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Old 29th Mar 2020, 7:41 pm   #1
qualityten
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Default Output transformer resistances - use a digital or analogue meter?

When measuring primary winding resistances between the CT and the screen tap and anode of some push-pull UL output transformers with my Fluke 87 meter, I have sometimes found a higher than expected result, or the digital display has flashed without a result.

But when I do the same measurement with my AVO 8, I get the kind of results I would expect. Is there a reason why digital meters seem less reliable for making these resistance measurements of transformers?
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Old 29th Mar 2020, 7:48 pm   #2
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Default Re: Output transformer resistances - use a digital or analogue meter?

Hi I have noticed this before and put it down to range changing and not enough settling time with the inductive load? Try a fixed range rather than auto and see how it responds.
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Old 29th Mar 2020, 8:07 pm   #3
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Default Re: Output transformer resistances - use a digital or analogue meter?

Just been through my transformer collection with a fluke 179 and an LG DVM and all read ok , that is I cant repeat the non settling display I recal seeing in the past. I keep a lab book but broke the rule "record everything you notice so that you can repeat the experiment"!!
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Old 29th Mar 2020, 8:10 pm   #4
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Default Re: Output transformer resistances - use a digital or analogue meter?

I think it's been reported on the forum previously that some digital meters use what is pulsed DC for resistance measurements. That's fine for a purely resistive component, but won't be happy with an inductive element.

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Old 29th Mar 2020, 8:26 pm   #5
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Default Re: Output transformer resistances - use a digital or analogue meter?

I've found the same, and put it down to the high inductance confusing the autoranging system.

Might be worth connecting a resistor of the expected value to the meter, and 'scoping the voltage across it, to see if it's continuous or pulsed.

One thing you can do of course, is short-circuit another, isolated, winding. That'll kill the inductance.

I would, however, rather trust an analogue meter in this instance. Measure with DMM, and do a ball-park confirmation with Avo.
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Old 29th Mar 2020, 9:37 pm   #6
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Default Re: Output transformer resistances - use a digital or analogue meter?

Thanks for these helpful replies. It is reassuring to understand this issue better. I was measuring two identical Vortexion transformers and thought one one was faulty till I remembered to try the AVO.

I'm so used to using my digital meters, that I didn't think about how they measure inductive loads, nor did I think of trying to use a fixed range.
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Old 30th Mar 2020, 11:08 pm   #7
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Default Re: Output transformer resistances - use a digital or analogue meter?

If they are identical transformers, then they should behave the same. A shorted turn will kill the inductance and may have negligible effect on the resistance.
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Old 31st Mar 2020, 12:02 am   #8
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Default Re: Output transformer resistances - use a digital or analogue meter?

I agree with Bazz as to the methodology of auto ranging DMMs with regard to high value inductance.
Go the trusty AVO meters!
An interesting experiment, if you have the kit, is to try to measure the resistance of a high impedance head set while wearing them.
Try the DMM on the auto-ranging setting and then on a manual setting.
Interesting to actually hear the efforts of the meter trying to get a resistance reading.
Please stay safe everybody.
Cheers.
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Old 31st Mar 2020, 3:49 pm   #9
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Default Re: Output transformer resistances - use a digital or analogue meter?

Quote:
Originally Posted by TrevorG3VLF View Post
If they are identical transformers, then they should behave the same. A shorted turn will kill the inductance and may have negligible effect on the resistance.
Thanks Trevor, but I'm asking what might be the underlying reason why the resistance between the CT and the Anode tap of one of the identical transformers should not be measurable with a digital meter, but measure fine with an analogue one. Would that condition necessarily suggest a short in that winding?
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Old 1st Apr 2020, 3:47 pm   #10
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Default Re: Output transformer resistances - use a digital or analogue meter?

An analogue meter will measure incorrectly if you read it quickly. A 4H choke may take a second or so to settle down. A digital meter may use a very short measuring time.
Meters are available at reasonable cost which can measure inductance. A shorted turn will kill the inductance. The bad transformer is likely the one which measures resistance properly.
You could parallel the two transformers and feed them through a lamp limiter. A shorted turn will allow the light to shine bright and the transformer with the shorted turn will run hotter than the other or put them in series and measure the votage across each. The transformer with the shorted turn will have a much lower voltage.
Where did the transformers come from? Were they discarded because one channel of a stereo amplifier was not working?
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Old 1st Apr 2020, 11:10 pm   #11
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Default Re: Output transformer resistances - use a digital or analogue meter?

That's very helpful info Trevor. I appreciate your advice on how to identify shorted turns. The transformers were bought at different times on eBay. Both sold as 'working'. The one that I'm not sure about was bought first (about 8 years ago), the other last year.

I have an Atlas LCR40 that can measure inductance and will do so tomorrow.
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Old 2nd Apr 2020, 10:07 pm   #12
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Default Re: Output transformer resistances - use a digital or analogue meter?

I've just measured the two transformers in question and one or two other older Partridge ones that I have. (The latter, as expected, have almost matching inductance on both sides.)

Regarding the two Vortexion ones, both have good inductance in the range expected, but the questionable transformer has a higher inductance (9H rather than 7H) on one side. I don't know if this imbalance would significantly affect performance?
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Old 2nd Apr 2020, 11:45 pm   #13
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Default Re: Output transformer resistances - use a digital or analogue meter?

Are these for ultra linear operation with a tap for the screen? Make sure the inductance is measured relative to the centre tap and to the appropriate anode or screen terminal.
How about a picture?

A different inductance means a different turns ratio. This would have an effect on distortion.
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Old 3rd Apr 2020, 8:56 am   #14
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Default Re: Output transformer resistances - use a digital or analogue meter?

Some transformer windings have enough inductance to shock you when disconnecting a resistance measurement. I prefer to use olde worlde passive analogue meters for this task. My AVO 8 is ideal. Electronic meters might or might not be well enough protected so I choose not to find out which.

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Old 3rd Apr 2020, 10:22 am   #15
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Default Re: Output transformer resistances - use a digital or analogue meter?

Quote:
Originally Posted by qualityten View Post
Regarding the two Vortexion ones, both have good inductance in the range expected, but the questionable transformer has a higher inductance (9H rather than 7H) on one side. I don't know if this imbalance would significantly affect performance?
The inductance of an output transformer primary winding has long been generally measured at a voltage level in the range from 5Vrms to about 20Vrms. An AVO 7 on lowest current range can just make that measurement using a 5V heater supply, although it gets marginal for a Partridge for Williamson amp. As the inductance is significantly influenced by the test voltage, it is best to state the test conditions as well as the inductance value.

For class A operation, it is the end-to-end inductance that is of most influence. For class AB, the half winding inductance and leakage inductances become significant.
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Old 3rd Apr 2020, 11:43 am   #16
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Default Re: Output transformer resistances - use a digital or analogue meter?

Digital meters are excellent for measuring voltages (or resistance) where you know roughly what the measurement is going to be, but you need to know exactly what it is, eg for setting up battery chargers, or checking devices against a spec.

For anything unknown, eg when probing faulty equipment or unknown devices, I much prefer a decent passive analogue meter.

For most fault finding, you rarely need the accuracy or resolution of a digital meter. With an analogue meter, you can see out of the corner of your eye when the meter is showing for example 2.8 5V not 2 8 5V. Often, you don't even need to wait for the meter to settle, except for example, to see the difference between base and emitter voltages on circuits such as totem pole output stages.

Then add the robustness of analogue meters, for instance to back EMF as mentioned by David, the predictability (no erratic readings with a failing battery or in the presence of RF), then for me, the default is an analogue meter. And when you get to tuning for a maximum or minimum, then it's no contest.

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Old 3rd Apr 2020, 12:03 pm   #17
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Default Re: Output transformer resistances - use a digital or analogue meter?

The Vortexion transformers are the same as those on this page. https://reverb.com/item/26361849-vor...ut-transformer

Trevor, I am measuring the inductance as you describe in post #13.

David, I did get a tingle when measuring the resistance of a Partridge transformer with my Fluke 87.

trobbins, how would I connect up a 5V heater supply to the primary windings and then measure inductance?

Thanks for all comments and advice. This is a new area for me. I have a new respect for my AVO 8.
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Old 3rd Apr 2020, 8:57 pm   #18
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Default Re: Output transformer resistances - use a digital or analogue meter?

That looks like a mains power transformer with a valve base for the rectifier attached. The wire marked 0V would also indicate this.

Look at the connections to the base and see if they comply with a 5Y3 or 5R4 rectifier.

If all else fails, you could take the sides off and see where the wires enter the spool.
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Old 3rd Apr 2020, 10:52 pm   #19
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Default Re: Output transformer resistances - use a digital or analogue meter?

No, think it is a push-pull output transformer. It has the same markings at the one in this servicing post: https://vintageaudioworkshop.blogspo...-photoset.html. I think the valve base is for selecting secondary impedances, but I agree that it's puzzling. I still haven't found a circuit diagram for this mic-mixer with amplifier.
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Old 4th Apr 2020, 6:06 am   #20
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Default Re: Output transformer resistances - use a digital or analogue meter?

Back in the 1940's an AVO7 on 5mA-AC current range could discern 50uA (minor scale division increment). 5Vac applied across a 100H primary winding (with meter in series) would cause about 160uA (50Hz mains, based on current = voltage / impedance, where impedance is 2.pi.f.L and is substantially inductive ohm as meter and winding ESR is only about 0.5%), or a deflection of three minor divisions, so just discernible although not quite the accuracy one may expect nowadays.

What meter do you have that can measure AC rms current, and do you know the internal impedance of the meter when measuring AC current on a practical range (the AVO7 is about 60 ohm on 5mA range).
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