Ok, thanks for all the testing suggestions and comments. Below I will
try to answer all the questions and suggestions so far. I also find it
hard to believe it is produced by the amp, but am running out of other
causes.
I have tracked the spur from 21000 to 21450 (tx from 14000 to 14300) it
is always exactly 1.5 times the tx frequency, the frequency of it is not
affected by the amp tuning controls. The onset point does vary with
load, near 14000 the swr is about 2:1 and the spur doesn't start till
about 1kw output, at 14300 the swr is 1.2:1 and the spur is there at
about 200w output from the amp but is much more sensitive to the amp
tune control.
I have generated it with and without the input bandpass filter and the
output stubs in all possible combinations with 2 different radios.
I unfortunately don't have an adequate dummy load right now, but even at
almost 1:1 swr at the best frequency for the stack the spur is still
there as loud and predictable as ever. (note the stack is 300' from the
shack, with the shack off the side of the beams, and there is no other
sign of rf feedback in any equipment, all grounds have been checked)
I have swapped the ft-1000mp for a ts-870, there was no change in the
spur at all.
The amp does not have a qsk board.
There is nothing in the rx antenna jack on the ft-1000mp or the ts-870.
I can't believe it is a design flaw as the amp functioned normally for
several years with no problems. And this is so repeatable now that I
can't believe we didn't notice it or that someone else hasn't also seen
it in multi station contest operations.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
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