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transformer burning up

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Newbie level 4
Aug 1, 2009
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I bought a 9-0-9 vac transformer to replace the original for a vintage Ashly 1/2-space noise gate (might have been from someone on this list, I don't recall). The gate was working again, but the transformer is getting Chernobyl hot and smoking a bit, which describes me sometimes, but is probably not good for a transformer.

The original was for 600mA current; this one is over 1 Amp so it can't be trying to draw too much.

As you can see in the picture the secondaries are red, blue and yellow.

on the side of the tranny it says: pri 120v WH-BLK. Sec 18vct 18VA Red - blue. Center tap yellow.
The primaries COULD be poloarized white and black but the wall socket plug is not polarized so I doubt it matters on the input side.

Because of the whacky-ass plug on the low voltage side (they use either a phone connector or a Cat 5 connector, not sure which; certainly nothing standard like a barrel.) the original transformer had 3 secondary taps OK, but each had two colored wires leading to the device (all PSU components other than the transformer are on the gate circuit board, which is why I was surprised it kept on working without blowing, smoking or melting). Black and yellow are 0 volts, so I connected those to the Yellow secondary.

Blue/white and Red/green are the 9 volt taps and I connected them to the blue and red secondaries respectively. That must surely be correct, although it does not say on the transformer case which might be +9 and which -9 volts.

Any ideas please? Maybe the transformer itself is just blown/shorted or however they die, but although I can measure voltage, that's about all the test gear I have.

Your picture isn't clear enough to identify what is primary and secondary windings on the transformer.

I think you might have exchanged primary and secondary windings, and that will cause the transformer to burn.

The promaries are black and white and you can't see them because the plug is not polarized, so it doesn't matter how the primaries are wired. That's why I just showed the secondaries.

However, because they are black and white leads on the primary side, you have made me wonder if it SHOULD have a polarized plug (in the USA you can put an unpolarized two pin plug into a polarized 3-pin socket with no problem. I doubt you can do that anywhere in Europe). Usually black is "hot" and white is neutral.
I will get hold of a polarized plug and try wiring it correctly and report back.

Added after 55 minutes:


Well, I put a polarized plug on it, but it didn't seem to make any difference; it's still too hot to touch for more than a second or so. Again, the device is still working, so I can only assume the transformer itself is messed up.

I can't find a supplier for one in the USA, incredible as it seems. My usual suspects: Digikey, Mouser and ApexJr do not have one. Any help locating a 9-0-9 supply would be gratefully received.

Added after 49 minutes:

I believe I have found the correct part at Jameco in the USA.
Part 105532 would appear to be correct.
I can't "confuse" the primary and secondary wires because of their color. Besides, had I wired them completely reversed I would have had a strange voltage that I'm sure would have taken out the electronics, which it hasn't.

If you had transposed pri and sec it would have blown a fuse somewhere (the sec wouldn't cope with the high voltage).

The primary is not polarised. It doesn't matter which way round it is connected.

You don't say why you changed the tranny in the first place. If it was because the original had failed, maybe there is something wrong with the load that is drawing too much current (while still appearing to function correctly). You say you can measure voltage so try measuring sec voltage when disconnected. You should get maybe 12VAC between red and yellow and the same betwen blue and yellow. Transformers are rated at full load, i.e. these voltages should drop down to 9VAC when the load is 1A (your spec. is 18VA for an 18V winding).

Centre-tapped secs are usually configured to give full-wave rectification (yellow goes to 0V, blue and red each go to a diode. Outputs of diodes are connected together and are +VE (probably 12VDC after a smoothing capacitor does its bit). If it is wired like this, check the diodes to see whether one has shorted; that could give the symptom you describe.


For the next deciding I wuld propose for you to measure 3 resistances on your transformer.
A primary has surely higher values as booth seconder! The both similar and clear lower resistance values referenced to a "middle" wire gives you the system of secs wireing.
Colors are_as I belief_ no matter, then from another producer is possible to become a tranny in some other coloring...

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