Posted by
aj4tf on
Jun 11, 2011; 8:42pm
URL: http://elecraft.85.s1.nabble.com/K2-Update-K2-100-loss-of-power-control-long-post-tp6465960.html
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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