White noise in preamplifier

Discussion in 'General Electronics Chat' started by aGpLT, Aug 23, 2010.

  1. aGpLT

    Thread Starter Member

    Jan 21, 2010
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  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Is it really noisy, or just that old "transistor" sound (background hiss)? It may take an oscilloscope and some time to run back from the output and see if you can find a clean signal.

    One source of white noise in a transistor circuit is a reverse biased junction. Look for the white noise generator in the link to see one - http://www.electricalfun.com/workbenchfun/Unusual_Components_TThomas.aspx

    Troubleshooting may be a bit tedious - checking biases with a meter or the like. I did not see voltage values in the power supply or anywhere else, so a good bit of guesswork may be needed to determine proper operation. Make sure the power supply output is clean. Replacing electrolytic filter capacitors can be worthwhile, as they deteriorate with age.
     
  3. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
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    Well there's the problem! The P's are flipped around and some of the the R's are backwards!

    [​IMG]

    [​IMG]

    Jeez. It's a wonder it EVER worked! ;)
     
  4. marshallf3

    Well-Known Member

    Jul 26, 2010
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    That will be a pain to chase down without a good scope, even the multitude of website forums dedicated to servicing old equipment such as this are probably going to have the same answer - scope it out.

    One chance you do have that has worked in the past is to pick up a can of freeze spray and selectively cool components one by one. When you hit the one that's generating the noise it will probably decrease significantly.
     
  5. 311499

    New Member

    Apr 29, 2010
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  6. aGpLT

    Thread Starter Member

    Jan 21, 2010
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    Ok, i get that i need scope :> Sorry for one thing i forgot to mention, all capacitors are changed (electrolytic). And when i remove timbre block the noise disappears. So the problem is in timbre block.
     
  7. marshallf3

    Well-Known Member

    Jul 26, 2010
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    If you've leveled it down that far then I'd invest in a can of freeze spray, shouldn't take long to find the offending component.
     
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