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

raypsi wrote:
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
I've sealed rewound filter chokes by dipping them in a bucket of
polyurethane varnish.

A good trick for the lams is clear nail polish as you reassemble them.
Just a quick wipe, as it'll spread as you squeeze in the last few lams
under a "Quick-Grip" clamp.

Cheers!

I'll second that.

Varnish is the best thing. Warm the varnish and the
transformer first before you dunk it. That will reduce
the viscosity and improve penetration. A little thinner
in the pot may help too. Vacuum/pressure impregnation is
better than dipping... but dipping can work well on a buzz.

There is even special dipping resin to reduce noise that
leaves the part looking like it's been dipped in a candy
coating. It's a soft, rubbery, epoxy coating.

Oh, and the teflon isn't so great as a shim. You might try
nomex paper, G-10 PC board scraps, fish or kraft paper even
typing paper, calendered cardboard (like shoe box cardbord)
or wood. Teflon will withstand the heat but it tends to
cold flow which makes it dimensionally unstable. The amount
of mechanical force between the core halves can be surprising.
Though in your transformer teflon may be be OK. In a choke
it can be a source of bewilderment as the gap decreases. Yes,
I've had this happen.

Any material that will soften when it's hot, like perspex/plexiglas
isn't a good choice either. I found this in an inductor for a
10KW boost converter that an otherwise reliable supplier built
for me.

For varnish, I like Dolphs (Von-Roll.) Failing that I use
marine spar varnish. Some varnishes are corrosive to copper
and over time will cause windings of fine guage to open if
any copper is exposed.




ken scharf wrote in
:

I've sealed rewound filter chokes by dipping them in a bucket of
polyurethane varnish.

"ken scharf" wrote in message
. ..
raypsi wrote:

n8zu
I've sealed rewound filter chokes by dipping them in a bucket of
polyurethane varnish.

This would work even better if you could pull a slight vacuum
on the bucket to get the air bubbles out. I believe that's how
the manufacturer's did it. There was a thread on this over on
AMFONE.NET a few month's back.

pete k1zjh

Uh! Sorry Ray, it was near 2AM and I misread this. Of course
there's no gap in the core it's a linear AC transformer. No
DC in the windings and so no gap.

Usually I'd do everything I can to reduce any gap, inclduing
interleaving the laminations, to reduce the magnetizing current.

I just wasn't thinking, that happens sometimes.


raypsi wrote in news:ea79280c-6148-42a5-be23-
:

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.

As was suggested by Grumpy... missed it first time.

Pedro

"Tio Pedro" wrote in message
...

"ken scharf" wrote in message
. ..
raypsi wrote:

n8zu
I've sealed rewound filter chokes by dipping them in a bucket of
polyurethane varnish.

This would work even better if you could pull a slight vacuum
on the bucket to get the air bubbles out. I believe that's how
the manufacturer's did it. There was a thread on this over on
AMFONE.NET a few month's back.

pete k1zjh

If you have bolts thru the lams have you torqued them to the point of
twisting them off? If you haven't you might want to loosen then and add
some more varnish to the lams while they're loose and then torque them.
Another possibility: Many of the transformers in microwave ovens have a
bead of weld across all the laminations and I've seen this on rectifiers
used in Telco applications. Keeps them quiet and I and has some
negatives like letting line spikes thru but on a filament that is
meaningless. Lam eddy currents area dead issue as I've heard that the
old Bell Labs had accepted this practice.
I had a 30S-1 with minor buzz but when I added 100 volts to the screen
the hum was about to drive me nuts but torquing to lams solved the
problem, I twisted off a couple of bolts.

--
73
Hank WD5JFR
"raypsi" wrote in message
...
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

Hmmm!

Bolts through the laminations should have insulators
(usually fiber board washers) under the heads. Just
one end will do, there's no need for them under the
nuts. But shoulder washers are best so the bolts don't
short any laminations together deeper into the core.

If uninsulated the bolt can form a poorly coupled shorted
turn and that in itself can cause noise, heating of the
core and the hardware as well as spray flux around where
you wouldn't have expected any. How bad it is depends
on the locations of the holes in the core.

A bead of weld across the ends of the laminations won't add
to the eddy currents significantly. Steel isn't a great
conductor. The weld's cross section is small and it's very
poorly coupled to the primary.

Why would a weld across the laminations let line spikes
though?




"Henry Kolesnik" wrote in
:

If you have bolts thru the lams have you torqued them to the point of
twisting them off? If you haven't you might want to loosen then and add
some more varnish to the lams while they're loose and then torque them.
Another possibility: Many of the transformers in microwave ovens have a
bead of weld across all the laminations and I've seen this on rectifiers
used in Telco applications. Keeps them quiet and I and has some
negatives like letting line spikes thru but on a filament that is
meaningless. Lam eddy currents area dead issue as I've heard that the
old Bell Labs had accepted this practice.
I had a 30S-1 with minor buzz but when I added 100 volts to the screen
the hum was about to drive me nuts but torquing to lams solved the
problem, I twisted off a couple of bolts.

Grumpy
Years ago in 93 or so on a Telco fiber system by Alcatel we took severe
hits when the air conditioner came on and it was traced to the station
rectifier. The Gould rectifiers had a weld bead and when we replaced
them another make which I can't recall that didn't have weld beads the
problem went away. Its quite strange since the rectifiers floated big
batteries but a vco in system was sensitive to the spike. After doing
some research I found an explanation that I can't recall because I was
too busy with other problems and mine had gone away. Alcatel spent
several hundred thousand trying to find the problem and I stumbled on to
it by shear luck.

73
Hank WD5JFR
"Grumpy The Mule" wrote in message
...
Hmmm!

Bolts through the laminations should have insulators
(usually fiber board washers) under the heads. Just
one end will do, there's no need for them under the
nuts. But shoulder washers are best so the bolts don't
short any laminations together deeper into the core.

If uninsulated the bolt can form a poorly coupled shorted
turn and that in itself can cause noise, heating of the
core and the hardware as well as spray flux around where
you wouldn't have expected any. How bad it is depends
on the locations of the holes in the core.

A bead of weld across the ends of the laminations won't add
to the eddy currents significantly. Steel isn't a great
conductor. The weld's cross section is small and it's very
poorly coupled to the primary.

Why would a weld across the laminations let line spikes
though?




"Henry Kolesnik" wrote in
:

If you have bolts thru the lams have you torqued them to the point of
twisting them off? If you haven't you might want to loosen then and
add
some more varnish to the lams while they're loose and then torque
them.
Another possibility: Many of the transformers in microwave ovens
have a
bead of weld across all the laminations and I've seen this on
rectifiers
used in Telco applications. Keeps them quiet and I and has some
negatives like letting line spikes thru but on a filament that is
meaningless. Lam eddy currents area dead issue as I've heard that
the
old Bell Labs had accepted this practice.
I had a 30S-1 with minor buzz but when I added 100 volts to the
screen
the hum was about to drive me nuts but torquing to lams solved the
problem, I twisted off a couple of bolts.