Whats gone wrong with this transformer ?

[N_Cook]

Squashed profile toroidal to get in a 1U case of a 1992, Ampeg B1 amp,
export version, internally set for 240 V use, blew (violently) the mains
fuse. No burnt smell or obvious damage to the transformer.
Disconnected and removed there are 2 seaparate primaries , >1000 Meg , 500
volt isolation between them and the secondaries.
3 wires to each primary , presumably 110V and 120 V each pair. Measures
about 3 ohm over each "120V " pair of separate primaries which seems about
right for a good 450 VA transformer.
Connect either primary to a 20 volt ac variac supply and they draw 1 amp of
current. I could understand a failure in one primary but both failed the
same ? Could there be failed VDRs or something buried inside the transformer
?

 

[N_Cook]

Shorted turns in either primary will show up as a high current when driving
either one since the non-driven one acts as a secondary.

Or shorted turns in a secondary.

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Combining the priomaries and running varaic at 27 volt, again high current
drawn, there is about 1 volt from 1 secondary and 2 volt from the other. One
secondary is for main power rails and the other for HT supply for a pair of
valves. Its a job to say which has some shorted turns.
DC resistance of the secondaries is 0.5R and 4.1R
I will try feeding low voltage AC in the secondaries and see what happens.

 

[Samuel M. Goldwasser]

Squashed profile toroidal to get in a 1U case of a 1992, Ampeg B1 amp,
export version, internally set for 240 V use, blew (violently) the mains
fuse. No burnt smell or obvious damage to the transformer.
Disconnected and removed there are 2 seaparate primaries , >1000 Meg , 500
volt isolation between them and the secondaries.
3 wires to each primary , presumably 110V and 120 V each pair. Measures
about 3 ohm over each "120V " pair of separate primaries which seems about
right for a good 450 VA transformer.
Connect either primary to a 20 volt ac variac supply and they draw 1 amp of
current. I could understand a failure in one primary but both failed the
same ? Could there be failed VDRs or something buried inside the transformer
?

Shorted turns in either primary will show up as a high current when driving
either one since the non-driven one acts as a secondary.

Or shorted turns in a secondary.

--
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ignored unless my full name AND either lasers or electronics is included in the
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[N_Cook]

There is a light gauge secondary, presumably about 18V ac , full wave
rectified to give 25.2 V dc for the seriesed heaters of 2 ECC83, CTs N/C.
The main secondary winding supplies the TO3 o/p devices and a diode pump for
the valve HT. Not determined where the opamp supplies are from.

Putting 20V ac on the heater supply gave 50 V on main and 96V on each
primary
Putting 20V on the main gave 7.6V on the heater one and 38 V on each
primary.

I assume from this the primaries are ok and something wrong with the main
secondary, perhaps 2/3 of the run is shorted somewhere, 1/3 only functional.
Luckily they always dilute the epoxy infill with chalk or something so easy
to chain drill out. Nothing obviously wriong with the outer layer of
secondaries but I assume the problem is due to only one layer of polyester,
half overlapped on the outside and only that separates squashed secondaries
right up against the metal top of the 1U case. Rubber disk on the bottom
face of the toroidal but nothing at the top other than the standard toroidal
polyester wrap strip. At least they did not decide to pass the coach bolt
through top and bottom of the case. If the secondaries are re-windable (if
no primary damage) then some extra padding will have to go in over the Tx
and some extra space found in the cab.

 

[Samuel M. Goldwasser]

Combining the priomaries and running varaic at 27 volt, again high current
drawn, there is about 1 volt from 1 secondary and 2 volt from the other. One
secondary is for main power rails and the other for HT supply for a pair of
valves. Its a job to say which has some shorted turns.
DC resistance of the secondaries is 0.5R and 4.1R
I will try feeding low voltage AC in the secondaries and see what happens.

My wild guess would be the HT secondary since that has the most turns
of fine wire with the msot stress on it, but who knows? I would think
that it will be rather difficult to sort it out in a well made
transformer without knowing what the original specs were exactly,
including resistances, or unwinding it.

What are the spec'd voltages on the outputs?

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[N_Cook]

happens.

My wild guess would be the HT secondary since that has the most turns
of fine wire with the msot stress on it, but who knows? I would think
that it will be rather difficult to sort it out in a well made
transformer without knowing what the original specs were exactly,
including resistances, or unwinding it.

What are the spec'd voltages on the outputs?

--
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Don't know the specs. What is Murphy's Law - If something can go wrong it
will go wrong at the most inopportune time or the most awkward place. I
always thought the primary was buried on the inside of transformers -
conventional and toroidal. Not this one, the primaries are on the outside
and the main secondary is on the inside, presumably to save on the expensive
large gauge copper wire. Basler Electric, Highland, Illinois

 

[N_Cook]

Can a toroidal former become permanently magnetised and cause excessive
current draw. ? This one does not attract a small piece of iron. I don't
know what the main
secondaries are supposed to provide, but can give 300 watts into 4 ohms.
I've wound off the primaries to try and
understand what is going on with this transfornmer. Applying 70 volt ac ,
from a variac, across series secondaries is ok but increase to 75 volt and
the current draw goes up to 1/4 amp. Should be able to take twice that
voltage. Absolutely no visible problem , to the wiring or the coating of the
toroid and both secondaries measure DMM the same DC. Tomorrow will apply
something a bit over 70 volt with temperature sensitive labels fitted
around. I will even try a TV degaussing coil.

 

[Samuel M. Goldwasser]

Can a toroidal former become permanently magnetised and cause excessive
current draw. ? This one does not attract a small piece of iron. I don't
know what the main
secondaries are supposed to provide, but can give 300 watts into 4 ohms.
I've wound off the primaries to try and
understand what is going on with this transfornmer. Applying 70 volt ac ,
from a variac, across series secondaries is ok but increase to 75 volt and
the current draw goes up to 1/4 amp. Should be able to take twice that
voltage. Absolutely no visible problem , to the wiring or the coating of the
toroid and both secondaries measure DMM the same DC. Tomorrow will apply
something a bit over 70 volt with temperature sensitive labels fitted
around. I will even try a TV degaussing coil.

No, it isn't magnetized.

That fact that you can now apply 70 VAC to it without significant
current drawn suggests that whatever short was present is now gone.

How many turns were on the original primaries? Call that "n".

Wind a few turns on the core, call that "m".

Apply the rated input voltage x m/n to your winding and report back.

--
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Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
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[bz]

Can a toroidal former become permanently magnetised and cause excessive
current draw. ? This one does not attract a small piece of iron. I don't
know what the main
secondaries are supposed to provide, but can give 300 watts into 4 ohms.
I've wound off the primaries to try and
understand what is going on with this transfornmer. Applying 70 volt ac
, from a variac, across series secondaries is ok but increase to 75 volt
and the current draw goes up to 1/4 amp. Should be able to take twice
that voltage. Absolutely no visible problem , to the wiring or the
coating of the toroid and both secondaries measure DMM the same DC.
Tomorrow will apply something a bit over 70 volt with temperature
sensitive labels fitted around. I will even try a TV degaussing coil.

The magnetic field on a toroid is contained within the toroid.

You will have a hard time degaussing it unless you wrap wire through it.

You will need at least a single turn of wire THROUGH the toroid to check to
see if it 'remembers'.

See how core memory works:

http://ed-thelen.org/comp-hist/navy-core-memory-desc.html






--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

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