[Elecraft] K2/100 down power on 20M

[Michael Carter]

Hi Jerry,

I agree that the relatively low TX power output on 20M
originates in the KPA100 module.  Did you build that
module and install it yourself?  The LPA circuit in the
base K2 seems to operating normally, and your
peaking of the BPF confirms that.

If you have capability to sweep the 20M/30M LPF
section on the KPA100 for insertion loss, that would
provide more insight.  However, those LPF sections
are also inline when the KPA100 is nominally
bypassed - it's only the amplifier circuit that is
bypassed.  If there was a significant problem in the
LPF on the KPA100, it would have been evident in
your tests on the 10 watt TX power setting.

I would check the ALC indication on each band, but
set the Power control for 100 watts rather than full
clockwise rotation.  You could also monitor the
ALC voltage on the Control board as you change
bands to see if for some reason the ALC control
is reducing TX power on 20 meters.  There is no
band-dependent circuitry in the ALC system,
but the SWR forward/reverse power detection
circuit on the KPA100 does have a trimmer cap
to balance the bridge.  It would be unlikely for
a resonance condition in the bridge to cause
an elevated VRFDET voltage on one band alone,
but I am curious if the ALC system is doing something
odd on 20 meters only.

73,
Mike, K8CN