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[AMPS] SB220 Xformer labodomy

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Subject: [AMPS] SB220 Xformer labodomy
From: (Phil T. (VE3OZZ))
Date: Thu, 18 Feb 1999 19:50:25
Len,  one cause of hummy power transformers is loose laminations and
sometimes loose windings.  Loose laminations can be  cured by tightening
down the screws or bolts that hold or clamp the lamination stack together.
The fact that someone's been in there already suggests the possibility that
they didn't tighten things up completely when they were done.

Loose windings will also vibrate  and hum.  This usually isn't a problem
because most windings are tight to begin with, and if the unit has been
vacuum impregnated with varnish, the varnish 'glues' everything in place
fairly well. If the previous owner disassembled and reassembled the entire
transformer, he may have forgotten to re-install the wedges between the
winding tube and the core.  The wedges are pieces or strips of fiber board
that are jammed in between the inside of the winding tube and the faces of
the core leg.  If you're able to wiggle the whole winding assembly while
its in the core, its too loose; look for missing wedges.  I'm not familiar
with the SB-220 transformer but its possible that wedges were used.

If some of the individual turns in the windings became loose and therefore
noisy over time, the previous owner may have been trying to correct it with
the duct tape (if that's what it is). It is normal to see the occasional
1/2" or 3/4" wide piece of cloth tape in the windings.  These are used to
anchor the first and last one or two wraps on a winding.  They may also
cover a soldered-on lead comming out of the transformer.

Shellac can be used but its normally not recommeded because of moisture
content. Admittedly, commercial shellac (like Zinsser's Bullseye shellac)
is made with very high quality denatured alcohol that has only minor
mositure.  Good quality "oil based" polyurethane varnish is a better choice
but it can take several hours to dry.  You can identify oil based varnishes
(just in case you're not familiar) by reading the instructions on the can.
If it isn't identified as oil based on the front label, you'll know its oil
based if the clean-up instructions tell you to use "mineral spirits" (ie,
Varsol, paint thinner, etc).  The suggested drying times for oil based are
usually 4 hours or more.  Water based polyurethane varnishes - which you
don't want - will either say 'water based' on the front label, or the
clean-up instructions will tell you to use water.  Water based products
typically have drying times of only 1 or 2 hours (or less) - which is
another clue that its water based.

Rich may reply with advice about potting, which is something I haven't
tried yet.  But potting would certainly quiet loose winding-turns as long
as the potting material can reach the noisy area.

Phil T

At 04:56 PM 2/18/99 -0500, you wrote:
>I have a very noisy SB220 power transformer that sounds like a bad
>florescent ballast. Other than being noisy, the amp works fine. I've taken a
>good look at the transfomer and noticed that the back surface of the
>tranformer has been hacked off. It looks like someone was trying to make
>more clearance between the transformer and the back plate of the amp but I
>doubt that that is the reason for cutting the transformer. I removed the
>back plate and pushed asside the paper insulator on the windings and noticed
>that there is some grey tape ( looks suspicially like duck tape) wrapped
>around the windings.
>I'm tempted to just loosen the screws, paint the laminations with some
>shellac and see if I can get rid of the noise. 
>Anybody ever hear of this kind of transformer surgery or why anybody would
>do it? 
>Thanks in advance.
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