Pulling in stereo radio on 500-C - very distorted

Discussion in 'Fisher' started by thornev, Jan 23, 2018.

  1. thornev

    thornev Active Member

    Messages:
    249
    Location:
    Mid Hudson Valley, NY
    Ah OK, gadget. I understand. I depend on my ears for my music "career" so I trust them. I absolutely understand depending on repeatable results. My day job is a software tester and I depend on automation to test which is all about repeatable results. My MPX is in the hands of an expert so I should be getting back an MPX board that is optimized. This is my first foray into electronics and it has been a good learning experience. I'm going to need to find a new project. Maybe something on one of my guitars or amps (although they are not "tubed").
     

     

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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,303
    Location:
    Ball Ground, GA
    For all who have tried to align a MPX unit by ear (I know I did many, many, many moons ago!), Thorne did better than most. I haven't actually aligned his unit yet, but I thought some might be interested in knowing just what the status of his adapter was when received before I do align it. The scope shot shows the stereo output from the adapter with the scope set for dual trace mode, when being fed a Left Channel Only 1 kHz Composite signal from the Fisher Model 300 MPX Generator. The sine wave with the greatest amplitude is in fact the Left Channel stereo output from the adapter, while the sine wave with lower amplitude is the Right Channel stereo output. Properly aligned, the Right channel should show no output at all (basically a straight line), or more practically, an output that is 35 db or more below the output of the Left Channel in this scenario. The measured separation between the two wave forms in the scope shot presented has the Right Channel producing a signal so great, that it is just 10 db below that of the Left Channel.

    Like the early "Matrix" style decoder designs, proper operation of Time Division style decoders that Fisher uses depends heavily on proper cancellation (as necessary) between the main and stereo information sub-channel to produce the FCC mandated minimum separation of 30 db between the two stereo channels. No doubt, Thorne adjusted his adapter to perform much better than it originally did. But therein lies the problem: Adjustment by ear lets you easily enough adjust away from "horrible" performance, but invariably never lets you get to optimum performance, because the closer you get, the harder it is to distinguish the improvements made. The result is that the best ear adjustments invariably have the adapter ending up performing somewhere in between, but typically closer to the lower performance end of the scale -- exactly how this one performs as received.

    Dave

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  3. thornev

    thornev Active Member

    Messages:
    249
    Location:
    Mid Hudson Valley, NY
    Dave - I'm not sure if I'm getting a compliment or not. LOL It doesn't matter. I'm confident my MPX is in good hands and will perform wonderfully when you're done with it. Thanks so much for your willingness to work on it. Thorne
     
  4. thornev

    thornev Active Member

    Messages:
    249
    Location:
    Mid Hudson Valley, NY
    Larry - Back in post 35 you suggested replacing 2 resistors, R139 and R143. After having the MPX realigned and doing the IF transformer adjustments to optimize signal strength, do you still recommend the resistor replacements? I don't want to mess up work already done since I don't know how the various components interface with each other if at all. Thanks, Thorne
     
  5. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,212
    Location:
    Glen Burnie Md.
    R143 and R139 are the Main power resistors between the 1st(C98) and 2nd(C91D) (R143) and the 2ndC91D and 3rdC91C(R139) sections of the Main filter caps. The originals tend to get heat stressed and operate at a high percentage of it's rating. Changing R143 to a 10W vs. a 7W puts the operating range back in the center of it's operating envelope. Changing R139 to a 2W vs. 1W does the same thing. IIRC R143 is dissipating 5W or slightly more. Increasing to 10w gives you some headroom wattage wise and puts it more in the SOA. SAME with R139. It won't change anything regarding the MPX/Stereo/FM reception as far as signal strength or sensitivity. It will help stiffen the POWER SUPPLY.
     
  6. vintelectra

    vintelectra Active Member

    Messages:
    440
    Hello , I would like to pose a question to this august panel: I am getting audio distortion( on scope) coming from the ratio detector stage V6:
    IF strip well aligned, new tube, replaced 8uf cap. What suggestion would you have to proceed to troubleshoot this stage? Thanks Martin
     
  7. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,212
    Location:
    Glen Burnie Md.
    I don't know about august panel. Maybe February or March Hare panel. Check the diodes inside of the Ratio Detector 1st. Take the coil out of circuit and note the ohms and compare with another(if you have one or just swap them). Check the condition of the slugs too. You might have a cracked one. Dave is on walk-about with new wife, will be back late in the week. Maybe. If he doesn't get lost in the outback or eaten. Don't know what size knife he's got either.
     
  8. Dave451

    Dave451 AK Subscriber Subscriber

    Messages:
    930
    Location:
    Philly area
    Agree with Larry. When I had distortion on a 400 with an otherwise aligned IF it was because of a bad ratio detector coil--had to replace it. Diodes were OK in that case, but worth checking. If this circuit is not operating symmetrically or is otherwise out of adjustment/whack, you will get distortion in the FM.
     
  9. vintelectra

    vintelectra Active Member

    Messages:
    440
    Thank gentlemen, will follow advice and let you know. Martin
     

     

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  10. deeezy

    deeezy Active Member

    Messages:
    406
    Location:
    Pittsburgh, PA
    I have had a similar problem with my 500c as well, with the stations sounding distorted on the "stereo" setting. I employed a foolproof 2 step method that should be much easier than the aforementioned solutions:

    1. Set your knob to "FM Mono"
    2. Don't give a crap so much about stereo

    I'm a simple man. All kidding aside, I am kind of surprised no one has suggested this yet, even sarcastically. Anyway, good luck, I'll be keeping an eye on this thread for improvements to my suggestion.
     
  11. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,212
    Location:
    Glen Burnie Md.
    deeezy. With the current state of Radio station mixing and transmitting, every thing is set to 11, with no dynamics to speak of, with the exception of very few select Classical or Jazz NPR stations. Mono is starting to sound better to me at times (except when I'm playing a tape (R2R) or on the phono).
     
  12. vintelectra

    vintelectra Active Member

    Messages:
    440
    Still working on the problem:checked detector stage diodes etc, OK.
    I found there is no voltage @the matrix network, I am tracing this.
    My problem is a distortion @ the CD-R out audio when feeding fm modulated signal. When switch selector is placed in stereo, open relay, the pwr returns to the matrix, but when the relay is closed, the pwr is grounded
    And the matrix network does not return. Caps replaced, checking resistors.
    Suggestions are welcomed.
     
  13. vintelectra

    vintelectra Active Member

    Messages:
    440
    Correction ..... distortion in recorder out jack.
     
  14. thornev

    thornev Active Member

    Messages:
    249
    Location:
    Mid Hudson Valley, NY
    deezy, Always a good discussion at parties. I enjoy both. I like stereo for the clarity of the individual instruments (it's actually dual mono, isn't it?) and the added variable of staging within the stereo image (which is up to the composer/engineer). I like mono for taking OUT that variable which provides for total focus on the music (and perhaps EQ and other effects) and not having to play with a balance control to ensure I'm getting all the audio content.
     
  15. fred soop

    fred soop Super Member

    Messages:
    1,878
    Using the scope in x-y mode and looking at separation, the only station in the Chicago area that has program content with good separation is the sole remaining classical station, WFMT. Even though many other stations broadcast in stereo, the crap music has very little separation, so there is almost no need for stereo. In fact, there is little need for the FM tuner any more. One could just stream the one station from the phone into the amplifier.

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