Power transformer primary is open.

Cassiel · 2nd December 2011, 01:22 AM

Winding is open somewhere between the 125V tap and the 150V tap. I have bridged both taps and secondary voltages seem OK. It does buzz a little, very little....I have to bend down and get my ear close to it to hear the buzz. That may be normal - transformer is old. It doesn't overheat (unloaded). Primary sees 230V (240 tap). Should I worry? Will it fail on me?

aardvarkash10 · 2nd December 2011, 02:44 AM

the buzz may indicate a shorted winding. I'd take a cautious approach and rebuild or ditch it... Fires are expensive.

Cassiel · 2nd December 2011, 02:51 AM

I don't know how some PT's work, I mean I don't understand how the primary can work with all those voltages with only one winding. Am I right if I assume that the 240V tab now it is a 215V one?

Quote:

I'd take a cautious approach

Yeah, that's why I'm asking, to be on the safe side. Rebuild it it's not an option.

cerrem · 03:11 AM

Quote:

Originally Posted by Cassiel

Winding is open somewhere between the 125V tap and the 150V tap. I have bridged both taps and secondary voltages seem OK. It does buzz a little, very little....I have to bend down and get my ear close to it to hear the buzz. That may be normal - transformer is old. It doesn't overheat (unloaded). Primary sees 230V (240 tap). Should I worry? Will it fail on me?

Most likely it can be fixed.....The most likely place for the break is in the lead outs from the windings....there will be 2 wires coming out at the 125 tap, the 110 finish and the 125 Start, based on your description it would be the 125 Start lead that is broken.... Or it will be at the finish lead of the 125 where it joins the 150V start lead...
You can look into the side of the paper sections to see if you find this break...you may get lucky and be able to repair it...

Chris

Cassiel · 2nd December 2011, 03:21 AM

Yes, I did that before posting. No luck - no access to the break point.

payitforwardeddie · 2nd December 2011, 12:18 PM

Quote:

Originally Posted by Cassiel

I don't know how some PT's work, I mean I don't understand how the primary can work with all those voltages with only one winding. Am I right if I assume that the 240V tab now it is a 215V one?.

Yes. The slight buzz may be caused by having 230v on what is now a 215v primary. The magnetic core may be saturating on the peak voltage. Do you have access to an oscilloscope? If you do, check the waveform on the secondary winding with a light load. If the sinewave is flat topped then you need to reduce the input voltage somehow to 215v. A wirewound resistor will do the trick. The value depends on the load to the transformer. Once done, it'll be fine.

Cassiel · 2nd December 2011, 12:20 PM

payitforwardeddie - Thank you very much.

payitforwardeddie · 2nd December 2011, 12:26 PM

Quote:

Originally Posted by Cassiel

payitforwardeddie - Thank you very much.

My pleasure. That's what we're all here for.

2nd December 2011, 02:44 AM	#2
aardvarkash10 diyAudio Member Join Date: Mar 2007 Location: Auckland, NZ	the buzz may indicate a shorted winding. I'd take a cautious approach and rebuild or ditch it... Fires are expensive. __________________ Yes, conservatism thrives on low intelligence and poor information. But the liberals in politics... continue to back off, yielding to the supremacy of the stupid. It's turkeys all the way down. - George Monbiot, guardian.co.uk, 6 Feb 2012

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2nd December 2011, 03:21 AM	#5
Cassiel diyAudio Member Join Date: Sep 2004 Location: Madrid	Yes, I did that before posting. No luck - no access to the break point.

2nd December 2011, 12:20 PM	#7
Cassiel diyAudio Member Join Date: Sep 2004 Location: Madrid	payitforwardeddie - Thank you very much.

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