Unequal resistance on output transformer primary

[bobloblaws]

Neither of those readings can be considered "good" or "normal". They either indicate a short in the primary or very bad quality output transformer.
The low output mentioned in the first post combined with those crappy reading indicates a short in the primary which means loss of primary inductance which and as a result the low output. The easiest way to check is to measure the primary inductance - end to end and from CT.

As a matter of fact, I just fixed a problem in the pre-amp and I'm getting a lot more output now, but not necessarily full output expected for 2 EL-84's, it's hard to tell just from listening, and I haven't gotten around to any of the tests already suggested. How can I measure inductance?

[Enzo]

May I humbly suggest do the simple things first before measuring inductance.

[R.G.]

it's hard to tell just from listening, and I haven't gotten around to any of the tests already suggested.

Um... yes, it is indeed hard to tell just by listening. That is ...why... those tests were suggested to you, to let you have some sort of measurable way to distinguish between busted and not busted.

How can I measure inductance?

Two ways. In the time domain, V = L * di/dt, or L = V(t)*dt/di. In the frequency domain, Xl = 2*pi*F*L. So L = Xl/2*pi*F. All inductor measurers work on one or the other of these two principles.

But as you're now thinking, how the devil do I actually do one of those? There are many, many ways, and some of them are in the tests you didn't get around to yet. Quite a number of the test processes involve buying a test instrument, probably not what you were looking to do. An oscilloscope can tell you pretty directly, but you have to know how to use it and set up the test. The neon bulb test is actually a quick and very dirty way of measuring the energy stored in an inductor, using the V = L*di/dt test.

It's probably quicker to do some of the suggested tests than to spend hours foraging through the internet or questioning a dozen or so technical forums.

[Gregg]

Next time when buying a DMM I would suggest to anyone to consider one that can measure inductance as well. Very handy in situations like this.
No, they are not expensive - even some cheap chinese ones for 40-50$ can do that.

[Mick Bailey]

Neither of those readings can be considered "good" or "normal". They either indicate a short in the primary or very bad quality output transformer.
The low output mentioned in the first post combined with those crappy reading indicates a short in the primary which means loss of primary inductance which and as a result the low output. The easiest way to check is to measure the primary inductance - end to end and from CT.

A short in the primary won't show higher resistance with a DMM. A major short that involved a large section of the winding would much more likely show a lower resistance. A higher than expected resistance doesn't immediately say shorted turn to me, though it does suggest some problem that needs investigating. Sometimes a spot develops which can give a compound fault - a high resistance area that breaks down to a short under operating conditions.

Measuring inductance is OK, but I have a transformer on my bench right now from a JTM45 that measures fine at the voltage that a digital inductance meter uses to test. I happen to know it has a shorted turn, but this doesn't show up until the transformer is stressed in the amp. There are numerous fault scenarios with transformers, and no single test will necessarily identify all faults. Testing inductance is like testing capacitance - it's a static value at a low voltage and can't be relied upon to validate a componnet. In many ways the neon test is a much more reliable method, as the flyback voltage is much higher.

[Gregg]

A short in the primary won't show higher resistance with a DMM.

No it will show lower DCR. I'm not aware how a primary can develop higher resistance but I don't know everything. Either way the OT is toast unless it's something special which will be worth the effort to investigate further and maybe attempt a repair which in most cases is not possible.
There's also the scenario when both halves measured DCR is OK but there's a short to the secondary and again no inductance.
A short always "shows up" after taking few simple measurements. What you described is very unusual case. I've seen intermittent short though if that's what you meant.

[g1]

If one of the connections to the actual winding is iffy, you will have a high resistance reading. In this case the transformer itself is fine and just needs a connection re-done. Sometimes accessible, sometimes not. In any case not always 'toast'.
Mick explained the scenario and potential simple fix right off in post #2.

[Mick Bailey]

No it will show lower DCR.

It will, but not be detectable with a DMM. A single turn has a resistance that is so low that whether it's shorted or not will not make a scrap of difference outside of lab conditions. What would be the comparator - how would you know whether a reading indicates a short or not? You would have to take a precision resistance reading at a stabilized temperature when that specific transformer was 'good', to know when it was 'bad'.

Exactly the reason to use the neon test method, or a ring tester.

[SoulFetish]

It will, but not be detectable with a DMM. A single turn has a resistance that is so low that whether it's shorted or not will not make a scrap of difference outside of lab conditions. What would be the comparator - how would you know whether a reading indicates a short or not? You would have to take a precision resistance reading at a stabilized temperature when that specific transformer was 'good', to know when it was 'bad'.

Exactly the reason to use the neon test method, or a ring tester.

If you are inclined, I would like to know more about the ring tester you built.

[Gregg]

A single turn has a resistance that is so low that whether it's shorted or not will not make a scrap of difference outside of lab conditions.

You're right but USUALLY it's not just a single or a couple of turns shorted.

[J M Fahey]

You're right but USUALLY it's not just a single or a couple of turns shorted.

Not disagreeng but thinking for a short to develop, we need copper to copper contact (through perforated/burnt insulation) so it´s either one turn touching the one right by it, same layer, or on next layer above or below.
Single turn short will cause a negligible resistance change; layer to layer might mean 5% or 10% .
Even a single shorted turn is enough to kill transformer magnetic field, both killing power transmission (what we notice and makes us test it) and inductance.
So confirming massive inductance loss (the neon bulb test is simple and precise), in the same act we are confirming that transformer fails to properly couple plates to speaker.

Is multilayer failure possible?
Unfortunately yes.

[Mick Bailey]

You're right but USUALLY it's not just a single or a couple of turns shorted.

Even if the winding resistance was compromised by a few percent, you have no way to determine whether the resistance you're reading is good or bad. Given that most OT windings are not bifilar or comprising separate identical bobbins for the primary, then a considerable difference is to be expected. So, say you have a transformer that reads 38 Ohms one side, and 42 Ohms the other (so a roughly 10% difference), is that good or bad, in your opinion?

I've made a habit of carrying out a post-mortem on every failed OT for a number of years to better understand the failure modes. I can say that out of the ones that were shorted, the fault was usually hardly discernable and had localized damage involving adjacent turns. Some that had serious damage and were shorted had obvious signs of burning through a few layers, but even so the difference in DCR between the two halves of the primary was not remarkable and could not have been an aid to diagnosis.

A common complaint here is that people post to say "The OT primary reads X Ohms one side and Y the other, so is it bad?" The answer is always that you cannot use DCR to determine whether a transformer is faulty, and this is difference is entirely normal.

I'll post some pics of my test unit in operation when I get chance, and a link to the PCB artwork.

[Gregg]

Even if the winding resistance was compromised by a few percent, you have no way to determine whether the resistance you're reading is good or bad.

You're generally right about the resistance that's why I do inductance measuring.

So, say you have a transformer that reads 38 Ohms one side, and 42 Ohms the other (so a roughly 10% difference), is that good or bad, in your opinion?

The answer is: depends on the winding configuration of that particular OT. If for example it's a 1/4+1/2+1/4 primary then equal DCRs are expected and that reading is bad. If it's a simple 1/2+1/2 primary then such reading can be expected.
In both cases the inductance of the primaries halves MUST be equal though. If you have a short in one of the halves the readings will be different (except may for the exception when only one turn is shorted) .
In any case it's always better to do DCR and inductance measurements which will provide you with more information for the head scratching that could follow

[nosaj]

Even if the winding resistance was compromised by a few percent, you have no way to determine whether the resistance you're reading is good or bad. Given that most OT windings are not bifilar or comprising separate identical bobbins for the primary, then a considerable difference is to be expected. So, say you have a transformer that reads 38 Ohms one side, and 42 Ohms the other (so a roughly 10% difference), is that good or bad, in your opinion?

I've made a habit of carrying out a post-mortem on every failed OT for a number of years to better understand the failure modes. I can say that out of the ones that were shorted, the fault was usually hardly discernable and had localized damage involving adjacent turns. Some that had serious damage and were shorted had obvious signs of burning through a few layers, but even so the difference in DCR between the two halves of the primary was not remarkable and could not have been an aid to diagnosis.

A common complaint here is that people post to say "The OT primary reads X Ohms one side and Y the other, so is it bad?" The answer is always that you cannot use DCR to determine whether a transformer is faulty, and this is difference is entirely normal.

I'll post some pics of my test unit in operation when I get chance, and a link to the PCB artwork.

Anxiously awaiting this. Also I think you had the ring tester that was modded(could you post the mods also?)
Thanks,
nosaj

[nosaj]

Next time when buying a DMM I would suggest to anyone to consider one that can measure inductance as well. Very handy in situations like this.
No, they are not expensive - even some cheap chinese ones for 40-50$ can do that.

I picked up a DER 5000 for about 60. Not a DMM at all just LCR.
Been happy with it.
nosaj