fisher MPX decoder troubleshoot

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

  1. gadget73

    gadget73 junk junkie Subscriber

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    maybe this sounds too easy, but what about just replacing it with another unit? The 500C uses one of the sub-chassis type units that come out fairly easily right? I've bought those things in working shape for under 30 bucks. Heck, I've got one sitting on the bench right now, waiting for me to build a box and power supply for it so I can use it as an external decoder.

    I applaud your tenacity with troubleshooting this, but personally I run out of patience with things at some point. You're well past the point I'd have fixed it with a hammer. That would have probably happened like 4 1/2 years ago.
     

     

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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    If the right ring is an original Fisher ring, and it's producing just 6 mV of imbalance, then that is the standard to shoot for -- otherwise, you have excessive noise in both outputs. The NTE 109 is a Germanium diode, and likely the cause of the excessive imbalance you're seeing. Fisher always used Silicon small signal diodes which are much more closely matched in general production. The Fisher diode crosses to an NTE 177 -- not the 109.

    Dave
     
  3. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    What about using a 1n4148 X 4 ??? Over on Sansui Forum a lot of V1112's are replaced with 1n4148's on amplifier boards, etc.
     
  4. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    That works great too. I use the 1N459A as well, which like the 1N4148, is a low leakage, hi conductance silicon diode.

    Dave
     
  5. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    The reason I ask is the NTE spec's are overly broad and cover too many parts.
    I think their philosophy is probably;" Will it work? Probably yes. Will it work good? Maybe, Maybe not. Will it work as good as the original? Who cares, it works doesn't it?!!
     
  6. vintelectra

    vintelectra Active Member

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    455
    Gadget: I agree, and have a good MPX ready to go, but my object is the exercise, not the end result.

    Dave & larry: I will order those diodes and try. meantime ,I gather, replacing the left ring with an old original Fisher is the next step. yes?
     

     

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  7. vintelectra

    vintelectra Active Member

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    455
    If I am taking a RT ring from an old mpx to replace a left ring, the output of the rt ring goes to same position on the left channel , or should I flip the rt ring?
     
  8. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    A ring is a ring is a ring -- they're all the same. The ground lead is always grounded. The modulator/demodulator input/output lead always goes to which ever input of V102 it's servicing. That leaves just the two 38 kHz leads. connect them one way to the 38 kHz source, and the left channel is decoded. Connect them the other way, and the right channel is decoded. Connect them which ever way is correct for the channel being serviced.

    Dave
     
  9. vintelectra

    vintelectra Active Member

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    455
    Will do, let you know. Martin
     
  10. gadget73

    gadget73 junk junkie Subscriber

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    Gotcha, understand entirely. I have fixed things just to see if I could before. I have also occasionally given things the finger and just replaced it.
     
  11. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Got any fingers left???? (Da Debble made me do it!)
     

     

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  12. gadget73

    gadget73 junk junkie Subscriber

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    I'd give you the finger for that but I'm down to my last few here :)
     
  13. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    LOL! :D:D What I really should have asked was "Replaced the gear or the Finger(s)??"

    (Ducking thrown Tubes and Transformers)
     
  14. vintelectra

    vintelectra Active Member

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    455
    Left ring replaced with original fisher (, 3rd left ring). Left mpx output pic#1 with no drive: sine 1.5 v 38K.
    seems like there must be another fault. What should I check next?
     

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  15. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    We're back to something I've always championed: Something you think is true, is not. If:

    1. The CT of the 38 kHz transformer is effectively grounded AC wise, and

    2. The output voltage and waveform from each side of the 38 kHz transformer is balanced relative to ground, and

    3. The integrity of the ring demodulator is good, then the demodulator will balance out to a 38 kHz null with no audio applied to its input, period.

    So something in these above statements that you say is true, cannot be. I would guess is will be somewhere in #2. I would check that the output on each alternating half cycle of 38 kHz energy is equal and balanced to ground from both output legs. If they are balanced to ground on one half cycle, but not on the next half cycle, then one demodulator will null, and the other will not.
     
  16. vintelectra

    vintelectra Active Member

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    455
    1. I had checked B+ AV grounding with added electrol cap= no influence
    2. Both legs of 38K tranf put out clean clean 15V sine
    3.I tried 3 demodulator rins on the left. What I havent done is replacing the Right ring, but this one puts out 6mV at its output.and you suggested take this one as standard.
     

     

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  17. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Go back to post 418. You proved that the right modulator balances to 6 mV regardless of which way it was connected across the 38 kHz output. Did you verify that the left ring produces 150 mV regardless of which way it is connected across the 38 kHz output as well? Have you got a bad ground that the left ring is connected to? There is nothing else to check. The problem must be in this area.
     
  18. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Would a bad tube section(shorted or near shorted) on V101 do about the same thing?? I'm probably talking out my a** again, tho.
     
  19. vintelectra

    vintelectra Active Member

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    455
    Reviewing my notes , flipping left diode only the output of left ring goes from 200mV to 45 mV flipped. The right ring untouched goes from 7 mV to 150 mV when flipping Left only. Does this interaction tells you anything?
    Replaced V101 , no change
     
  20. vintelectra

    vintelectra Active Member

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    455
    Looks like a good grnd ,common to both rigs.
     

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