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Old 10th September 2006, 04:32 PM   #1
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Default shorted turns in primary?

I have a Onkyo amp that the filters vent. I replaced them once and the replacements vented too. So this time, I measure the voltage. It's +/- 45 Volts. The caps working voltage is 35v!! Okay, so they're venting for a reason

The original caps were 35VDC rated.
The power transformer has a 2-wire primary.
The back of the set say's "US 120VAC 60Hz".

I've checked the rectifiers, they're okay.

The only thing that I can think of is that the transformer is for Japan market (100V), or that the transformer primary has shorted in a way to give a different turns ratio. ...A turns ratio that produces greater secondary voltage. I haven't seen this before. Is there something else that could cause the large secondary voltage? Has anybody seen this type of failure?
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Old 10th September 2006, 11:54 PM   #2
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Default AC line

Have you measured your AC line coming from the wall? Or has someone brought this amp to the US from another Country?

Shawn.
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Old 11th September 2006, 12:06 AM   #3
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Quote:
Have you measured your AC line coming from the wall? Or has someone brought this amp to the US from another Country?
Yes. I've got the AC feeding the unit going through a VariAc with a voltmeter on the output. I've also got a current meter in series with the load.

It looks like the unit was setup only for the US market. There's no indication that the primary can be wired in any other way (2-wire, no taps or jumpers). It doesn't look like the power transformer has ever been replaced.

I don't think it's a Japanese market transformer put into a US unit. I found while measuring the DC voltage on one of the filters, that only 58 VAC in turns into a DC output of 35VDC, ...the working voltage of the cap. So something is really messed up in this picture. I think I'm gonna use an isolation transformer on this one. Something is really weird.
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Old 11th September 2006, 12:16 AM   #4
clem_o is offline clem_o  Philippines
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Any shorted turn would cause the trafo to draw huge amounts of current - either the trafo gets REALLY hot or the fuse blows. Since that doesn't seem to be what's happening, must be something else...


Could the thing somehow turned into a voltage doubler circuit...


edit: otoh it could be that the short is from one turn to some other point "far' away. That would make sense...
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Old 11th September 2006, 12:41 AM   #5
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Disconnect the XFMR AC output from the amp. Isolate the transformer. Measure it (ohms and volts). Do you have a voltmeter on your variac? Is the variac telling the truth? Was the amp brought to you broken this way? I have never heard of a damaged power supply delivering more voltage when it is broken, it kinda goes the other way, ya know. You sure do have something weird there.

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Old 11th September 2006, 12:43 AM   #6
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Originally posted by TomWaits
...I have never heard of a damaged power supply delivering more voltage when it is broken, it kinda goes the other way.

Shawn.

Well that is not entirely true but for a simple XFMR/Bridge/Filter type power supply, I have never heard of your problem.
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Old 11th September 2006, 01:52 AM   #7
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Quote:
Disconnect the XFMR AC output from the amp. Isolate the transformer. Measure it (ohms and volts). Do you have a voltmeter on your variac? Is the variac telling the truth? Was the amp brought to you broken this way? I have never heard of a damaged power supply delivering more voltage when it is broken, it kinda goes the other way, ya know. You sure do have something weird there.
Yes,a good to test a XFMR,since there aint any blow fuse symptom.
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Old 11th September 2006, 01:58 AM   #8
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Did you measure both output voltage ( plus & minus ) in a standard configuration they should remain equal if they are unequal you may have a ground or a center tap trouble
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Old 11th September 2006, 02:57 AM   #9
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Quote:
Originally posted by audiofan
Did you measure both output voltage ( plus & minus ) in a standard configuration they should remain equal if they are unequal you may have a ground or a center tap trouble
Yes, do all the measurements you can, with power, without. XFMR connected XFMR disconnected. Give us the scoop.

Cheers,

Shawn.
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Old 11th September 2006, 03:02 AM   #10
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Has the unit accidentally been rewired incorrectly by someone?
A simple power supply will have a XFMR like yours? say 20-0-20 VAC coming out but if wired wrong, you could drive 40-0 VAC into the rectifier and then reach the caps with a big DC voltage? I'm speculating of course. Perhaps someone else messed around with it before it reached you?

Shawn.
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