This problem has developed sometime in the last 6 months or so, but has
taken some time to pin down as it seemed intermittent and was being
blamed on other things. It did not exist when I first installed the
amp.
First, the amp is on the 20m position of my station, there is an ice
bandpass filter between it and the ft-1000mp exciter. There is a pair
of stubs after it. I have bypassed the stubs with no change in the
symptoms.
What I am seeing is this... with the radio set for 14024.5 I tune the
amp and all appears normal. Except when going above about 800w a strong
spur suddenly appears on 21036.9, almost exactly 1.5 times the
transmitter frequency. The spur can be moved by tuning the 20m radio.
It is not moved by adjusting the tune or load controls on the amp. With
the amp tuned to 1500w output on 20m the spur can be turned on and off
with fairly small adjustments of the amp tune control (5-10 degrees
max), it does not change over fairly large adjustments of the amp load
control. The onset point of the spur is not stable, sometimes it
doesn't start until the amp is up to almost full power, sometimes when
turning the tune control the start and stop points are not the same.
But once started the spur is very stable and its frequency is not
affected by the tune or load controls, not what I would expect from a
parasitic. When keying the radio with CW it creates large clicks on
15m, probably because of some instability when suddenly increasing
through 800w. There is also no discernable change in grid current when
the spur starts and stops as I would expect from a parasitic.
Maybe unrelated.. in one test I had my 6m radio turned on and when I was
turning the spur on and off by adjusting the amp tune control. When the
spur stopped on 15m I could hear it sweep through 6m, with some practice
I could tune it to the 6m frequency using the amp tune control. This is
more what I would expect of a parasitic. This was very weak and
probably only detected because the 6m antenna is only about 5' above the
20m one.
Any ideas where to start with this one???
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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