On Oct 27, 11:29*pm, Stray Dog wrote:
On Sat, 25 Oct 2008, raypsi wrote:
Date: Sat, 25 Oct 2008 10:25:23 -0700 (PDT)
From: raypsi
Newsgroups: rec.radio.amateur.homebrew
Subject: I built a 7.5VAC 21Amp transformer now it has lots of buzz

Hey all:

I tore down a transformer I got from ebay it was a Zenith TV power
transformer from days gone by. Re wound it with 17AWG wire on the
secondary and primary. It powers up my 4-1000A filament nicely with
7.25VAC from 120VAC mains. I used a penta-filar winding on the
secondary. Shimmed the bobbin in the *core window *with some 1/8 thick
virgin teflon. Epoxied the windings to the bobbin. This was a 240VA
transformer and now is a 83watt buzzer / 157watt filament transformer.

So what is the cheapest I can gits away with sealing these laminations
from buzzing, I used up all my epoxy sealing the windings.

73

n8zu

This might be a little "far-out" but, besides just buying more epoxy and
just doing it right, maybe you could find a can or metal box, put the
transformer into the box, and fill the box up with enough sand that you
have 1-2 inches all around the transformer, and that might muffle the
buzz. You'll need to worry about heat dissipation through that sand which
will act like an insulator, but if you are on the air only a couple of
hours per day, it should not get dangerously hot (however, many of the old
hermetically sealed transformers were burried in a waxy-tar that was
quite hard at room temperature).

Your other option is to figure out some clamping arrangement so that the
laminations are held close and tight.

Re-winding transformers is fun, but next time you want some off-beat
voltage, maybe scrounge up higher voltages (eg. car battery chargers)
and use either a variac or power resistor (with sufficient heat
dissipation) to bring the voltage down to where you want it and save
yourself the headaches and earaches. Or, don't take the laminations apart
but feed the wire (stranded) through the holes that are left.

Or, other kludge-cheapskate approaches.

Good luck.- Hide quoted text -

- Show quoted text -

I have always dunked them in varnish but only got some success with
this by
drawing a vaccum then applying pressure. I used an old compressor
from
a refrigarator to do this. The tank was made from from pipe and
fittings. The best
results I got was winding the core on a block of wood cut the same
size as
the core wrapping the wood with cardboard then windng the coil then
pushed
then put the core together. I was 18 at the time and a very talented
man
named Olin Griggs helped me with this project and let me use his
shop.
Olin was a retired engineer that had worked for GE at their
Bloomington plant.
Since then the onl transformers I have wound that havent bussed to
much
was by modifying a ready built transformer. USually this meant one
that I
on which I could keep the primary winding intact. Lately I have gotten
interested
in SMPSs for doing this sort of thing. It is very easy to see why
industry has
has made the move to them.

Jimmie