[Elecraft] K2 / KPA ALC problem. Are power levels sensed at the same point?

[Rob KK4R]

Chris,

I have been monitoring this thread, since I have had some similar problems
with my K2.  My recent experience may help you.

In the K2, amplifier gain is in the ALC loop.  When you switch from low
power to high (go from less than 10W to greater than 10W), the KPA100 gain
changes the ALC behavior and may be the source of many of the differences
you see between low and high power operation.  If the KPA100 needs some
adjustment, that could also be a factor.  In my case both
calibration/alignment and gain were causing problems.

I recalibrated the KPA100 first and noticed some improvements immediately in
output power on the high bands and in the agreement between set power and
output power.  Note that I did this even though I believed that I had taken
care in the original calibration.  Maybe things change over time, or perhaps
I did a better job this time, but the effort was rewarded.

Once the recalibration was done, I followed Don's suggestion and adjusted
R98 to reduce the ALC loop gain.  Prior to adjusting R98, my K2 would hunt
endlessly at some power levels, jumping up and down by significant amounts
and never converging to the set power level.  At other power levels, it
might settle down at a value that wasn't as close to the set value as you
might like.  After adjusting R98, the K2 always converges to the set value
with a relatively small error, because the ALC steps are smaller.

You mentioned that you don't have a power meter to use for adjustments to
the KPA100.  I have two, but after a comparison, decided instead to use a
voltage measurement across a good dummy load to calculate power.  This
method seemed to give me the most reliable result across the range of
interest.  If you have a good dummy load, you can use a scope to measure
peak-to-peak voltage or use a diode detector to get a DC voltage off the
dummy load.  The Elecraft 20W load is very good and has a built in provision
to take a DC voltage measurement for conversion to power.  With that taken
care of, you can adjust your amp with little trouble.

Rob



--
View this message in context: http://elecraft.365791.n2.nabble.com/K2-KPA-ALC-problem-tp7581353p7581458.html
Sent from the Elecraft mailing list archive at Nabble.com.