Update... I removed D16 and D17. D16 appears to be defective, it measures about 40 k ohms when reverse biased.
(At least I think it's D16, I kind of lost track of which one was which after I cut them out...) In any case, that isn't good... that's about 30 uA leakage at 1.2V (the OC voltage of the DMM on the 200K scale). The spec on a 1N5711 is 0.2 uA leakage at 50V. New diodes are on order. Thanks again Don, 73, 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 > > > > > > ______________________________________________________________ > > Elecraft mailing list > > Home: http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/mmfaq.htm > > Post: mailto:[email protected] > > > > 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:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
