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[TenTec] RFI to Ten Tec power supply

To: <tentec@contesting.com>
Subject: [TenTec] RFI to Ten Tec power supply
From: rohre@arlut.utexas.edu (rohre)
Date: 13 Oct 1997 10:07:44 -0500
Hi Webster,
Now take a moment to consider the symptom you have, and your question if a
part went bad in the power supply.  But first, check all the physical
connections to and from the supply, then open it up and look for obvious
problems, like bad solder joints, totally unsoldered joints that were only
being held by mechanical twist of the wire, etc.  A magnifying glass and
strong light are a great help to this exam.

Power supplies are pretty basic, and a main feature is a filter capacitor on
the output, which though not perfect at RF frequencies is there to filter
everything and should make the output mainly DC.  Thus, if it fails, you will
have OTHER symptoms, such as hum reported on your signals.  This is a way to
logically analyze a circuit.  Consider what the components are, and what the
absence of one working will do not only as to your observed symptom, but what
others, and look for them.

Let us back up further.  Have you changed anything in your setup to the power
supply such as increased the length of the DC leads?  

Are the DC leads twisted pair?  If not that is a good start.  

Now, you do not say if your station is on ground floor or elevated, ie how far
is it to earth?  Just because you have a wire to earth, depending on its
length, and material and gauge, you may NOT have a good RF ground.  Few hams
do at all frequencies.  

Does this problem show up at all frequencies?  Have you checked the tightness
of all DC and other connections associated with the power supply?  

Have you checked the DC voltage output?  Look at it with a scope if you have
one, or the AC and DC ranges of your meter.  You might have a weird problem
like one leg of a bridge rectifier failing, and that will affect your DC
output purity, but of course that also might have been noticed on your voice
With the supply off, use the diode check or ohmeter to check the forward and
back conduction and resistance of the bridge diodes.

How old is the supply?  Electrolytic capacitors could be suspect after 10
years, but often if kept formed are good much longer.  Most other bypasses for
RF, like ceramics or others fail only open or shorted.  Adding a good RF
bypass capacitor right at the supply output terminals is a good idea anyway. 
A disk ceramic of 0.1 mf is a good start.  Other tricks to exclude RF are
passing the DC leads thru as at least one turn on a Ferrite core such as Radio
Shack and others sell.  One core on the AC power lead could be used as well. 
Put as close to the cabinet of the supply as possible.  Or, if that does not
seem to do the job, slide the core along the power lead to see if there is a
point of optimum placement.  A core as close as possible to the RF source is
indicated as well.

My concern is that your post implies that it has worked before, and thus, I am
inclined to think you have made some small change that has not seemed to you
to be a connected event, but could be. Or, some failure has happened that only
manifested the symptom you have noticed so far.  A good test is to take your
supply elsewhere and try it with your rig that gave trouble at your location. 
And in a double substitution, borrow a supply or a battery from someone to try
at your location.
Let us consider your earth connection again for a moment.  Have you changed
anything to do with station hookup, earth or antenna leads?  You might have a
loose connection acting as a rectifier and producing RFI, if a loose
connection exists.  Has the weather been so dry that your earth ground might
be less effective?

Finally, along the lines of considering ALL changes in the vacinity of the
station, has anyone installed metal gutters nearby, or siding, or any roofing
of metal?  You see, the changes are the key.  Think back to what were things
like before the problem, and then when did the problem first show?  After a
lightning storm?  Thus, you will hopefully narrow down the logical steps to
solve the symptom and to fix the root cause.  Sometimes a matrix chart or a
list of what have you checked, will focus on what remains and what could be
the problem.

Good Luck, and let us know the answer.  A nearby ham "elmer" can be a big help
in seeing something we consistently overlook.  Try to detach yourself in the
same manner, and do not say, "OH I KNOW the AC power is ok," etc. Approach the
process with open mind and perhaps a rereading of the how it works section of
the manual on your power supply.

73, Stuart K5KVH

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