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
     
  2. vintelectra

    vintelectra Active Member

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    373
    #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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    373
    Hi Dave: I got the answer, are you there??
     
  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Location:
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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:
    373
    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
     
  7. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    8,985
    Location:
    Ball Ground, GA
    Glad you found it!

    Dave
     
  8. audmod01

    audmod01 AK Subscriber Subscriber

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    Tioga, TX
    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
     

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