fisher MPX decoder troubleshoot

Discussion in 'Fisher' started by vintelectra, Apr 21, 2013.

  1. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    The de-emphasis networks normally won't cause distortion. How does the regenerated 38 kHz sub-carrier look?

    Dave
     

     

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  2. vintelectra

    vintelectra Active Member

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    #1--- distorted left only output. #2 left ring output.---#3 38K subcarrier. thanks
     

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  3. vintelectra

    vintelectra Active Member

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    Hi Dave: Did you get the pics? Martin
     
  4. vintelectra

    vintelectra Active Member

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    467
    Hi Dave: I got the answer, are you there??
     
  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Well now I am -- I've been out most of the day. With a problem like this, there's only a few things that it can result from:

    1. The composite signal must be present, undistorted, and of sufficient amplitude.

    2. The sub-carrier must be present with sufficient amplitude, and of the proper phase relative to the composite signal, as presented to properly functioning demodulator diodes.

    3. The matrix output stage must be functioning properly to match the amplitude of the L+R audio signal and the L-R sub-carrier signal so as to achieve individual +2L and +2R output signals.

    For one channel to not work properly, something has gone wrong at some point in this process. That usually implies something in the demodulators or the matrix stage, but phase issues can cause these types of issues as well.

    So what did you find?

    Dave
     
  6. vintelectra

    vintelectra Active Member

    Messages:
    467
    OK, having found nothing wrong with the mpx itself, I thought if the load to it could couse to distort its output, so by cutting the left output connection,
    the mpx returned to a normal , non distorted output with a 35 dB separation. I traced back the circuit to main chassis , checking diodes resistors etc
    and I found a mistake I had made in the replacement of the mustard caps ( shame on me). Thanks very much for your help again. Martin
     

     

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  7. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Glad you found it!

    Dave
     
  8. audmod01

    audmod01 AK Subscriber Subscriber

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    vintelectra;

    You did well!:beerchug:Finding one's own mistakes is sometimes difficult. We have all been there and done that.

    Joe
     
  9. vintelectra

    vintelectra Active Member

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    Amen , Joe. Martin
     
  10. vintelectra

    vintelectra Active Member

    Messages:
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    Hello Dave :Hope everyone well. This other MPX with good 19K clean lissajous has a spurious triangular output signal on left output only of 38k 80 mV without any input signal applied ( lug one opened)
    ( new tubes, new electrol and new audio path caps. does not respond to input signal other than interference and distortion) Any thoughts greatly appreciated as usual. Martin
     
  11. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Location:
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    Significant 38 kHz energy in the output typically smacks of a failed diode in one of the ring diode demodulators. But I'm confused: How can the sub-chassis give a nice, clean Lissajous pattern (which requires an input be applied to Lug 1), yet the unit does not respond to input signal.......?

    Dave
     

     

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  12. vintelectra

    vintelectra Active Member

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    Thank you Dave.I will check diodes and let you know.
    I ment clean lissajous when input is applied to lug 1,
    Obtaining the signal to scope from Z , but the spurious signal
    Obtained from left output terminal is essentially independent
    From any input or no imput at all at lug one. Martin
     
  13. vintelectra

    vintelectra Active Member

    Messages:
    467
    Hi Dave: Diode rings good. What kind of signal if any is expected at output with no input and lug one opened? Pic shows left channel output
    with no input drive and lug 1 opened. Thanks Martin
     

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  14. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    What does the output of the other channel look like under the same conditions, and what does the signal look like at the two grids of V102?
     
  15. vintelectra

    vintelectra Active Member

    Messages:
    467
    pic #1 right ch out, only 3 mV rms (left out was 100 mV)
    pic#2Left grid of V102....35 mV
    pic#3 Right grid of V102....40 mV
     

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  16. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Location:
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    Short the two grids together. Do the left and right outputs appear similar then?
     

     

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  17. vintelectra

    vintelectra Active Member

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    Yes, just 3mV noise in both
     
  18. vintelectra

    vintelectra Active Member

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    pd the previous signals gone
     
  19. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Then the source of the noise is the ring demodulators which are not balancing to minimal output when no audio is applied. You might check that as well. Make sure nothing is coming through the composite coupling cap that is causing the demodulators to unbalance. Ground the demodulator side of the composite coupling cap, and then see what is on the audio outputs. If the noise is still there, then one of the demodulators is not balanced. If the noise is gone, then find out where the signal is coming from (through the composite coupling cap) that is unbalancing the the one modulator.

    Dave
     
  20. vintelectra

    vintelectra Active Member

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    467
    Which one is the composite coupling cap?
     

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