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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    120 -- Outside of the multiplex chassis itself, the basic FM tuner must be aligned properly, as a poorly adjusted IF strip (most importantly, the Ratio Detector transformer) will not allow even a properly functioning multiplexer to work properly.

    As for the multiplexer itself, the usual suspect is the 1 uF 350 volt electrolytic composite signal coupling cap. If this goes out, the stereo production will be poor. Also, with the separation control located where it can easily get disturbed, that is also a likely possibility.

    Of course, with the 500C and 800C, you also have the automatic stereo-mono switching matrix circuit that can develop its own issues as well, not allowing the circuit to switch properly when called upon to. There are a couple of electrolytic caps in that circuit, as well as some resistors that are underrated in earlier versions of these units that are the usual culprits.

    So there are really three general areas where most issues with FM Stereo MPX reception can originate from -- and, as you can see, they don't always originate at the multiplexer itself.

    The good news is that if it is the multiplexer, it is often possible to remove the multiplex chassis from the receiver/tuner it is installed in (just 7 wires), and send it off by itself for repairs/alignment. These units were manufactured and aligned separate from the tuner/receivers they were installed in, and can be repaired the same way. That way, you avoid the time/expense/material and worry of shipping the entire unit for service.

    I hope this helps!

    Dave
     
  2. vintelectra

    vintelectra AK Subscriber Subscriber

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    So far I have worked in about 30 mpx"s and of those which caused problems, not two had the same fault. Some man made. Lets hear what Dave has to say. Martin
     
  3. vintelectra

    vintelectra AK Subscriber Subscriber

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    Hey Dave: I got 1 uV on the 20dB test. What d"ya think. Martin
     
  4. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Very good, Grasshopper!! :)

    Dave
     
  5. vintelectra

    vintelectra AK Subscriber Subscriber

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    Hi Dave, are you there? Martin
     
  6. vintelectra

    vintelectra AK Subscriber Subscriber

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    Hi Dave: Are you there? Martin
     
  7. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Last time I checked! :)

    Dave
     
  8. notdigital

    notdigital AK Subscriber Subscriber

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    :rflmao:
     
  9. vintelectra

    vintelectra AK Subscriber Subscriber

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    Just checking,no interesting questions this time! Martin
     
  10. vintelectra

    vintelectra AK Subscriber Subscriber

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    Hi Dave, hope you are doing well: I am picking on your MPX wisdom again. Thoroughly read your posts of 2013 and I am still learning: This fisher 500-c adapter is giving me a reversed 22 dB separation???
    Recapped 1uf cap and audio path caps. stable 2;1 Lis , good sine with 38Kh filler at modulator output, 7 vac pp, reviewed connections and diodes,. Driving mpx directly with 300 gen. Any suggestions?
    Thank you in advanced. Martin
     
  11. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    I'm not sure what you mean by "reversed separation" -- unless you mean that you're sending (for example) a known accurate L only composite signal to the decoder, yet primarily receiving a R channel output. If that is the case, then that can only be caused by an improper phase relationship between the output of the on-board 38 kHz oscillator, and the pilot signal. If these two signals become 180 degrees out of proper phase which each other, separation can be made to be excellent, stable 2:1 Lissajous pattern and all, except that the channel identity will be reversed. Since it is the phase relationship between these two signals that establishes channel identity, the only way that identity can change is if the phase relationship has changed -- all assuming that the composite signal is accurately being generated.......

    Dave
     
  12. vintelectra

    vintelectra AK Subscriber Subscriber

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    Thank you for replying, Yes, the channel identity is reversed. There is a good composite from both 300 gens. I thought about the relative phase you are indicating albeit in a more basic form, which led me to observe this
    reversal in dual channel scope while tuning Z101 back and forth to no avail. Martin
     
  13. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    If Z101 is not affecting the relationship of the two audio channels to each other, then that would suggest that the 38 kHz oscillator is not locked to the pilot signal -- which would suggest that the 2:1 pattern would not really be stable, either.......
     
  14. vintelectra

    vintelectra AK Subscriber Subscriber

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    But it does affect both channels with 2 points close to each other where the separation is best , although reversed. And I tested several times the 2;1 liss and locked normally, and stable from 30mv to150 mv 19kh signal.
    Would tinkering with the 19 k z100 coil help? Martin
     
  15. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    The 38 kHz transformer can be adjusted for either correct or reversed channel identification. Have you checked for that?

    Dave
     
  16. fred soop

    fred soop Super Member

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    The final step in the alignment instructions for the 600-T (may or may not be applicable in your case) is to check for proper channel output and actually swap the wires on the MPX board if the channels are reversed.
     
  17. vintelectra

    vintelectra AK Subscriber Subscriber

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    I ran the full course of Z101 and only at the same position as previously mention i get a separation, reversed identity. 22db is low, my usual number is around 35 dB. Swapping the terminals would imply this is a factory fault, since the adapter, appeared untouched until I recaped, corrected de-emphasis and tuned with fisher protocol. Martin
     
  18. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    Martin; Just an FYI! FISHER tuned the MPX units separately from the tuner/receivers and then installed them in the unit. It's possible that L&R outputs were reversed (it's probably a post holiday, Monday morning unit ). at the factory and not picked up on QC Inspection.
     
  19. vintelectra

    vintelectra AK Subscriber Subscriber

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    Possible Larry, but the left yellow output and the right black output terminal are in normal position. The composite input (black) is also in normal position. Which other connection could be accidentally swapped?
     
  20. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    Could it be between the diode bridges and the output's? That would be the most likely area. But it's so tight in there that finding something reversed would be a nightmare.
     

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