Dear Boris,
This may not be entirely scientific, but I had a similar problem
using old television receiver transformers that made noise.
What I did was to put a 1/8" sheet of aluminum (or copper
if I could find some), just a bit larger than the transformer
footprint, under the transformer. I used machine nuts
both on top of and underneath the aluminum plate,
with flat washers and lockwashers in all sixteen positions.
My friends used to call this humming "Yarf" for some reason that
I can't remember, but it was a hurdle all of us who used to scarf
surplus xfmrs on Radio Row in NYC had to jump when building
HB gear.
Hal
W4HBM
On Sun, 19 Jun 2005 06:14:13 -0700 Borislav Trifonov <bdt@shaw.ca>
writes:
> I had a 1.5 kW microwave oven transformer (MOT) that I took apart,
> removed the magnetic shunts, and when putting it back together, I
> interleaved the laminations (originally, all the Es were together as
> one
> E; likewise the Is). The transformer hums quietly on its own, but
> when
> I put it in my project chassis, which is steel, I found out that it
> had
> huge leakage as it made the chassis hum very loudly. Knowing that
> MOTs
> are made with the bare minimum of materials, I figured that the core
> was
> saturating and added 15% more turns to the primary (about as much as
> I
> could fit). That helped a bit, but it was still pretty bad. I also
>
> added a copper tape around the windings, outside the core, as the
> 'flux
> band' sometimes seen in transformers (especially in audio
> equipment).
> That made a small difference. The chassis still hums too loudly.
> I've
> put in too much work in the current project chassis to replace it
> with a
> non-steel one. So I'm wondering about magnetic shielding. Mu-metal
> has
> very high permeability but saturates easily and is only useful for
> weak
> fields. Often I've seen tube audio amp transformers either
> completely
> encased in, or at least wrapped around the edges of the EI, with
> silicon
> steel or soft iron, which reduces leakage flux. However, I actually
> was
> not able to find any appropriate material. Any suggestions?
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>
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