fisher MPX decoder troubleshoot

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

  1. gadget73

    gadget73 junk junkie Subscriber

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    Sounds like they need to go on the Chopin block.
     

     

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  2. vintelectra

    vintelectra Active Member

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    455
    Swapped rings rt to left: Now no separation and contamination in both
     

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  3. vintelectra

    vintelectra Active Member

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    455
    Which are the best small electrolytic?Nichiton?
     
  4. vintelectra

    vintelectra Active Member

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    455
    Now I have reversed separation and the most contamination follows the left ch: " the problem stays with the channel" ( if we ignore the minor contamination on right ch
     
  5. vintelectra

    vintelectra Active Member

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    455
    This are the pics for post#464
     

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  6. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    OK. So when you reverse the signals applied to C218 and C220, the left channel signal -- which now appears at the output originating from pin 1 of V102 -- still contains the heavy 38 kHz modulation?
     

     

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  7. vintelectra

    vintelectra Active Member

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    455
  8. vintelectra

    vintelectra Active Member

    Messages:
    455
    sorry, Nope. The left mpx output remains heavily contaminated, not the right output, but the gen output has to switch to to right to obtain the contaminated left signal, and to left to obtain a clean sine on left output.
     
  9. vintelectra

    vintelectra Active Member

    Messages:
    455
    So the problem stays with the left output, suggesting is the left network the culprit
     
  10. vintelectra

    vintelectra Active Member

    Messages:
    455
    The left trap is 10 ohm lower than the right one.
     
  11. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Check the caps in the trap and de-emphasis network. Ceramics always tend to measure low in my experience. I've also seen the chokes ave shorted turns. Best to measure them with an impedance bridge which will be a much more accurate test that using simple DC resistance. Or, another approach is to simply swap the chokes between channels as well. That will tell you too.......
     

     

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  12. vintelectra

    vintelectra Active Member

    Messages:
    455
    Hi, Im back;: Rebuilt left network, super clean sine in both!! Chokes ok 20mH. Cant pin point fault. But now I am stuck with No separation. rechecked work seems ok.
    At this time , and to relieve frustration, would be great to hear Dave expand on the inner workings of the trap and de-emphasis network Thanks
     
  13. vintelectra

    vintelectra Active Member

    Messages:
    455
    The right ch network is a little dirty, Should I rebuilt this one too?
     
  14. vintelectra

    vintelectra Active Member

    Messages:
    455
    Right network rebuilt, clean sine, but still no separation. Will check diode ring outputs again.
     
  15. vintelectra

    vintelectra Active Member

    Messages:
    455
    Hi Dave: Puzzle: Pic 1 is 300 gen composite direct output, pilot suppressed ( perfect)

    pic 2 is same output but thru input network. ( same with two input networks).

    pic 3 signal at splitter.

    WHY? your response will be greatly appreciated. Martin
     

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  16. vintelectra

    vintelectra Active Member

    Messages:
    455
    pic 1 Rt demodulator ring output( nice)

    pic 2 left demodulator output ( bad)

    Left ring bad AGAIN??
     

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  17. vintelectra

    vintelectra Active Member

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    455
    pic 1& 2 rt & lft ring output in good working unit.
    Will replace left ring with matching 1N459A to the rt one. martin
     

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  18. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    I'm not sure what you mean by the term splitter, although various signals are in fact split out at different points in the circuit. The input network (180pF, 12K) alters the phase of the signal coming from the 300 generator so as to mimic the phase of the signal as ultimately received by the adapter from the ratio detector when installed in a Fisher tuner or receiver using the Wide Band IF circuitry.

    The amount of times you've replaced the left ring is a complete mystery!
     
  19. vintelectra

    vintelectra Active Member

    Messages:
    455
    Thank you for your interest. "Splitter" was your own word ealier in this thread back in 2013, indicating the neg terminal of the coupling electrolytic.
    Now I remembered the effect of the input network we discussed earlier thanks.
    I ordered another set of Fairchild In 4598 to match the right one. Since they are sold in sets of four, I don't think I need to match them with 1meg resistors.
    This will be the fourth left ring,Mpx from hell!!
    Question: the fisher diodes measure in mi tester o.5 range, while the 4598s range 0.25 significant?
     
  20. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Location:
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    I wouldn't think so.
     

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