Can you be a little more vague, Stuart?
Our club lost an Astron supply every time there was lightning in the county.
After grounding the negative post, they never lost another. Be darned
careful when you advise people to take action contrary to what the
manufacturer advises. You hurt people in your attempt to sound erudite.
Mike N4NT
----- Original Message -----
From: "Stuart Rohre" <rohre@arlut.utexas.edu>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Tuesday, November 29, 2005 1:55 PM
Subject: Re: [TenTec] Comments on "Repairs"
Some comments on the summary of comments and suggestions on the Omni VI
problems.
1. Everyone needs a dummy load! I held out for years, but then you have a
bad coax jumper cable or antenna cable and the dummy load and known good
jumper coax to it and the SWR meter and its coax jumper become "ground
truth". If you have RF feedback problems in a dummy load set up with known
good cables and SWR meter, then you have a rig problem. If NOT, then the
problem lies in the coax switches, jumpers to them, jumpers thru the system
and the antenna coax and connectors themselves.
Several times I have had RF feedback to a rig causing audio distortion of
lack of ease of getting contacts, and it was traced to loose coax plugs, or
bad shield connection on a coax cable, or one side of the feedpoint broke
connection on a doublet, etc.
Corrosion, water in the coax, and similar problems can be isolated by having
a known good dummy load.
When everything works on the dummy, then you know to look to the downstream
connections toward the antenna.
It is well known that the floating negative of the Astron 35 Amp power
supply causes RF to get back into the regulator causing problems. Many
ground the negative lead right to the chassis of the power supply without
exploring why they have a critical RF condition, which also depends on how
your at the rig grounding is done.
This can be because of the total length to earth of any intervening chassis
grounds, bus bars, and final earth rod lead length. I well remember having
my first external ground rod rendered totally useless for RF by having the
down lead 8 feet long, a perfect 10m quarter wave high impedance at the rig
chassis!
Grounding the Astron negative post can have a downside. In our club
station, that was done, then lightning surge used that path to get back to
AC ground as the preferred path, after burning out the RF ground conductor!
So it is not a universal solution and might be a bad "fix". Study and
diagram out your whole RF grounding scheme to look for sneak loops and paths
back to AC ground, telephone ground, etc. any ground in addition to your
driven shack ground you installed. Find out how the rest of the building is
grounded and bond all outside to meet the newest electrical code.
Stuart
K5KVH
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