I would like to thank all those who took the trouble to answer my query. Much
appreciated.
I think that I now have a fix that satisfies me. I started thinking about why
the rfi filter would produce the cure, and realised that it had both X and Y
capacitors* whereas my decoupling attempts had only had X. So I made a little
board up with both X and Y capacitors that mounts on the terminals of the HV
transformer. Complete cure. I am fairly happy with this as I don't think I am
hiding some nasty behaviour but suppressing it. I surmise that the problem was
some sort of ground loop and perhaps causing common mode on the coax - the
amplifier is grounded only through the power ground and the coax connections.
This will be rectified!
(* X and Y capacitors are designed for direct connection across AC mains and as
the names suggest the X goes across the terminals while the Y go from each
terminal to ground/chassis.)
I must admit that Manfred's explanation made perfect sense - after all that is
the precise mechanism of most power line arcing. But I am pretty confident that
that is not actually the case here.
73 and thanks again.
RogerVE3ZI
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