[Alinco] Parasitic oscillation in DX-70??

[Albert Woodhull]

I have a DX-70 which has been a satisfactory rig for me for many years. For 
mobile use and because I power my home station with solar charged batteries I 
set the internal switch to run 50 watts rather than 100.

I was experimenting with adding a 20 meter dipole in parallel with an 80/40 
meter trap dipole, in order to make it a 3 band antenna. I checked the SWR on 
the three bands and it looks like the idea will work OK, the 80 and 40 meter 
bands tune almost the same as before and the 20 meter lowest SWR is as close as 
I ever get to where I want it the first time I put up a new wire antenna.

But... a strange thing happens on 40 meters, it looks like a parasitic 
oscillation. When I press the key the reflected power jumps up momentarily and 
then settles down to what is should be, which is zero when I have adjusted the 
tuner for the best match and satisfactorily close to zero when I take the tuner 
out of the circuit. Furthermore it looks like the reflected power momentarily 
jumps up when I release the key. I did not see this behavior before I added the 
20 meter dipole to this antenna.

It looks like a reactive load is causing a parasitic oscillation when the output 
power is rising or falling. A jump in reflected power most likely means power is 
going out which is not at the frequency for which the impedance is matched. It 
happens whether I am running at the 10 watt or the 50 watt level. I don't see 
this if I put a 3 db resistive pad between the rig and the antenna tuner. The 
tuner is an MFJ 949E which I have had for more than 10 years and have used with 
several rigs with all sorts of antennas that were not pure resistive loads.

I wonder if anyone on this list has seen this kind of thing or if anyone has 
suggestions for further investigating what is going on and/or curing it.

Albert S. Woodhull
Leyden, Massachusetts
Amateur Radio: N1AW