 0.6V
>>> U5-1: 0.6V
>>> U6-6: 1.62V
>>> U6-3: 0.81Vmeter showed>40W and 1:1 SWR.
>>> U5-5: 2.55V
>>> U5-7: 2.55V
>>> U5-3: 0.6V
>>> U5-1: 0.6V
>>> U6-6: 1.62V
>>> U6-3: 0.81V
>>>
>>> 9) set power to 40W, at this point, I was able to re-adjust R26/R27 to
>>> match the externally measured output power to the indicated power
>>> (within a few watts, and limited by the accuracy of the external power
>>> meter).
>>> After adjusting R26/R27:  Indicated power 42W,  indicated 1.8 SWR
>>> reading from display, external wattmeter showed ~40W and 1:1 SWR.
>>> U5-5: 3.76V
>>> U5-7: 3.76V
>>> U5-3: 1.15V
>>> U5-1: 1.15V
>>> U6-6: 2.37V
>>> U6-3: 1.18V
>>>
>>> 10) set power to 60W, Indicated power 60W,  indicated 2.3 SWR reading
>>> from display, external wattmeter showed ~60W and 1:1 SWR.
>>> U5-5: 4.4V
>>> U5-7: 4.4V
>>> U5-3: 1.72V
>>> U5-1: 1.72V
>>> U6-6: 2.78V
>>> U6-3: 1.39V
>>>
>>> So, after all of this,  my conclusion is that D16 and D17 may be leaky,
>>> or somehow defective, and providing invalid voltages to the measuring
>>> circuit.  I do not believe that U5 or U6 are defective, and it appears
>>> that the microcontroller is controlling the power based on the voltages
>>> it is seeing.
>>>
>>> I would appreciate any other opinions or insight, or any other
>>> suggestions on what else to check.  In the meantime I'll go ahead and
>>> order some replacement 1N5711's.  Static electricity?   Lightning?
>>>
>>> David AJ4TF
>>>
>>>
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