fisher MPX decoder troubleshoot

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

  1. gadget73

    gadget73 junk junkie Subscriber

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    I tried to avoid using the second filter cap after the drop resistor once, it did not work correctly. I presume this is why. Adding the cap back in made it work properly again.
     

     

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  2. vintelectra

    vintelectra Active Member

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    Thank you.That should be the reason! I have converted an old Collins power supply to operate these mpx ( for fun) with voltage reduction netwoks for mpx 65 and WX.
    I added a 40 uf elecrol filter at the output of 165 v ,& did the tric. May be too high capacitance? Will try lower,
    What I find interesting is that the 275 v output does not have a ripple, while the165 V does, although both come from the same Collins PS terminal .
     
  3. gadget73

    gadget73 junk junkie Subscriber

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    I don't know that the value of that cap after the drop resistor is all that critical. It just needs to be big enough to decouple anything from the first voltage stage.
     
  4. vintelectra

    vintelectra Active Member

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    You are right, I tinkered with different caps with no change , I will settle for a 40 uf. Finish calibration of mpx 65 with max separation 37 dB. Now I will test the new box adaptor with a WX mpx Thanks for insight and help. martin
     
  5. vintelectra

    vintelectra Active Member

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    467
    Hello Boss and Fisher fans: Finished this beautiful 500-C (new everything) . On stereo reception only forced or auto, I pick up a subtle noise interference in both channels , not in Mono. This noise that is exacerbated by
    high frequency vocals, is not attenuated by the mpx filter. Almost sounds like some am interference. Tried tweaking Z-100 (withing 2:1liz tune) to no avail . I am using a indoor dipole, and occurs in all stations. MPX specs great and clean, separation better than 35 dB. Any suggestions? Many Thanks for your interest. Martin
     
  6. thornev

    thornev Active Member

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    I can't help other than to say I'm also using an indoor dipole and have no such problem on my 500-C. I had an expert tune my MPX-64. Thorne
     

     

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  7. vintelectra

    vintelectra Active Member

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    I tuned this mpx with a fisher 300 and specs optimun
     
  8. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Sounds like a slight echo distortion? Make sure the tubes are good. With equal sections for gm and emissions. Did you recap the MPX-65? Check all solder joints for cold ones.
     
  9. vintelectra

    vintelectra Active Member

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    467
    thanks. tubes balanced new tungsol. recapped audio path only. will recheck solder joints
     
  10. gadget73

    gadget73 junk junkie Subscriber

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    Try old production tubes in the MPX unit if you haven't. I've found some of the new tubes behave differently enough to cause issues in some circuits.
     
  11. thornev

    thornev Active Member

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    gadget - That was my question in my recent thread "Tuning MPX-64... is it tuned to the tubes?" i.e. can swapping out tubes in MPX cause audio issues and one response was that it is not a feasible explanation. Let's see if anyone else chimes in.
     

     

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  12. gadget73

    gadget73 junk junkie Subscriber

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    The new production ones sometimes behave differently than old ones, but I don't know enough about the specifics of the MPX unit to tell you if that is one of the spots they may act funny in. All 3 of mine use whatever random old tube I had within reach when I made them work. I wouldn't figure swapping any old, good tube for any other old, good tube would cause any problems at all, or at least it has not for me. I've just never tried new production tubes in an MPX unit.
     
  13. vintelectra

    vintelectra Active Member

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    Will do and let you know, thanks
     
  14. vintelectra

    vintelectra Active Member

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    Replaced new tung sol for good used fishers and it made no difference, But what solved the problem were several factors: Tinkered with the trap L100 by ear ( as supposed by fisher instructions) and it helped. Then replace one tunning trimmer cap in FM section and tinkered with L4 by ear. I will let it run with the new tubes for break-in. Thanks All!
     
  15. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    The old production tubes vs new production tubes I don't feel is a problem in the MPX. I run new and old production tubes in my various units with no problems, and most of them haven't seen the inside of a shop/lab since they were built in the 60's. The problem with new productioin tubes is in the Phase Inverter, more so with the noose than without. The removal of the noose increases the chances that a new production tube will work without going into a non linear area of it's envelope. But pushed too hard (volume to 11) and they will cause drop outs or higher distortion.
     
  16. gadget73

    gadget73 junk junkie Subscriber

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    Thats also the place I've had issues with them, but not in a Fisher. Dropouts were exactly the problem I had too. I've just never tried new production tubes in enough places to know for sure where they misbehave.
     

     

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