While doing the transmitter alignment on K1 #00074 (page 46), I noticed that the rf output indicated on the K1 internal meter did not agree with my WM-2 wattmeter.  My version of the K1 is configured for 40 meters and 20 meters.  I also found that the maximum output on both bands was only about 5.0 watts, even when the K1 output (OUT setting) was set at 7.0 watts.   There appeared to be a problem with the accuracy of the K1 rf detector.

I sent a note to the Elecraft email support address on Sunday 11/12/00 and received an answer in under an hour!  Talk about service!  Gary Surrency responded to my inquiry and indicated that he had experienced the same thing I described.  He suggested experimenting by inserting different values of resistors in parallel with R37 on the rf board.  R37 is part of the K1 rf detector.  He also indicated that the K1 ATU unit will have a more accurate rf detector and would resolve this problem. 

I decided to try a few different values of resistors and see what the affect was on the K1 output.  The original value of R37 was 226 ohms.  I decided to try paralleling values of  3.3K, 2.2K, 1.5K and 1.0K and tabulate the results.  This was equivalent to values of 212, 205, 196 and 184 ohms for R37.  Since the rf detector is not linear, I would pick the best overall results achieved.   Below are the tables showing the results that I experienced.

The first table shows the original results measured on both bands.  As you can see the accuracy was very poor.

This table shows the results with a 3.3K resistor in parallel with R37 (equivalent to 212 ohms).

This table shows the results with a 2.2K resistor in parallel with R37 (equivalent to 205 ohms).

This table shows the results with a 1.5K resistor in parallel with R37 (equivalent to 196 ohms).

This table shows the results with a 1.0K resistor in parallel with R37(equivalent to 184 ohms) .

Based on the above test values, I chose the results using the 1.5K resistor since it provided reasonable accuracy at the 2.0, 5.0 and 7.0 watt settings.  The actual value of the combination of the two resistors was approx. 196 ohms (measured with DMM) so I used a 1/4 watt 200 ohm 2% resistor for the new value of R37.  I measured the actual value of this resistor with my DMM and it was 199.6 ohms.  The last table shows the final results with the new value for R37.

The overall results were slightly better with the 1.5K resistor in parallel with the 226 ohm resistor (approx. 196 ohms), however, the closest standard value resistor was 200 ohms.  The new value improved the overall accuracy of the rf detector, compared to the original readings,  and improved the maximum output from 5.4 watts to 6.6 watts on 40 meters and from 5.0 watts to 5.5 watts on 20 meters.  I have graphed the before and after results for both bands as measured on my WM-2 wattmeter.  In the graphs, the box and diamond symbols show the actual measurements and the lines show the trended results.  Since I did not take sample readings at 1 watt increments, I trended the results to show a smoother K1 performance curve.

After calibrating the rf detector, I did some additional experimenting to try and increase the maximum output power that I had obtained.  I adjusted the windings on transformers T3 and T4 on the rf board  and the windings on low pass filter toroids on the filter board.  I also readjusted the Toko coils on the rf board.  This increased the maximum output from 6.6 to 6.9 watts on 40 meters and from 5.5 to 6.0 watts on 20 meters.

This was an interesting and beneficial exercise which increased both the accuracy and the output of K1 #00074.   If you have any questions concerning this process, please send them to me via the email link below.

73,
WT9W

 e-mail WT9W