Testing for shorted turns in tube output transformer

[Dreams]

I've been thinking about this and researching it, and I have yet to come up with any viable test to tell me whether or not a transformer has shorted turns. So I came up with this method, and I'd like to see if it makes any sense to anyone else

My test rig is as follows:

120v Mains AC ---> Lightbulb Limiter ---> Variac ---> Transformer Primary ---> Unloaded Secondary

My thinking is as follows:

If there are any shorted turns in the secondary, they will act like a load (?) and cause current to flow in the primary, and the lamp will light.

I suppose I'd have to swap the Pri and Sec to test for shorted winding in the Pri, because, as set up, a shorted Pri winding would only mess with the turns ratio? (Since it's an output transformer, I probably wouldn't put more than 20v on the Sec because I'm not trying to light a neon sign {or kill myself})

Anyway I have more questions than anything at this point, so let me know if I should just shut it down right now, etc.

Thanks

 

[CJ]

that method probably will not work for an audio output,

it would work for a big industrial power transformer where they have big wire and a lot of current, and a core big enough to provide enough linkage to get a lot of current flowing in the shorted turn, usually the transformer comes back with an open winding, after you tear it apart you will find one char broiled turn that conducted a lot of current then burned open,

you might try a leakage inductance test, of it is a push pull OPT, then the side with the shorted turns will have more leakage, talkin bout the primary here, do you have an inductance meter?  then cli[p  it to the sec and short each half of the pri, they should have about the same leakage, if not, you may have some shorted turns, is it a Dagnal as used in a Marshall by any chance?

 

[mjrippe]

Here's a neat method from R. G. Keen: http://www.premierguitar.com/articles/the-super-secret-transformer-tester-1

I have used this one on an Ampeg B15 output transformer in the past.

 

[Dreams]

I'm talking about a few transformers, really, but no, no Dagnals. For the sake of being clear, I'll focus on an OT from an Ampeg V4.

I've done the neon lamp test on this OT, and it seems to pass. On the Primary.
When I do it on the Sec, no-go, but I wonder if that's because the flyback voltage is not high enough to light the neon?

How likely is a shorted secondary turn anyway? The voltage is way lower than what develops across the primary, right?
CJ, is that why the leakage inductance test you suggested I do involves only the primary?

Speaking of which, I do not have an inductance meter.  But I did set up a circuit where a Cap is in parallel to one side of the primary so that I could calculate the inductance of each side. I could type out what I did if it seems important, but I came up with:

23.5 H for one side
and
21 H on the other

I don't know what those numbers mean.

How much inductance SHOULD each side have, or am I just trying to see that they match pretty well?

 

[joaquins]

As CJ said, probably the leakage inductance is a good way to test it, measuring inductance at the primary with the secondary open and shorted. If there are shorted turns inside the transformer it won't change much, if there are it will change a lot. The actual values and relationship will vary with each transformer but I would expect at least 2 orders of magnitude in a good transformer, less than 1 order of magnitude in one with shorted turns.

JS

 

[MicDaddy]

If there are shorted turns inside the transformer it won't change much, if there are it will change a lot.

Should it read, "..If there are shorted turns inside the transformer it won't change much, if NOT (no shorted turns) it will change a lot"  ??

 

[joaquins]

If there are shorted turns inside the transformer it won't change much, if there are it will change a lot.

Should it read, "..If there are shorted turns inside the transformer it won't change much, if NOT (no shorted turns) it will change a lot"  ??

That's what I meant, you're right...

JS

 

[MicDaddy]

If there are shorted turns inside the transformer it won't change much, if there are it will change a lot.

Should it read, "..If there are shorted turns inside the transformer it won't change much, if NOT (no shorted turns) it will change a lot"  ??

That's what I meant, you're right...

JS

;D you were right, I'm only trying to follow along and keep up.