Identifying a Transformer

[sonolink]

Hi
I need a little help here:

I got an 18-0-18 @30va BUT it's an old TX and 2 of the secondary cables are discolored. On the TX the secondary windings described are RED-YELLOW and BLACK-WHITE. I can only identify RED and BLACK. The other 2 are discolored. Let's call them D1 and D2.

I connected the TX to mains and read voltages across

Red - Black : 18.8 vac
Red - D1: 21 vac
Red - D2: 0.6 vac
Black - D1: 1.5 vac
Black - D2: 21 vac
D1-D2: 21 vac

Red-D1: continuity
Black-D2: continuity

Am I right to think that the windings are Red-D1 and Black-D2? Shouldn't I get 18 across each? Or that's after rectifying?

Finally, if I join D1 and Black I should get 36 vac across Red and D2 (or across Black and D1 if I join Red and D2), correct?

Thanks for your time and help
Sono

 

[Whoops]

Red-D1: continuity
Black-D2: continuity

Am I right to think that the windings are Red-D1 and Black-D2? Shouldn't I get 18 across each? Or that's after rectifying?

transformers have 15% to 20% higher value tolerance, so 21v for a transformer rated for 18V seems correct

After rectification is 21VAC x 1.414 = 29.69 VDC

If you have continuity between Red-D1 and Black-D2 and you have the right voltages between them 21 VAC it's safe to say those are your secondaries

 

[sonolink]

Thanks for your quick reply Whoops

So I should be OK wiring them in serial to get 36 vac joining D1 and Black for instance ?

Thanks !

 

[Whoops]

Thanks for your quick reply Whoops

So I should be OK wiring them in serial to get 36 vac joining D1 and Black for instance ?

Thanks !

Not 36VAC,
if you wire them in series you will get 42 VAC

Do it, I'm not sure about the polarity though, someone has to chime in on that

 

[sonolink]

You're right. 42 vac

I don't think there's polarity being a Transformer (AC). My concern was to properly identify the windings to avoid shorting anything.

Thanks a lot for your help!
Cheers
Sono

 

[Whoops]

I don't think there's polarity being a Transformer (AC).

Please read on alternating current:
https://en.wikipedia.org/wiki/Alternating_current

Audio when captured with a microphone is AC voltage. Dont you have a polarity/phase reverse button on the console input or mic pre or daw?

I guess if you want for example to create positive and negative DC voltages you would need to know the phase of the transformer secondaries.

 

[sonolink]

Oh I see what you mean and you're right about the phase problem. I was thinking negative polarity as 0v in DC. My bad

Well I think I'll have to wire it to the bipolar PSU and check what DC comes out.

I guess at worse I'll get the wrong polarity and will have to reverse wires right?

Cheers
Sono

 

[Whoops]

Oh I see what you mean and you're right about the phase problem. I was thinking negative polarity as 0v in DC. My bad

Well I think I'll have to wire it to the bipolar PSU and check what DC comes out.

I guess at worse I'll get the wrong polarity and will have to reverse wires right?

Cheers
Sono

do you have a schematic of the PSU?

 

[sonolink]

Yep!

http://www.expataudio.com/diy/uberpsu/Expat%20Audio%20Uber-Power%20Supply.pdf