Pulling in stereo radio on 500-C - very distorted

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

  1. thornev

    thornev Active Member

    Messages:
    109
    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").
     
  2. dcgillespie

    dcgillespie Fisher SA-100 Clone

    Messages:
    9,163
    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:
    109
    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:
    109
    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 You can be sure if it's Westinghouse??????? Subscriber

    Messages:
    20,781
    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.
     

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