RE: Drifty power k2-100

Posted by Ron D'Eau Claire-2 on Aug 07, 2005; 10:37pm
URL: http://elecraft.85.s1.nabble.com/Drifty-power-k2-100-tp381065p381066.html

John K5MO wrote:
I recently began driving a linear and noted that the output power drifts
quite a bit. This is not something that's noticed during "barefoot "
operation but becomes noticeable when driving an amp.

If I set the power at about 70watts, and TUNE/Display into a dummy load for
about 10 seconds, I can see the power drift to 72, 74, 69, 72 all within
that span of time. It seem random, and is not a drift, but rather jumps up
and down every few seconds. I note the difference on the K2 panel as well as
the output wattmeter.

----------------------

It sounds like you're seeing the quantizing error in the K2 DAC's in the
power control circuit, John. The DAC's check the power level and when
occasionally report a different level, especially when the level is close to
a quantizing step in the DAC.

Wayne, N6KR, who designed the K2 published a "fix" that will help reduce
this tendency to jump around back in a message to the reflector back in 5
Aug 1999. It's currently on the web site under the builder's resources
section. Here's the procedure for any of the newer K2's (If someone reads
this who has an older "field test" K2, there are a couple of other points to
check - see the original message):

1. Temporarily insert a 1K potentiometer at R98 (use leads of 1" or less).
Set the pot to mid-range (500 ohms).

2. Switch to 10 meters and set power for 10 W (or your desired maximum power
output on 10 meters).

3. Your goal is to find the largest value of R98 (up to 1 K) that allows for
the desired maximum power output on 10 meters using TUNE. Try various R98
settings until you find this value.

4. Once you've found the highest usable value for R98, switch back to 40 m
and see if this has improved power setting resolution using TUNE at 2 W, 5
W, etc.

5. Replace the pot at R98 with a fixed resistor. Use a standard resistance
value that is equal to or lower than your final pot value.

Ron AC7AC



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