V76 input transformers acting weird

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Murdock

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Hey folks,

I got three V76 input transformers BV511. They came from a spare department of a radio station as far as I now. So they should be unused.
One of them works like a charm and is flat from 10 Hz to almost 50 KHz...
The other two have low end loss. Top end seems to be ok. When looking at an oscilloscope with two probes in "differential mode" the low end drop starts at around 300Hz. But is flat to almost 45Khz.
I also have a Psophometer (AC voltmeter 15Hz - 100KHz) and there it's even weirder. It's flat from only 10khz to around 25khz. Signal drops fast under 10khz.

Thought it could be the infamous magnetization problem and tried to demag it with a 3v 30Hz signal slowly decreasing level but that didn't work.

Does anybody have a clue what the problem could be?
 

emrr

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That sounds more like what i see with a partially open winding. The treble may be leakage.
 

Bonnie1

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Hello Murdock,
What's the pri./sec. L and dcr between the good and not so good units?
 

Murdock

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All three have primary DC of around 6 ohm when wiring the windings in series. So 3 ohm each.
I can't get a reading of the secondary. Even with the good one. But with the good one the DMM just can't find the right range I think.
While with the "bad" ones it says "OL". So it seems open winding.

I cant get a exact L reading on the secondary of the good one.
The primary is around 2.7H on the good one and around 2.1H on the bad ones.


But both transformers exhibit almost exactly the same problem. Could the windings be broken in the same place?
 
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Bonnie1

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Try setting your DMM to K ohm range, if its a Fluke DMM (flashing OL =can't auto range)
Solid OL= open connection.

BV511 has 6 secondary coils, any one of these 6 coils likely has a break if your DMM is solid OL.

Inductance measurement comparison is also useful in evaluating the healthy status of the transformer, when there are no open windings.
 

Murdock

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So this is evidence for a broken winding?
Probably because of corroding wires?
It's weird, because they are impregnated with wax or something.
It can not be another problem? Thought it could have something to do with the transformers beeing on the shelf for so long... That they have to "burn in" or something. Can Mu-Metal lose it's magnetic properties over time?
 

mjrippe

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Possibly they were sitting on the shelf because they were bad. Seems unlikely that a radio station would have expensive input transformers sitting unused.
 

emrr

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Tightly wound wire in transformers can eventually break simply from decades of atmospheric pressure changes, temperature shifts, humidity, etc.
 

Moby

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Just to repeat it. Open windings 99%. Probably someone skilled can rewind it.
 

Bonnie1

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So this is evidence for a broken winding?
Probably because of corroding wires?
It's weird, because they are impregnated with wax or something.
It can not be another problem? Thought it could have something to do with the transformers beeing on the shelf for so long... That they have to "burn in" or something. Can Mu-Metal lose it's magnetic properties over time?
Hi Murdock,
If your DMM is not measuring Sec. resistance (solid OL), then yes you have an open winding. The cause could be various possibilities. Transformers sitting on the shelf generally don't just go bad. Granted BV511's were made 60+ yrs ago.... some were made w/magnet wire wrapped with silk.
Transformers are passive devices, they either work or don't. There's no "burn in" time, etc.....

The BV511's should not be impregnated with wax or any other goop. If your's are, they are not original.
Laminations can lose permeability if dropped, bent, age etc.... Re-annealing is sometimes necessary.

Experience, a ton of patience with steady hands, and skill are necessary to repair these to original factory spec's. (winding the coils is the easy part)

-dmax
 

Murdock

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Thanks for the infos.
I've shipped the two defective transformers back and the seller send me two other ones.
Same thing...
What's weird, is that they were from a spare department of a radio station. Why should they keep defective transformers?

@ Bonnie1: you say they generally don't get bad just sitting on the shelf. But also, that the laminations can loose permeability with age. Could that also maybe be the cause?
Because on the oscilloscope I get low end but it is lower in amplitude as everything over 100Hz.
And I read once a post from CJ, that he couldn't measure anything from a plate choke he wound only with a specific inductance meter. Anything else would give him no readings...
 

rock soderstrom

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Why should they keep defective transformers?
Because they still have value.

The high number of faulty transformers could also have something to do with your measurement method, but I don't think so.

I also believe that transformers can become defective over time. The coating of the enameled wire can become brittle.
 

Murdock

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If there is a broken winding wouldn't I get no low end signal at all?
Or would there still be some kind of coupling?
 

emrr

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capacitive coupling = treble

disconnect one side of a winding on sonething else and listen.
 

MaxDM

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The connections for these transformers are not straightforward at times

You need to be sure that the wires are connected properly
 
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MaxDM

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Open it up and make sure your secondary is wired properly by tracing the wires

Given that all three behave like an open winding, it's likely that you have improper connection


I once sold a pair of trannies from the v76 measuring version, 1:10 and the buyer in Asia took a fit because he said the windings were broken.. he had it wired up incorrectly
 
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Murdock

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capacitive coupling = treble

disconnect one side of a winding on sonething else and listen.

That was a good tip!
I tried a Hammond 812 input transformer which has two secondaries. I disconnected them and send a signal through it. Exactly the same behavior as the broken bv511...
I thought an open winding would give no low end signal at all.

Thanks to all of you!
I will send them back.
Or does anyone know someone who is able to rewind them?
 

MaxDM

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That was a good tip!
I tried a Hammond 812 input transformer which has two secondaries. I disconnected them and send a signal through it. Exactly the same behavior as the broken bv511...
I thought an open winding would give no low end signal at all.

Thanks to all of you!
I will send them back.
Or does anyone know someone who is able to rewind them?
Have you opened them up and checked the connections?
 

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