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Old 2nd October 2009, 06:45 PM   #1
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Question Power Transformer Kaput ???

I have Pioneer SM-83 vacuum tube amplifier, and few days ago I have noticed smell of burning oil or whatever else material used for paper impregnation. Pioneer power transformer uses a lot of layers of oil-impregnated paper between layers of winding (this is 120/220V power transformer, not audio output).

Amplifier always run very hot, but I never noticed any smell except stream of hot air.

I measured output voltage, it was normal. However, when idle, amplifier have drawn only 80mA from 220V power outlet (below 18W). With power tubes removed - just 20mA.

I have disconnected all tubes, and run amp "empty", after some time transformer becomes slightly warm, but slight amount of smell was still present. I took infra-red digital thermometer with laser pointer, and noticed interesting thing. Transformer core was only 35C (95F), yet in one area close to the core temperature was 100C (212F)!

If there were shorts between different turns of winding, I assume transformer should draw a lot of power. However, it does not. Could it be arching between layers of winding, or whatever else?

Thanks in advance for any help.
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Old 2nd October 2009, 07:10 PM   #2
disco is offline disco  Netherlands
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Only 17.6 VA when active? Seems low to me.
Look for shorted laminates in the hot spot.
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Old 2nd October 2009, 09:00 PM   #3
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Check the lamination screws holding the sections and lids together haven't perished; one side should have isolation fiber washers to eliminate eddy short circuit currents when screwed to chassis and it's suprising how much current can circulate. I had to do this on my Viceroy amp. Pic given

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Old 4th October 2009, 12:54 AM   #4
kevinkr is offline kevinkr  United States
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I think your primary side ac current measurements are wrong, I would expect the current to be much higher and not to vary more than 25% of so from no power to full power. (Depends on how the amplifier output stages are biased, class A will result in no variation at all, whilst the further the amplifier is operated into class AB the greater the variation will be.)

What you describe sounds a lot like a short in one of the windings - your symptoms closely mimic my experiences when this has happened to me. Have you measured all of the transformer output voltages - this will usually provide a clue. Shorts in the high voltage secondaries are quite common in over stressed power transformers.

All of the above presupposes that you have done the usual things and recapped the amplifier - if not you might want to look at this as well.

An unloaded or lightly loaded transformer will have no blatant hot spots unless there is a short somewhere in the transformer or in the case of a low voltage winding perhaps an external short.
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