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Can anybody advise how to measure the specs of an SUT transformer?

H

hififansince1965

Member
A few years ago I bought a pair of moving coil transformers, without an enclosure, RCAs etc. They have sat unused for a long time! They were marked 'EMT T90/3' but I have been in touch with EMT and they don't recognise the code.
So, now I want to put them to use.
They are transformers mounted in a typical steel cylinder, with a set of 6 flying leads:
Blue, White, Red, Yellow, Black and Green.

So, How do I measure the specs safely and come up with a wiring diagram?

I'm not sure if I want to use a multimeter to measure the coils in case it damages the windings.

Can anyone advise me please?
 
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A multimeter will not damage the coils. If you want to be tin foil hat paranoid, put it on a higher resistance range, where it will use a low probe current. So a range that measures to say 200K would be a neurotically cautious choice. On that range, I would expect to see the secondary(ies) show up as a few KOhms, and the primaries to look like dead shorts - but you will be able to sort out what is connected to what. I am guessing that there are some extra windings to allow for different step up ratios.
 
PigletsDad said:
A multimeter will not damage the coils. If you want to be tin foil hat paranoid, put it on a higher resistance range, where it will use a low probe current. So a range that measures to say 200K would be a neurotically cautious choice. On that range, I would expect to see the secondary(ies) show up as a few KOhms, and the primaries to look like dead shorts - but you will be able to sort out what is connected to what. I am guessing that there are some extra windings to allow for different step up ratios.
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Thank you! That's reassuring. I will give it a go in the next couple of weeks and see what I can measure.
 
I have a pair of EMT STX-20s. Red and green are inputs, blue and white are outputs, yellow and black are connected to ground.
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Ok, I bought a multimeter and tried all possible combinations of wires.

This is what I'm getting (checked on both the transformers, which are both close in values):

Red to White reads approx 125 ohms
Blue to Yellow reads approx 105 ohms
Black to Green reads approx 240 ohms

So are the red/white and the Blue/yellow two sets of primaries? And the Black and Green the outputs?

Thank you in advance for any advice, wiring diagrams, clues, etc!
 
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hififansince1965 said:
Ok, I bought a multimeter and tried all possible combinations of wires.

This is what I'm getting (checked on both the transformers):

Red to White reads approx 125 ohms
Blue to Yellow reads approx 105 ohms
Black to Green reads approx 240 ohms

So are the red/white and the Blue/yellow two sets of primaries? And the Black and Green the outputs?

Thank you in advance for any advice, wiring diagrams, clues, etc!
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I think that sounds likely - I am a bit surprised the primary resistance isn't lower, I would have expected just a few ohms. I think the next step would just be to try them, assuming you have a MC cartridge to hand, and see if they work OK.
 
Surely you need impedance values, not DC resistance?

If so, forget measuring anything at home.

Having used quite a few different SUTs, I can assure you that the numbers and manufacturer's claims mean nowt. Only what your ears tell you counts.
 
PigletsDad said:
I think that sounds likely - I am a bit surprised the primary resistance isn't lower, I would have expected just a few ohms. I think the next step would just be to try them, assuming you have a MC cartridge to hand, and see if they work OK.
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Thank you.
Is there an easy way for me to measure / calculate the impedance?
Ok, I'll wire up a few RCA sockets over the next few days and report back.
 
I don't think those numbers make sense (yet). What sort of multimeter did you buy?

Anyway, I would make a big table, measure and record red to everything, white to everything etc. Perhaps that will make things clearer.

It's curious that the colours match the wiring for the STX-20 but nothing else does.
 
Paul R said:
I don't think those numbers make sense (yet). What sort of multimeter did you buy?

Anyway, I would make a big table, measure and record red to everything, white to everything etc. Perhaps that will make things clearer.

It's curious that the colours match the wiring for the STX-20 but nothing else does.
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I'm not myself convinced that it is an EMT transformer. The fact is that somebody used sticky back vinyl to put the term" EMT T90/3" on the side of the casing. So I don't know whether that is relevant or not. I'm sure it won't be original labelling.

It has some of the hallmarks of a Jensen casing, but they haven't responded to my enquiry. The only official identifying mark on the casings is a dot matrix printed number, which is 48.

I have even opened them up this morning, and there is no name on the transformer inside the casing.

But it did confirm that the black and green wires come from one side of the laminations, and all the other wires come from the other side.

I did make a table of the results, and checked it several times. The multimeter is just a common or garden typical £10 one from eBay.
 
I think we'd expect the secondaries to have 10x or more the dc resistance of the primaries, so on the face of it your measurements look odd. And for a step up transformer I'd expect only a single primary. Are the three pairs you've identified isolated from each other? It's intriguing.
 
I wonder if it is some other sort of transformer entirely - might be a balanced to unbalanced converter, for example. The winding resistances would be plausible for say a 600 Ohm system, as used in recording studios, and the two lower resistance windings might be used form a center tapped side, with the other winding being a floating single ended one. That would be the sort of thing that would be used on say the output of a mixing desk.

Trying it with a MC will be by far the easiest test for you. It either works well enough, or it doesn't.
 
There are no other connections or readings between any other combinations of the wires. So maybe it isnt an MC step up after all. The contents of the can are not potted. Its just a small laminated tranny. I’ll take some pics. I’m beginning to think its for a different purpose, though.
 
awkwardbydesign said:
So that's what they look like. I wonder if my EMTs are similar, but I won't be opening them to check!
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As I say, I have no guarantee if they even ARE emt's! Going to rig up some RCA sockets to one this morning and try it in place of my current high quality SUT, which is based on a pair of Hashimoto transformers.
 
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