Transformer Questions

[Scruffie]

I have little experience with power and output transformers but have 2 at the moment with no datasheets and need to know which connections are which.

First up, I have an old EHX pedal (240V UK version) with the plug missing, it has a blue and brown wire which by the old coding system, brown would be live and blue would be neutral but, just to be safe that the colours here aren't arbitrary though, how do I check before I wire up a new plug?

Second, I have an old radio output transformer, I know the impedances are about 7000:4 but which lugs are which?

Be grateful for any advice for what is potentially a stupidly easy question.

[R.G.]

First up, I have an old EHX pedal (240V UK version) with the plug missing, it has a blue and brown wire which by the old coding system, brown would be live and blue would be neutral but, just to be safe that the colours here aren't arbitrary though, how do I check before I wire up a new plug?

No good way to tell.
Is there a green or green/yellow wire in the cord? If not, the two remaining wires must be treated as though both are "hot", and which is which doesn't really matter, GIVEN that you use your ohmmeter and check for
(1) under 1k ohms between the two when the unit is not plugged in (easy as it is now!) and
(2) infinite resistance to any user - touchable metal when the power switch is turned on.

Second, I have an old radio output transformer, I know the impedances are about 7000:4 but which lugs are which?

Feed it a signal into one winding, then measure the voltages on the other winding. 7000:4 = turns ratio of 42:1. The low-resistance winding is probably the low impedance winding. If you feed 1Vac into that, you ought to get 42Vac at the other side. If you get 1/42V, you guessed wrong.

[Scruffie]

First up, I have an old EHX pedal (240V UK version) with the plug missing, it has a blue and brown wire which by the old coding system, brown would be live and blue would be neutral but, just to be safe that the colours here aren't arbitrary though, how do I check before I wire up a new plug?

No good way to tell.
Is there a green or green/yellow wire in the cord? If not, the two remaining wires must be treated as though both are "hot", and which is which doesn't really matter, GIVEN that you use your ohmmeter and check for
(1) under 1k ohms between the two when the unit is not plugged in (easy as it is now!) and
(2) infinite resistance to any user - touchable metal when the power switch is turned on.

Second, I have an old radio output transformer, I know the impedances are about 7000:4 but which lugs are which?

Feed it a signal into one winding, then measure the voltages on the other winding. 7000:4 = turns ratio of 42:1. The low-resistance winding is probably the low impedance winding. If you feed 1Vac into that, you ought to get 42Vac at the other side. If you get 1/42V, you guessed wrong.

There is the green earth wire yes which is just wired to one of the jacks. I assume those tests are to check the transformer isn't broken, it was known working before the plug was removed so I assume it still is.

Will it matter that on the PCB the transformer is centre tapped but wired up to provide just positive voltage in to a regulation circuit? Just double checking before I blow up a vintage pedal.

Regards the audio transformer, so assuming I do that, how do I work out which is the common? I notice that 2 of the leads have black covering and 2 are bare wire which I assume is some indication.

[Scruffie]

Okay there's 1.4k between the 2 leads, what does that mean?

[PRR]

> radio output transformer, I know the impedances are about 7000:4 but which lugs are which?

Measure resistance. ""Generally"" an audio transformer winding's DC resistance is ""roughly"" 1/10th of its nominal audio impedance. So the 678 Ohm side is about "7K", the "sub 1 Ohm" side is around 4 or 8 Ohms. Inexact, but in this case there's little confusion.

The advice to feed 1V in assumes you have a beefy signal generator. Ideally the un-loaded transformer reflects infinity all ways, and you can measure voltage-ratio effortlessly. In reality transformers are full of losses sucking your test power. A "4 Ohm" winding may never get over 50 Ohms, and may be real-close to 4 Ohms in the bass and far-treble. If you have a 600 Ohm generator, or a sound-card which does not really like <2K (or has a weeny output cap), you may get false readings.

For many tube-folk, the handiest beefy few-Volt source is a 6V filament winding. Even better if it has a CT, so you can get 3V. While the frequency is far below optimum (losses are least around 700Hz, your 3V AC will be 50Hz), the beefy robustness is commendable.

Put that into your presumed 4r winding. *Beware!* If you get it right you will have over 100V at the 7K winding! Use Clip Leads, not fingers!! If you get it wrong the other winding shows <<1V. When you find step-up, get good voltage readings on both sides, do math.

You can NOT tell 7K:4 from 14K:8 or 3.5K:2 without additional clues. Knowing the original tube-type is a major help (OTs and PTs should never be orphaned without original usage notes!). Cross-check against DC resistances will cast doubt on some possibilities. If the apparent "2r" winding has 0.8 Ohms DC resistance, loss would be too high to be a likely design. (However standard ohm-meters don't read sub-Ohms well.) While frequency response runs may be helpful, you don't really know what the design numbers were, and may care much.

> 2 of the leads have black covering and 2 are bare wire which I assume is some indication.

AH!! Solved. The plate ("7K") leads run 250V and should be insulated. The speaker leads are 4V or 5V and do not require insulation. The "bare" leads are surely the actual winding (enameled wire) simply brought-out. The primary winding is much finer wire, and is internally spliced to the insulated leads for safety and robustness.

> 1.4k between the 2 leads

Ball-park we would guess "~~14K".

However 5K to 7K are FAR more common values for single table-radio tube loading. 1.4K DC resistance on a "7K" winding is 20% loss, high-ish, but SE needs more turns and anyway a radio designer accepts much loss of potential power to save a penny.

[Scruffie]

Great! Thanks, got it all sorted now and it did have the original usage notes with it (ECL86 to 3ohm speaker), just, no indication of connections.

I should have thought of that with the insulation, but the 4 lugs are in a row so I was assuming they'd just be in pairs, one black lead per common rather than the 2 outer being the primary and inner being the secondary.

The 1.4k was in reference to the power transformer on the EHX pedal that R.G asked me to check, not the output transformer so still need an answer on that one.

[Scruffie]

Oh and it turned out the insulated wire taps were actually the secondary! Checked with multimeter (368R to 3R) and tested at low voltage on an amp.

[Scruffie]

Can anyone offer verification on the Power transformer questions?

To reiterate, it's a vintage EHX 240V to centre tapped (12,15V?) transformer that uses both sides of the secondary to feed a 15V regulator circuit, there is a 1.4k reading between the 2 power leads, it has the earth wire attached to one of the jacks.

Does it matter which way I wire the 2 power leads to the plug (I don't want to feed a vintage pedal reverse polarity and kill it!) and if so, how do I work out which is hot and which is neutral and why was I measuring for less than 1k between the 2 leads?

[GibsonGM]

Reading a low resistance off a PT will tell you that you have the leads to THAT coil in hand.  R.G. may have been looking to see which coil you had - primary or secondary (guessing).      If you measured and got *no connection*, you would then know "these wires here have no continuity - they are not what I am looking for".

To find the hot and neutral, you - using clip leads as Paul said!!!!!  - measure AC voltage between the GROUND (Earth to you) and one of the wires.  The one that has voltage on it is your hot.  The other is neutral.  IF your wall outlet is installed correctly  

Can you post a schematic of your specific pedal, bud?  Or even the name if you can't find a schematic.  That would help....IS the center tap used (earthed)?    As AC changes polarity 50/60 times/second, you won't do a reverse polarity thing, but I would like to see how this is wired before giving you advice - it's old, we don't want to fry it....

[Scruffie]

Whoops, did he, I thought he was only referring to the radio transformer, should have read more carefully 

It's for my polyphase, the schematic may not be entirely correct (there's about 4 versions about and none are 100% I haven't confirmed my power section to any of them) but it should be close;



Pretty sure my wall outlet is wired correctly

[GibsonGM]

As long as you set up the center tap properly, which is going to earth and establishing reference for the 'top' and 'bottom' of the secondary, you are good to go.    The secondary center tap can be found (or you can 'be assured of which one it is') by measuring the resistance of the secondary wires, from center to either side.   When you have the center tap, the resistance to each of the other 2 wires should be nearly the same.    Additionally, using extreme caution and wire clips, you can measure AC voltage from what you think is the CT to each outer wire, and find that it's just about the same.   If not, you are not right about which is center tap....

Doesn't matter which way you connect the hot and neutral of the secondary to the 1N4001 diode rectifiers.   It's the CT that MUST be earthed back to its original location, with a well-soldered and solid connection (of course, all those connections should be well done, ha ha).         

Be sure your primary has the right fuse in line (don't see one in the schematic...) and be careful. 

[Scruffie]

The secondary on the transformer is already wired to the board as it was originally so I don't have to worry about that it's only the primary i'm concerned with.

Ah yes, that was my next question, what fuse value would be suitable, the original plug didn't contain one or have the earth wire connected, hence the swap over... I assume a standard 3A should suffice.

[Seljer]

3A will be better than no fuse at all but I expect the draw on the AC side for that circuit probably isn't more than 100mA.

[Scruffie]

3A will be better than no fuse at all but I expect the draw on the AC side for that circuit probably isn't more than 100mA.

I don't think it's even that much, about 40mA IIRC but I should have some 3As around as they're fairly common, if I have lower i'll use it.

[GibsonGM]

Actually, you want one that is close to the expected draw (of course, plus a little more).  If your circuit draws 40mA, and you allow it to draw 3A via a short or what have you, it's likely going to be smoked.   100mA would be a good fuse in that case.   I *believe* you are supposed to pick the fuse by going over rated current by 125%. 

For now, of course toss the 3A in there for safety, but find one more in line with what *should* be there.


If you're talking about just wiring in the primary, sorry!  Thought you had to do the whole transformer.   It's as simple as the secondary....the Earth wire of your cord MUST go to your chassis earth connection, of course.  Solid, start-washer type connection - most important connection in that whole circuit.  I would actually ADD a hole/bolt/locking star washer/nut and a lug on the ground wire to make sure this connection is true.    I know they didn't do that back then, and it's vintage, but it is safer this way.  If the jack came loose, no safety ground...
     
Doesn't matter which trafo wire you connect to which mains wire...as long as you connect the power to the coil and return it thru the other end of the coil (see schematic...) then you are ok.    This was the reason to test it with an ohmmeter....should have SOME (low) resistance there, indicating continuity.   There's no "polarity requirement" in this type of circuit...

Stand back when you plug it in    

[Scruffie]

She lives! As do I!

Earth wire is still on the jack but i'll get a washer and wire it to one of the transformer nuts and bolts in the future (don't want to make new holes in it!) anything is better than the no earth it had when I got it though.

Thanks for all the help everyone

[GibsonGM]

You're welcome, glad it worked!  For the ground, be sure to test that ground wire for continuity with the enclosure when you're done. 

"They" really don't recommend using a nut/bolt holding something else - a separate dedicated connection is desired - but that will be much much better than grounding to a jack!  Use a wire lug if you do that.    Remember, if the hot ever touches that enclosure (broken wire or something), the ground is the only thing between you and potential death...your call      

[Scruffie]

You're welcome, glad it worked!  For the ground, be sure to test that ground wire for continuity with the enclosure when you're done. 

"They" really don't recommend using a nut/bolt holding something else - a separate dedicated connection is desired - but that will be much much better than grounding to a jack!  Use a wire lug if you do that.    Remember, if the hot ever touches that enclosure (broken wire or something), the ground is the only thing between you and potential death...your call      

Tested for continuity before I boxed it back up and from the plug before turning on

Yeah i'm well aware of that, the power switch that the hot is connected to and soldering is 35 years old, it ain't great looking. Main thing was getting it going again and knowing I wasn't going to kill it, tidying up comes later