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Old 17th February 2011, 09:06 PM   #1
Eric W is offline Eric W  United States
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Default Physical Oscillating noise from phono stage

Hi Everyone.

I have an old Harman Kardon Trio tube that sat on a shelf for around 5 years. Everything worked fine at that point. Now there is a physical sound coming from the amp when i switch it over to the phono stage. As in an audible sound the amp itself makes, when not connected to any speaker. It sounds like an oscillating something. It starts at a frequency, then shifts until it reaches a balancing period/frequency and then stays at that level. I'm just wondering where I should start looking. The tubes, an old cap? I'm not even sure what you would call this noise in order to try finding out about it online.

Thanks.
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Old 17th February 2011, 09:22 PM   #2
Eric W is offline Eric W  United States
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I forgot to say that if a test driver is used, then the noise is amplified and the driver is pushed past any decent limit.
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Old 17th February 2011, 09:46 PM   #3
SY is offline SY  United States
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Both channels or just one?

Have the electrolytic filter caps been replaced? Tubes?
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Old 17th February 2011, 10:03 PM   #4
Eric W is offline Eric W  United States
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I believe, but am not positive, that the tubes were replaced about ten years ago. I haven't done a lot of heavy vinyl listening since then. I don't think the caps have been changed. The amp works great in non phono stage setups.

I just noticed that the oscillating lazer like noise either goes away, or becomes much quieter when you connect a driver to the left channel, but continues if you connect the right. the driver then starts to sound like a cheap machine gun toy.
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Old 17th February 2011, 10:37 PM   #5
SY is offline SY  United States
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What test equipment do you have? Voltmeter? Oscilloscope?

The likelihood is a decoupling electrolytic for the phono stage (they should all be replaced anyway). I don't have a schematic handy, but if your phono stage has any tubes which are common to both channels, that would be a suspect as well.
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Old 17th February 2011, 10:53 PM   #6
Eric W is offline Eric W  United States
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I believe a friend of mine has a voltmeter and an oscilloscope. He has a couple solid state amp projects going now, but no experience with tubes. I'll replace the caps and see if that solves the issue.

Thanks

Last edited by Eric W; 17th February 2011 at 11:01 PM.
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Old 17th February 2011, 11:05 PM   #7
SY is offline SY  United States
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Cool, if that doesn't do the trick, come on back and we'll go through it systematically.
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Old 18th February 2011, 01:32 PM   #8
kevinkr is offline kevinkr  United States
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The one thing no one has mentioned is to never run a tube amp without a load either resistors or speakers. Given that it is generating large amplitude LF pulses running unloaded is an excellent way to consign the output transformers to the scrap heap.. (Flyback effect)
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