http://elecraft.85.s1.nabble.com/K2-Update-K2-100-loss-of-power-control-long-post-tp6465960p6466209.html
Thanks for confirming my diagnosis. I *usually* remember to disconnect
> 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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