Measuring the coils of mic input transformer

[API]

This question comes out of something Geoff Tanner wrote over at his Neve forum.
He said that you should not measure the coil  resistance of an audio transformer with a normal multimeter since there was risk that you could magnetize the core.
And if you did, you could "zap it with a high voltage" to de-magnetize it.

I just bought a bunch of St. Ives 10468 transformers that i would like to measure for open windings etc and doing so with my Fluke would be the quickest and easiest way i could think of.

So, is this true, should i stay away with my fluke meter or is there a safe way of doing this?
And if i alreaydy measured some and happened to magnetize it, what voltage should i apply and for how long to de-magnetize it again?

Would love to hear your thoughts on this.

 

[ruffrecords]

Transformers are mode with 'soft iron' cores, in other words types of material that to not exhibit residual magnetism. This means it should not be possible to magnetise the core by passing dc through it for example by using a DVM to measure its resistance. Having said that, even soft iron materials have a small amount of residual magnetism - that's why you have to demagnetise tape heads. And you can demagnetise  a transformer in a similar way. Just feed it a sine wave, raise the level until the core saturates  then slowly reduce the signal to zero.

Cheers

Ian

 

[gyraf]

The old Danish "Jorgen Schou" mu-metal core microphone input transformers came with a specific warning against ohm'ing.

Though I never had any real problems from this.

On the other hand, I once had to disassemble a PV76 output transformer and individually deflux the laminations in order to get it working again. It had had a shorted output transistor in one side. Running signal through did not have any effect at all, secondary shorted or not, that's why the disassembley..

Jakob E.

 

[API]

Thanks for the replies.

So, it seems that its maybe better to not do it then, or do it and de-magnetize afterwards.

Is there any other safe way to check the transformer?

 

[emrr]

I think there are a number of older threads about this worth reading.

 

[dmp]

You could play music through them.  Make a couple custom cables with xlr going to alligator clips.
Listen with headphones.
Or a scope and signal generator would be my first test.

 

[abbey road d enfer]

This question comes out of something Geoff Tanner wrote over at his Neve forum.
He said that you should not measure the coil  resistance of an audio transformer with a normal multimeter since there was risk that you could magnetize the core.

I wouldn't go as far as saying it's an urban legend, but it's not far. There's no risk of magging with a DVM. I wouldn't recomment using an analog VM, though.

And if you did, you could "zap it with a high voltage" to de-magnetize it.

That is outrageous. That could really damage the transformer. You may demag the transformer by applying an LF signal  (20-50 Hz) at nominal level and decreasing progressively the level. Or use a demagnetizer - a contraption that existed in every studio when music was made with magnetic tapes...

I just bought a bunch of St. Ives 10468 transformers that i would like to measure for open windings etc

I don't think there are many "etc" you could measure with a VM; you can only measure the DC resistance of windings, unless your DVM has inductance measurement.

and doing so with my Fluke would be the quickest and easiest way i could think of.

I would not hesitate one second.

And if i alreaydy measured some and happened to magnetize it, what voltage should i apply and for how long to de-magnetize it again?

Demagging takes only a few cycles of sinewave so takes less than 1 second. What is important is to smoothly decrease the signal.

 

[RuudNL]

Why not check the continuity of the windings with an AC signal?
Simply run 'music' through the winding and listen if it plays at the other end.
No risks involved...

 

[CJ]

certain ohm meters can magnetize certain cores,

effect is a diminished bass response,

which meters magnetize the core?

meters which pass enough DC current thru the winding to magnetize the core,

how can i determine which meters magnetize the core?

pick out a resistor that is close to the coil resistance you want to measure,

a mic input pri will be around 10 ohms, sec maybe 50 ohms,

let us say you want to measure pri DCR,

hook a 10 ohm resistor up to your ohm meter, put a DC ammeter in series with the resistor, measure the DC  current, if it is a bench meter, set the ohms range to
200 ohms or lower if you have a lower range,

if your meter passes micro amps thru the resistor, then you will not magnetize the core, if you meter passes milli amps thru  the resistor, you might be magnetizing the core,

if you think you magnetized the core, pass a low frequency sine wave into the primary at a level that will saturate the core, saturation will be reached when the applied voltage levels as you increase the level,

or you can just pass music thru the coil at a good level and the transformer will gradually return to it's normal inductance,

why do you decrease the LF demag signal slowly? because if you disconnect the signal suddenly then there is a good chance that you will not be at the "zero crossing" point of the sine wave,  in other words, disconnecting the sine wave at either peak or on the way towards the zero crossing which will mean that the current will polarize the core once again since the current will be going one way during disconnect,

problems with cores getting magnetized are rare, but it is something to be aware of in case you notice a change in response after using the ohm meter,

usually any residual magnetism goes away just by normal use of the audio gear so no 20 Hz signal generator is required,

 

[API]

CJ, i never got around to thank you for the information and the drawing you made!
Amazing info and just what i was wondering about.

Now i am measuring coils safely and that leads me to my next question.
I looked at your spreadsheet for the Marinair 10468 input transformer and in it you wrote that the DC recistance should be ca 133 ohms for each coil on the secondary.
This is correct for many vintage 10468 and also Carnhills new version, but i have several transformers that have a higher reading which spans between 210 to 270 ohms.
Do you think these transformers are wound differently, or what could affected the DC recistnance.
And would it be possible to use two transformers with different readings side by side in a stereo preamp, or should they be somewhat matched?

Thanks for any input......pun intended...

 

[CJ]

hi nickel lams can screw up digital meters, get a hold of an old Simpson analog meter or remove the lams from the coil, but this requires invasive surgery, are you a licensed MD? FDA approved?  ;D

 

[API]

Great info!
I believe i have an old AVO somewhere, just need to check that its safe to measure with..... 8)