Hi David
First guess is that your FT-1000 has a spur at 7 Mhz and the amp is
making the third harmonic.
I take it that you are adjusting amp power by reducing drive from the
FT-1000?
That would make sense as the harmonic content would be worse at higher
power levels.
Try another exciter if you have one....... better yet if it has a tuned
tank output.
---
Ron
David Robbins wrote:
>
> 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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