Anyone who has rewound a power xformer

What were the results? Did you manage to get all the lams back in? Was
there any audible hum? I took apart a power transformer from an old tube
Tektronix scope and wound two secondaries with 13awg wire. Its for a
*gasp* solid state amp. Theres still a small stack of lams left over
that I can't get back in. The last few had to be tapped in with a
hammer. Unloaded the hum is very audible which bothers me. Am I wasting
time or should I spend the $50 on a ready made toroid?


Adam

"Adam Stouffer" <adam_stouffer@hotmail . com > wrote in message
news:wAwFj.1174$Qv5.1079@trnddc03...
> What were the results? Did you manage to get all the lams back in? Was
> there any audible hum? I took apart a power transformer from an old tube
> Tektronix scope and wound two secondaries with 13awg wire. Its for a
> *gasp* solid state amp. Theres still a small stack of lams left over that
> I can't get back in. The last few had to be tapped in with a hammer.
> Unloaded the hum is very audible which bothers me. Am I wasting time or
> should I spend the $50 on a ready made toroid?
>
>
> Adam

Hi Adam:

I suspect that your Tektronix transformer was wound on a paper core tube
rather than the coil having been wound on a plastic bobbin. With this type
of transformer, it's always pretty hard to get all of the original
laminations back in during re-assembly for three reasons:

1) Since the original transformer was impregnated, the laminations are
somewhat hard to remove during disassembly. As a result, some of them
usually end up slightly bent. Once bent, they're almost impossible to
straighten again and do not stack as tightly when re-assembled.

2) There is often some impregnating compound that sticks to the laminations
which also prevents tight stacking. This problem can be minimized by making
sure that you reassemble them in the same order that they were in
originally.

3) With two secondaries of 13AWG, there's a possibility that you have
deformed the core tube slightly, also making the stacking process more
difficult.

Now, if your Tek scope was originally a 50-60 Hz unit, you will be fine with
a reduced lamination stack (when operating at 60Hz only) provided that you
haven't reduced the stack height by more than 20%.

As far as the audible hum is concerned, you'll have to impregnate the
transformer again to reduce or eliminate this. One of the epoxy based
transformer varnishes is best, but for a home job, the following will work
nicely.

1) Re-assemble the transformer without its end-bells and use temporary bolts
in the four corners to clamp the laminations. They should be clamped
snugly, but not overtightened.

2) Heat the transformer up in your oven to about 200F.

3) While hot, dip the transformer into a can of spar varnish (the type of
varnish that never becomes as hard as a rock). Allow it to sit in the
varnish for at least 30 minutes. An hour is even better.

4) Lift out of varnish, allow it to drain, and when it has finished draining
stick it under an infra-red heat lamp for a day.

This has worked for me in a pinch many times. Better than a poke in the eye
with a burnt stick as the old saying goes.

Best Regards : Doug Bannard

Doug Bannard wrote:
>
> "Adam Stouffer" <adam_stouffer@hotmail . com > wrote in message
> news:wAwFj.1174$Qv5.1079@trnddc03...
> > What were the results? Did you manage to get all the lams back in? Was
> > there any audible hum? I took apart a power transformer from an old tube
> > Tektronix scope and wound two secondaries with 13awg wire. Its for a
> > *gasp* solid state amp. Theres still a small stack of lams left over that
> > I can't get back in. The last few had to be tapped in with a hammer.
> > Unloaded the hum is very audible which bothers me. Am I wasting time or
> > should I spend the $50 on a ready made toroid?
> >
> >
> > Adam
>
> Hi Adam:
>
> I suspect that your Tektronix transformer was wound on a paper core tube
> rather than the coil having been wound on a plastic bobbin. With this type
> of transformer, it's always pretty hard to get all of the original
> laminations back in during re-assembly for three reasons:
>
> 1) Since the original transformer was impregnated, the laminations are
> somewhat hard to remove during disassembly. As a result, some of them
> usually end up slightly bent. Once bent, they're almost impossible to
> straighten again and do not stack as tightly when re-assembled.
>
> 2) There is often some impregnating compound that sticks to the laminations
> which also prevents tight stacking. This problem can be minimized by making
> sure that you reassemble them in the same order that they were in
> originally.
>
> 3) With two secondaries of 13AWG, there's a possibility that you have
> deformed the core tube slightly, also making the stacking process more
> difficult.
>
> Now, if your Tek scope was originally a 50-60 Hz unit, you will be fine with
> a reduced lamination stack (when operating at 60Hz only) provided that you
> haven't reduced the stack height by more than 20%.
>
> As far as the audible hum is concerned, you'll have to impregnate the
> transformer again to reduce or eliminate this. One of the epoxy based
> transformer varnishes is best, but for a home job, the following will work
> nicely.
>
> 1) Re-assemble the transformer without its end-bells and use temporary bolts
> in the four corners to clamp the laminations. They should be clamped
> snugly, but not overtightened.
>
> 2) Heat the transformer up in your oven to about 200F.
>
> 3) While hot, dip the transformer into a can of spar varnish (the type of
> varnish that never becomes as hard as a rock). Allow it to sit in the
> varnish for at least 30 minutes. An hour is even better.
>
> 4) Lift out of varnish, allow it to drain, and when it has finished draining
> stick it under an infra-red heat lamp for a day.
>
> This has worked for me in a pinch many times. Better than a poke in the eye
> with a burnt stick as the old saying goes.
>
> Best Regards : Doug Bannard

I never bother to go to too much trouble with trannies that are hummy
or ones that seem to draw way too much magnetizing current with no load.


Getting all the lams "back into" a given winding after extracting them
painfully is something I gave up years ago.

At my house, I have an open log fire place, and a drop down door
that is lowered in front to seal off the fireplace from the room,
preventing
the warm fireplace sucking all the warm air out of the house after
midnight when the fire has burnt down and i try to sleep,
hopefully without nightmares about crook transformers.

Anyway, when I want to re-wind a tranny, I order new wire from the
wire supplier. I place the old tranny
which is to be re-wound in a fireplace and light a wood fire around it,
allowing it to just get up to red for 10 minutes.
This means the varnish and plastics and paper and enamels will all
be burnt off or vaporized or carbonized.
The iron is mildly annealed, and retains or has its magnetic properties
bettered.

Next day when everything is cool, the tranny is taken to a vice,
and copper cut off with an angle grinder.
Its removed from a pot if there is one.

If the tranny was bell ended, all bolts are removed and bell ends before
firing.

With copper cut off easily, the laminations all fall apart, and can be
cleaned
with a brush. The oxide coating is still there, and the lams can be
re-used,
and generally the same number can be jammed into the same sized bobbin
as on the original tranny if there was one.

New bobbins are not expensive, but sometimes I make the bobbins
after making a cardboard tube would around an accurately made timber
mandrel
to suit the stack height of the lams. If I do this I always make the new
bobbin with
some extra curve on the rectangular shape to cause less wire bend in the
first
few layers which will make it a lot easier to wind and keep the wanted
turns per layer
without gaps between wires.
I also make the cheeks higher than wanted, so the added hoieght for taps
doesn't
result in windings rising above the cheek height.
I drill lots and lots of holes for the wire to emerge from windings, so
wires coming out from the bobbin don't share the same hole in the cheek.

I varnish my trannies as i wind them using Wattyl 7008 floor coating.
Its a two pack mix and it painted on generously with a cheap thro-away
brush
or cloth daubing mop.
Its smelly and sticky work and you need a fan to blow fumes away,
and a can of methylated spirits to keep cleaning the goo of hands,
which turn black after the muck sticks to fingers.
Its harmeless to me, and the black wears off after 3 days,
but other folks might get a bad allergic reaction in lungs or skin.
If so, don't use this method at all.

I use onlt paper based insulation material betaeen layers of all neat
layer wound
windings for power trannies.
I apply varnish as the as the bobbin height increases, and mix only
enough varnish to last 1/2 an hour, or a few wire layers before mixing
another batch.
After say 6 hours winding on a large power tranny, and with maybe 2/3
the work complete,
I'll stop for the day and fit neatly cut blocks of ply or MFF
between the bobbin cheeks and cramp them up making sure the cramping
is too tight, but also that it acts to flatten the inevitable wire
bulges
develops as you wind around a rectangular bobbin.
This squeeses the still liquid varnish around inside the windings, and
ensures the paper layers are
well impregnated. I leave it crampewd for 24 hours and the 7008
will have cured to a semi hard and cramp can be removed the the final
layers put on and re-cramped
when it is all finished, without to much bulge at all.

After day 2, the bobbin is carefully removed from the lathe without
breaking any of the many
labelled wires coming out of it.
Its a little chorish because of the stickness of the varnish.

The lams are inserted, and plastic sheet scraps used ensure the stack is
tight in the bobbin.
The termination board is fitted if there is one, and insulated bolts
placed with open yokes
and bolts drawn tight while keeping the assembly square and plumb.
Then the wires are brought to terminals after stripping enamel off with
a box cutter knife.
Wires from the bobbin should carry woven heat resistant sleeving tight
around the wire
between bobbin and terminals, and wires with large voltage differeneces
routed to keep them apart.
The same woven sleeving will have been used wherever wires come out of
the bobbin,
so that the sleeving extends 20mm inside the bobbin, and this guards the
wire
from being bent badly and perhaps causing a shorted turn against a
neighbouring turn.
Once the tranny has been bolted up tight, and with all loosenesses
visible wedged up tight,
and with all wires wound twice around the terminals, it can be tested
for
voltage by applying 1/10 of the operating voltages to the primary,
and measuring all other voltages which should be 1/10 of the wanted
values and
with all phases all where you want them. Its easy to make mistakes.

When you are satisfied with the accurate terminations, and that you have
all your wanted voltages, solder up the terminals and draw the final
winding board diagram and double check
it and make sure any other dumbo can understand it in 10 years time.
The sleeving on tranny wires placed during winding is kept short
outside the bobbin.
During ternination of wires, the sleeving is extended to the terminal.
And a sleeve of large size placed over the lion in the tighter initial
sleeving butt join.
Where lots of sleeved enamel wires go from bobbin cheeks to terninal
boards,
they are bundled in groups for each winding and tied together with
string
and perhaps some blobs of high temp silicone used to keep bundles off
the core or other wire
bundles with a big voltage difference.

At this point there is still a possibility for vibration and noise,
especially
with a rectifier connected when the odd order harmonics in the
transformer current is huge,
causing magnetic force jolts to all windings and iron.

I found spray pack furniture clear enamel generously applied to all
lamination surfaces and
to lams where they enter the bobbin tends to work to lock up the lams
well
where the bolts don't fully manage to do.
This goop is well absorbed into tiny gaps between lams.
Place 3 coats of this all over the tranny, except to the terminal board
to which
you will need to bring your amp wires.

The sleevings of windings will be properly saturated with varnish.

If you don't varnish as you go, the next best treatemnt is the soak and
bake method.
This requires special electrical varnish used normally for motor
rewinding.
It resembles furniture varnish but is different, and NEEDS to be heated
to force it to become a
tough plastic that holds wires well gued together and to the insulation
layers.
The insulation properties of the paper when impregnated with any
insulant becomes much better than
if no impregnant was applied. Paper that is un-impregnated tends to
become moist,
especially if the tranny ends up being parked somewhere damp for a
length of time.

The tranny is soaked overnight in a vat of the varnish, preferably after
some
pre-heating, but that's not really necessary.
It will be pulled inside the tranny by capillary action and air will be
expelled.
But its not foolproof, because air lock pockets can form and varnish
just won't soak
into all areas, especially on fine wire windings.
After the soak, its left to drain excess out.
The tranny is then oven baked so its whole temp is kept
at 125C for 4 hours, when there appears to be no more terrible toxic
smell
generation. Finding a way of controllig temperature baking properly
is nigh impossible at home, and doing it in the kitchen stove will bring
a quick divorce.

Hence my dislike for the soak and bake method.

Makers get this process done much more reliably if they place the tranny
into the varnish vat
then apply a vacuum over the surface of the varnish. The varnish must be
a type that does not
boil or bubble up when a vacuum is applied, lest the varnish get into
the machine pulling the vacuum.
The vacuum needn't be perfect, 95% is OK, and so a small compressor can
be used to
suck rather than blow to give the vaccum.
The vacuum only need be held for 20 minutes to let air excape the core
and bobbin
cavities.
The air presure is then allowed back over the vat and the varnish rushes
in to fill the vacuum cavities
within the tranny. The process can be repeated a a few time to make
sure.
Then during the bake, any dry unvarnished areas will be wet with moving
fumes from nearby wet varnish
and this is the best way to get varnish to go where you want it to
apart from applying it while you wind.

The electrical varnish can be applied while you wind the tranny using
brush before and after every layer of wire
and insulation is placed, like I do with the 7008, but you'd still have
to bake the tranny afterwards.

One is not supposed to thin electrical varnish with anything like turps
or white spirits to make is less viscous,
or to have enough to submerge an item in. Once thinned, afaik, the
vacuuming is more difficult,
and the thinning agent fumes, and prevents an absense of pressure
developing.
I used an old frig compressor as a vacuum pump for awhile until it
became
ruined with varnish entry to its innards.
Then I used a larger compressor with a bottle and pipe to trap material
flow, and to show
me what was going on better, but really the vacuum and bake varnish
method is a very messy horrible
and anti social process.

A mains tranny with only a steel box around it will make that box hum
loudly from stray radiated
magnetic fields. A box or can when filled with solid material is called
a transformer pot,
and the the process of canning and filling the can is called potting.

The tranny should tested in the amp before potting is done, but with
steel cover box in place.
Don't be too worried abut the box hum.
The unloaded tranny must be fully tested for correct woking voltages
before potting.
No use potting something with a mistake present.

My typical potted tranny has the tranny bolted to the top of the pot,
and termination board on the tranny bottom, with terminals protruding
into
the underchassis space.

To get the sand in, the termination board is sized to allow it.
The pot & tranny is turned upside down and dry sand poured in to fill
the pot.
I use a wooden block and a hammer to gently hit the pot aboput 500 times
from all directions to vibrate and compact the sand so its level sinks
about 15%
then pot tacasanInl.


I am using well dried fine sand well compacted into the box except for
the last 15mm of pot height
when the tranny and pot are t.
and sealed down
with casting resin to stop the can humming and to enclose any
transformer noise.

The last recent result with a 350VA tranny used a 72mm stack of 51mm
tongue GOSS lams
for a PT used to power a 60W SET amp with 845 has worked out fine.
The pot is 1.6mm black steel sheeting, 135mm x 147mm x 170mm.
It hummed loudly without the sand, but with well packed sand its silent.

The sand and seal method means that if ever the tranny is to be
re-wound,
the seal can be broken out with hammer and chisel, sand drained out,
bolts undone, and tranny removed and burnt clean, and core and pot is
re-usable.

The other way to pot is with purpose made epoxy potting mix, and it is
the best job
for to quality gear, but its terribly expensive.
Fiberglass casting resin is also usable, and cheaper,
and can be used with sand, so you pour in the casting resin well mixed
with its hardener,
then pour in sand and let it sink into the resin. This process is done
in
25mm height sections, to avoid heating by the resin curing.
So this gives a saving of about 50% for the amount of resin used
otherwise.

But once its done, there is NO more access to wiring within the pot or
tranny.

Transformers made at home should of course be better made than those one
might purchase.

Why would anyone waste a lifetime in this horrid work when cheap
trannies are to be purchased?

Trouble is, many cheap trannies that might be purchased and appear
suitable could
be noisy and run hot.

Rarely does any ONE transformer have all the voltages one wants for say
an exotic
amplifier using say 845 or 211.

I decided to become a transformer winder so I could have anything I
want.

One might be able to use a group of several stock standard transformers
to
provide all the necessary voltages.
Hammond also make a range which are not as expensive as the diyer
solution.

And finally, a word of warning about potting and using old cores
and low turns per volt.

A typical older open frame un-potted mains tranny may get warm if not
quite hot
after several hours, and surely so in hot weather.

Placing

On 3=D4=C224=C8=D5, =C9=CF=CE=E71=CA=B138=B7=D6, Adam Stouffer <adam_stouf..=
.@hotmail . com > wrote:
> What were the results? Did you manage to get all the lams back in? Was
> there any audible hum? I took apart a power transformer from an old tube
> Tektronix scope and wound two secondaries with 13awg wire. Its for a
> *gasp* solid state amp. Theres still a small stack of lams left over
> that I can't get back in. The last few had to be tapped in with a
> hammer. Unloaded the hum is very audible which bothers me. Am I wasting
> time or should I spend the $50 on a ready made toroid?
>
> Adam



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