Some of you will recall my tale of the repeated failure of (it turns
out) diodes and other components in the driver stage of my Omni-6 Plus.
I have had it back to Ten-Tec 4 times for the same problem.
To be specific, this was (per the last service ticket) "D-2, 3, 4, 5, 6,
7, 8, 9 on the low-pass filter, R-10, 11, and D-7, 8, 9 on the bandpass
filter."
This appeared to be an oscillation that would take off and go out of
control while beginning to advance the power control, causing the
current meter to pin and component failures. I have since learned that
the "oscillation" I see is just the symptom *after* the failure, not the
cause.
Through help from many of you, I decided that what was needed was to
look at two things:
1.) RF getting in through the power supply, and any other cables
2.) installing a circuit breaker inline with my Astron RM-35 power supply
I also took the precaution of setting up so that I can adjust my tuners
via an MFJ analyzer before switching the rig over to them.
I did both, with the circuit breaker coming from Airpax and the choke
sleeves from Surplus Sales of Nebraska. They are installed on *every*
cable, including headphones and CW paddle. I got the rig back just in
time to start calling the guys at the South Pole on 30m, using a
ladderline-fed dipole with my Palstar balanced tuner.
20 minutes into calling, the meter went to zero, and I knew something
had given out. I couldn't get it to put out any RF at all on 30, so I
switched to 80m and my Hy-Tower, (which I think of as a very predictable
load.) As I advanced the power, at only 10-o'clock, the meter pinned and
the circuit breaker popped. Same as before, except for the circuit
breaker - but it had never tripped until that moment.
This time I had a long conversation with Paul at Ten-Tec. He told me a
few things that seem to narrow it down, but do not explain it by any means:
He says that there is no way it is oscillating - if it did, the finals
would go, and they never have.
He says that the finals do not care whatsoever about SWR.
He says that the only thing that can fry these driver components is
*something" getting *in* on the antenna connector, from the outside.
We went on to discuss the possibility of lightning (not likely in Ohio
in December or February, and certainly not likely to coincide with it
failing within hours each time) - or static buildup on the antenna from
wind. This, to me, is just as unlikely as lightning, and would be
extremely coincidental to be able to fry it within hours or days, each
of the 4 times. The antenna it was connected to at the time is just a
70' dipole at 40 feet. It wasn't even windy. I haven't measured any
static, but I just don't see it as possible to be generating the 1000+ V
that would be needed (per Paul) to fry diodes.
He also asked about inline remote antenna switches, or any other remote
devices, where I would be running a DC supply on the coax. I have
nothing of the sort. The only thing between my rig and the antennas is
either the Palstar or Nye tuners, and coax switches. There are no other
transmitters, at least none that I use while the Ten-Tec is turned on.
I measured DC and AC at the connector to the rig a few times, and found
nothing. As always, comments are appreciated.
--
Larry N8KU
w w w . l o n g w i r e . c o m
100% CW 100% HF
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