[Fourlanders] Radio problem

[Bob Lear]

Gentlemen, I've finally gotten around to testing my FT 920 that gave us
a problem at the beginning of the contest on 6m.  As you may remember,
it was producing garbled SSB output, so bad it was hard for our friends
to copy.  I disconnected it from the amp, computer and everything I
could on the mountain to no avail.  It had the problem at high and low
power, with different mikes, etc.  

When I got around to testing it, I hooked everything up, computer,
radio, PS, etc. and found no problems with it here!!!!!!!
Even with the same Samlex switching supply I was using on the mountain
and I even tried it with a variac on the input to simulate low line
voltage as suggested by Fred K4KAZ, it wouldn't even start to sound
funny until I got down to 50% input voltage, and then the PS was
groaning, not a problem that we saw on the mountain.  

I sure thought we had this figured out, but I'm at a loss as to what
happened.  We did the remainder of the contest with a borrowed FT897,
same PS, amp, computer, Headset, connections, etc, pretty much without a
hitch and certainly no distortion like I experienced there at the
beginning of the contest with the 920.

Any thoughts on what the problem might have been from anyone out
there!!  I'm glad my FT920 is OK, but I sure wish we knew what was going
on.  Neal is also going to test his 990 (with internal switching supply)
on a variac, but I'm not holding my breath for that to be a problem
either.  

It's looking like RF feedback, but why only with certain radios?  Are
switching supplies more susceptible to RF than linear supplies?  

Maybe we can set up the antenna out in the yard on the trailer sometime
and set one of the problem stations up again to see if we can duplicate
the problem at home and figure out a solution.  Not a lot of fun to try
to fix right after the contest has started.  BTW, my system was fine on
Friday night and Saturday morning.  It wasn't until we got the linear on
at the beginning of the contest that we had a problem.  And of course we
shut the linear down and took it completely out of line and still had
the distortion problem, and then it was OK with the 897 and same PS.

I'm tired of this 'VooDoo curse' we seem to find up on the mountain!!!

Any comments or suggestions are welcome.

73 Bob