[TenTec] Radio Shack 25 amp switching supply?

Mon Mar 10 18:16:36 EST 2003

Well yes, copper or aluminum works but that is just one part of the story.
The secret is to have the box RF tight, thus the covers need to be
adequately overlapped and electrically bonded.  I've seen some metal covers
where the inside of the case was painted as well as the outside.  There was
no electrical connection between the two even thought the outside case was
held on with screws.   Internal tooth lock washers under each screw and pan
head screws (not undercut) will often take care of this part of the concern.
Removing the paint around the inside edges will also work up to near
microwave frequencies.  Once there, finger stock is generally required.

Also, radiated noise may have been taken care of on the output lines with
various filter schemes.  But what about the AC line?  Did it have adequate
RF blocking devices?  And most of today's meters on the front panel are some
form of plastic.  There's  no shielding there.  Correctly, the meters should
have a metal shield to prevent stray leakage via the large hole they are
mounted in or behind.  Then of course the leads to the meters should be
adequately filtered as they pass into and out of the meter shield area.  And
then there's the cooling fan, its air passages in and out.  Another point
that will need addressing.  In some cases, the rocker style AC mains switch
may need shielding and properly filtered with respect to the leads to and
from.  Usually, the switches mount into a sizeable hole suitable for RF
leakage.

There's a host of specific issues to deal with.  Each design and
implementation of packaging will require its own set of corrective measures.

Just a few general thoughts on how to quiet a noisy switching supply.

73
Bob, K4TAX

----- Original Message -----
From: "Scott Rosenfeld N7JI" <ham at w3eax.umd.edu>
To: <tentec at contesting.com>
Sent: Monday, March 10, 2003 5:37 PM
Subject: Re: [TenTec] Radio Shack 25 amp switching supply?

> SHEILDING!  copper flashing, solder, etc.
>
> On Mon, 10 Mar 2003, Robert & Linda McGraw K4TAX wrote:
>
> > OK, that takes care of conducted noise on the output, now what about
> > radiated noise?  That is usually a significant part of the problem.
> >
> > 73
> > Bob, K4TAX
> >
> > ----- Original Message -----
> > From: "Gary Hoffman" <ghoffman at spacetech.com>
> > To: <tentec at contesting.com>
> > Sent: Monday, March 10, 2003 4:21 PM
> > Subject: Re: [TenTec] Radio Shack 25 amp switching supply?
> >
> >
> > > Hasn't anyone tried to ground/shield/filter these things ?  We used
them
> > at
> > > the place I used to work, and I got them quieted down to the point
where I
> > > couldn't measure any noise with a scope hooked directly to the output
of
> > the
> > > supply.
> > >
> > > 73 de Gary, AA2IZ
> > >
> > >
> > > ----- Original Message -----
> > > From: <al_lorona at agilent.com>
> > > To: <tentec at contesting.com>
> > > Sent: Monday, March 10, 2003 4:17 PM
> > > Subject: RE: [TenTec] Radio Shack 25 amp switching supply?
> > >
> > >
> > > >
> > > > I have posted on this topic here before. You may cringe at such a
> > blanket
> > > statement, but *all* switching mode power supplies generate noise. As
> > users
> > > of extremely sensitive RF receiving equipment, we must be prepared to
> > > tolerate at least a small amount of RFI from these devices. Show me
any
> > SMPS
> > > of any size from any manufacturer and give me a good receiver, and I
will
> > > let you hear the noise coming out of the supply. It's just a fact of
life.
> > > >
> > > > The noise you hear will be greater with unbalanced antenna systems
> > nearer
> > > the SMPS. Operate from a hotel room with a random wire and an SMPS one
of
> > > these days and you'll see what I mean.
> > > >
> > > > The advantages of a SMPS are small size and light weight, but the
> > > advantages of a linear power supply are its quiet-- if not
nonexistent--
> > RFI
> > > spectrum.
> > > >
> > > >
> > > >
> > > > > -----Original Message-----
> > > > > From: Paul Christensen, Esq. [mailto:w9ac at arrl.net]
> > > > > Sent: Sunday, 09 March 2003 8:18 AM
> > > > > To: tentec at contesting.com
> > > > > Subject: Re: [TenTec] Radio Shack 25 amp switching supply?
> > > > >
> > > > >
> > > > > > Mine is going back to the store today.  If I lived in a
> > > > > place where the
> > > > > > noise level was always S5 to S8 it would be fine, but it is
> > > > > not the case
> > > > > > at this QTH.  I hear the noise on 20 meters, now, since the band
is
> > > > > > mostly quiet this morning.  Give me back my linear supply.
> > > > >
> > > > > I generally stay far away from switching power supplies: My main
shack
> > > > > supply is centered around an Astron RS-35 linear supply and power
is
> > > > > distributed to many devices via a RigRunner DC distribution
> > > > > panel (which
> > > > > btw, is a fantastic product).
> > > > >
> > > > > However, about a year ago, I was looking for a lightweight
> > > > > supply that could
> > > > > handle my Triton IV at full 100W output power for portable
> > > > > operation.   I
> > > > > sheepishly purchased the Astron SS-25 after reading a QST
> > > > > product review of
> > > > > a similar Astron SS supply.
> > > > > As I recall, there was essentially no measured noise from the
> > > > > Astron SS.
> > > > >
> > > > > The SS-25 is very light in weight, takes up a relatively
> > > > > small footprint and
> > > > > doesn't even blush when pushed to 25 amps over extended
> > > > > periods of time.
> > > > > There's a small thermostatically-engaged whisper fan on the
> > > > > rear panel that
> > > > > produces nearly no audible noise.  I absolutely love the
> > > > > Astron SS amp.
> > > > > However, I still would not consider it for the main station
> > > > > where I leave
> > > > > the RS-35 linear supply on 24/7.  I simply never want to have
> > > > > to consider
> > > > > whether or not a spur that I'm hearing is being generated by
> > > > > the supply.
> > > > > It's difficult enough in trying to localize shack noise
> > > > > problems without
> > > > > having to examine the supply and rule that out as a potential
> > > > > noise source.
> > > > > But for intermittent and portable operation, I couldn't be
> > > > > more pleased with
> > > > > the Astron SS.
> > > > >
> > > > > Also, Ten Tec makes some nice switching supplies and I would
> > > > > bet that theirs
> > > > > is at least as clean as the Astron.  I know in my previous
> > > > > dealings with Ten
> > > > > Tec, that they have been very sensitive to the user's power supply
and
> > > > > potential noise generation.
> > > > >
> > > > > -Paul, W9AC
> > > > >
> > > > > _______________________________________________
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> > >
> > >
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> >
> >
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>
> --
> Scott Rosenfeld  ARS N7JI
> 541-684-9970  Eugene, OR  Land o' much rain
> If you find me on the air, I'm probably in my car
> ham at w3eax.umd.edu  http://w3eax.umd.edu/~ham
>
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