On a suggestion I checked the oscillation with a scope. It agrees
with my frequency counter - oscillation around 400 - 450 kHz.
I failed to add in my first post... when this problem occurred
during "normal" operation on 12M, the amp seemed stable during
transmissions, apparently taking off at the moment of transition
back to receive. That in mind, I am wondering if I should
investigate the timing of relay and bias switching. This behavior
may suggest the vacuum relay contact on the amp is opening before
cutoff bias is applied.
The other really ugly thing I failed to mention is that this
oscillation would then continue until I toggled the amp
operate/standby switch to standby. I suspect this LF RF is getting
into places where there should not be RF.
73,
Paul
--
Paul Kelley, N1BUG
RFI Committee chair,
Piscataquis Amateur Radio Club
http://www.k1pq.org
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