[Elecraft] Strange problem with K3s/10: could use some suggestions

[Jerry Moore]

I don't see what you have connected to the output past the meter dummyload
or antenna.

-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Bill
Leonard N0CU
Sent: Thursday, January 26, 2017 12:01 PM
To: elecraft at mailman.qth.net
Subject: [Elecraft] Strange problem with K3s/10: could use some suggestions

I am having a strange problem with my K3s/10: the max output power on 20M
drops from 11.5W to 7.5W as the freq is moved from 14.0 MHz to 14.35 MHz.
All of the other bands stay above 11W across their respective bands. The
other interesting characteristic is that if I lower the power to 5W, there
is no drop- it stays at 5W across the entire 20M band.

Other relevant data:

-All data is taken post Cal

-The drop on the 12V line when I key up the transmitter (set for 12W) is the
same at both ends of the 20M band: 13.9 to 13.4V as measured by the K3s.

-Output power is measured with an LP-100A and is the same as shown on the
K3s.

-The 20M BPF & LPF are flat to a few tenths of a dB and the SWR<2:1 across
the band.

-The voltages and waveforms on the drains of the two FETs are identical, and
look OK to me.

-The bias on both FET gates is ~2.6VDC.

-The RF input signal to the KLPA3A (as measured at the XVTR output) is the
correct level and remains constant on all bands as the freq is moved across
the band. All spurious are down >50 dB. I also verified that the signal
level remains constant over the 20M band when the output power is dropping.
This suggests that the problem is not with the AGC or with any components
ahead of the KLPA3A.

Since the problem does not occur at reduced (5W) power, it appears to be due
to some non-linearity. The FETs would be a logical candidate if it were not
for the fact that the problem only occurs on one band. I checked the temps
on all of the 20M LPF toroids and didn't observe any rise.

I have no idea where else to look for a band specific non-linearity and have
run out of ideas on what else to check. Any suggestions would be greatly
appreciated.

Bill  N0CU



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