Recently I asked about an instability problem after my 10 year old
HB 4CX1500B amp suddenly started destroying vacuum relays. Thanks
for all the advice! I discovered under certain conditions it was
going into LF oscillation around 400 to 450 kHz (with lo load
connected to the output).
I'm afraid there is more going on than originally thought. I used to
be able to keep a table of tune/load presets and never have to vary
from them unless my antennas were iced up. Now it seems the settings
are constantly drifting. I have checked all connections in the amp
and antenna system. Could this be a sign of tube or vacuum variable
failure?
On the higher bands I am also seeing instability when tuning. While
adjusting the tune and/or load caps, I'm seeing sudden jumps in
antenna SWR and power output. Obviously this is a stability /
parasitic problem. I am unable to determine if it is the same LF
oscillation since there is so much RF at the desired frequency under
these conditions.
Any new thoughts based on the above? This amp had been trouble free
for 10 years prior to the onset of problems a couple of weeks ago.
Something has clearly changed.
73,
Paul N1BUG
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