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

Posted by Don Wilhelm-4 on Jun 11, 2011; 10:13pm
URL: http://elecraft.85.s1.nabble.com/K2-Update-K2-100-loss-of-power-control-long-post-tp6465960p6466090.html

  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
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html