[Amps] HV transformer buzzing!! Help Please

[Pat Barthelow]

I am guessing a parasitic high Q (narrow band) mechanical resonance of the 
chassis/cover.... Excited by the 60 Hz mains frequency through the 
transformer, mounted to the chassis.

Refabricating the cover, possibly different thickness, stiffness, and 
especially with the offset vent holes, probably moved the HI Q resonance 
frequency of the cover away from the 60Hz transformer-exciter..no more 
resonance....

Sincerely, Pat Barthelow     aa6eg at hotmail.com
http://www.jamesburgdish.org
Jamesburg Earth Station  Moon Bounce Team
http://www.cq-vhf.com

>From: "k7fm" <k7fm at teleport.com>
>Reply-To: k7fm <k7fm at teleport.com>
>To: <amps at contesting.com>
>Subject: Re: [Amps] HV transformer buzzing!! Help Please
>Date: Sun, 10 Jun 2007 18:04:00 -0700
>
>This discussion of transformer buzz came just at the right time.  I already 
>came up with a cure, but I thought it was through a defeatist mode.  But, 
>some of Tom's comments at least reinforced my results.
A friend has two Ten-Tec 20 amp power supplies.
In a quiet room, one buzzes.  I somehow got sucked into helping him fix it.
The top is steel and it only buzzes with the top on.

The fellow sent it back to Ten-Tec, but it came back the same way.

A prior owner added some aluminum plates with adhesive.  That helped a bit.

1.  I removed the transformer and tightened everything.  Hum still there.

2.  I looked up mu-metal to install under the steel cabinet shell.  About 
$50, plus mu-metal may lose characteristics if bent or cut.  Abandoned that 
idea.
>
>3.  Removed transformer and dipped in varnish, then let it bake in the sun. 
>Replaced transformer.  Hum still there.

>4.  Found a replacement transformer in my transformer junk box.  Same exact 
>size and current.  Replaced transformer.  Hum still there.

5.  The transformer is on a sub-chassis.  I made a rubber barrier to
>eliminate transfer of buzz.  Remounted all.  Hum still there.

6.  I admitted defeat.  I replaced the original transformer.  No hum with 
steel cover off.  Put cover on.  Hum still there.

7.  Admitted defeat.  Found an old piece of aluminum and used my shear and 
brake to make a new cover.  I improved the ventilation holes, by putting in 
some low and some high - justifying the new cover as an inmproved 
circuilation method.  Installed new cover and NO HUM!.

>8.  I will prime and paint tomorrow and give it to my friend, along with 
>his old cover.  I was tempted to trade covers to see if this cover had been 
>altered with bad characteristics.  Also thought of running it through one 
>of the ship degaussers.  In the end, I will move on to the next project and 
>never know why this damn supply buzzed.

>73,  Colin  K7FM
>