Getting the Most From Fisher FM Stereo MPX Tuners and Receivers

Discussion in 'Fisher' started by dcgillespie, Mar 17, 2016.

  1. thornev

    thornev Active Member

    Messages:
    406
    Location:
    Mid Hudson Valley, NY
    Monk, I did this mod a while ago. If no one answers you, next time I open mine up, I'll let you know where it goes. If you can find R204 which is a 1M ohm resistor (use a multimeter to find it?), the 47K resistor is actually piggy-backed on top of it i.e. each arm of the 47K resistor is connected to each arm of R204. I thought I had a picture I could post for you, but I can't find it. Thorne
     
    Last edited: Mar 2, 2018

     

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  2. monkboughtlunch

    monkboughtlunch Super Member

    Messages:
    1,678
    Thanks. Here's an even closer view (with and without notations). Does the yellow dotted line and red circles represent the correct location of where to solder both leads of the 47k resistor?

    closeup2notes.jpg

    closeup2.jpg
     
  3. monkboughtlunch

    monkboughtlunch Super Member

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    1,678
    Last edited: Mar 2, 2018
  4. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,472
    Location:
    Ball Ground, GA
    That part works, and your connection points are correct.

    Dave
     
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  5. monkboughtlunch

    monkboughtlunch Super Member

    Messages:
    1,678
    I soldered this resistor in this evening. It fixed the "dancing eye" issue! :banana: But the interesting thing is my reception is improved. Before I had some static and hash in stereo on some stations (not all). Now all the stations across the band are locking in when in stereo (no garbled stereo stations). How did adding this one resistor fix that issue?
     
  6. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
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    Location:
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    If the strength of the 19 kHz pilot signal -- either due to reception issues or alignment of the adapter -- is such as to cause marginal syncing of the pilot with the 38 kHz oscillator, then distorted stereo will always be the result. The resistor acts to reduce the sensitivity of the noise detector, which allows the pilot amplifier stage to operate with more gain. This causes more pilot signal to pass to the trigger tube, causing the 38 kHz oscillator to then consistently sync with the pilot, where it didn't before. So besides greatly reducing/eliminating the eye tube dancing, the stereo reception will improve as well.

    Dave
     
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  7. monkboughtlunch

    monkboughtlunch Super Member

    Messages:
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    I still have the original EM84A magic eye tuning tube in my 400. If I replace it with an EM87, will the new eye tube behave any differently than the old EM84A (especially with the resistor installed to address the dancing eye fix)?
     
    Last edited: Jun 30, 2018
  8. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,665
    Location:
    Glen Burnie Md.
    Yes, Slightly. Read back in the thread about the sensitivity of the tubes, and changes needed for each tube.
     
  9. monkboughtlunch

    monkboughtlunch Super Member

    Messages:
    1,678
    Did the stereo beam "dance" on the 500B and 800B too? Or just on the 400?
     
  10. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
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    Location:
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    It is primarily a product of the MPX-65 decoders, used in the 400, 500C, and 800C. The WX decoders of the tuners and "B" receivers were much less prone to the issue.

    Dave
     
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  11. Kennyg2209

    Kennyg2209 New Member

    Messages:
    25
    Hi Dave, I've been reading your Thread and have been learning a lot. I'm a little confused about the V100 area and the V101. I have the MPX-78 in a Fisher Allegro. It appears that those components are not in this MPX? I'd like to get the best out of this tuner. What are your thoughts? Thanks you for this very informative article.

    Ken 20180714_122041.jpg 20180714_122006.jpg 20180714_121914.jpg 20180714_122041.jpg 20180714_122006.jpg 20180714_121914.jpg
     

     

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  12. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,472
    Location:
    Ball Ground, GA
    Correct. The MPX-78 does not employ the noise detector circuit, because there is no indicator output to drive an eye tube or stereo/mono switching circuit. With no indicator output, there is then no concern for any false triggering of the indicator by noise, so the noise detector circuit is eliminated on this model.

    Dave
     
  13. Kennyg2209

    Kennyg2209 New Member

    Messages:
    25
    Thanks Dave, I got it sounding pretty good. Just like to make it as good as it can be. I appreciate your reply.

    Thanks, Ken
     
  14. monkboughtlunch

    monkboughtlunch Super Member

    Messages:
    1,678
    I installed an EM87 in the 400. A few observations. The EM87 actually fully opens, going "dark" (no beam) when I move to a non FM source input. In contrast, with the EM84a (Fisher branded), the edges of the beams could still be seen slightly when a non-FM source was selected. So the EM87 actually seems to perform better than the EM84a in that respect. The EM87 does leave less of a gap when tuning stations than the EM84a.
     

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