> 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.
I keep saying this over and over. The biggest problem I have
seen when steel chassis and steel cabinets are involved is
flux leakage from the transformer. Even very good
transformers have this problem, and it is often a real
bitch to fix.
Now if you think a transformer is bad, a filter choke like
the Globe uses is significantly worse. The steel cabinet on
many Viking Valiant's, even though inches away from the
filter choke in the HV supply, actually physically moves
when HV is applied from choke flux leakage. This is because
chokes have to be intentionally designed to have significant
flux outside of the iron in the choke.
If you put aluminum in the field, the flux can pass due to
imperfect conductivity. If the aluminum was a very good
conductor the flux would generate a high eddy current and
the current would generate an opposing field. That could
significantly cancel the flux. If you put mu metal in the
field, the mu metal moves and much of the flux can still
pass anyway unless you do things really right.
One way to reduce external fields is a wide copper band
around the transformer that forms a shorted turn outside the
transformer window. I'm not sure how well that will work
with a choke, but it sure works with a sine wave
transformer. I expect it would work with a choke, although
the starting flux level can be significantly higher from a
Other things can happen too. For example the screws in
transformers and chokes have to be insulated from core
contact at least on every area except one point maximum.
This is so the screw doesn't form a shorted turn with the
transformer frame or any of the laminations. Sometimes on a
rebuild that gets messed up.
This is also dangerous business because the only real way to
find out what is buzzing is often to poke around inside with
some sounding device while running the thing.
Anyway if I wanted to cancel a time-varying magnetic flux
I'd use a thick (like .062) copper sheet. You want as much
conductivity as possible to encourage the formation of
maximum eddy currents. A wide copper band forming a closed
loop around the transformer outside would be best.
With the Ten Tec, he should just get an aluminum or
stainless cover. With the Globe, I'd embrace some hum. The
humming and smell and heat and glow is what it is all about.
I always enjoyed the kahthunk when I hit the HV switch in my
Viking Valiant, and the buzz as the choke vibrates the
cabinet on CW is almost romantic.
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