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stereo tuner only does mono

Discussion in 'Electronic Repair' started by Michael Salmons, Jun 16, 2004.

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  1. Hello,

    I've gotten my hands on a very nice older Sony FM tuner (ST-A6B). It
    has a few problems but the most troubling is it will only do mono, not
    stereo. It has a mode switch with three positions: mono, hi-blend and
    stereo. They are all identically mono EXCEPT in hi-blend, imagining it
    as a stereo image, the sound is moved over a bit to the left, but
    still with no channel separation.

    audio quality is excellent in all other respects.

    This, as I stated before, is an older tuner, probably made in the
    mid-seventies, all analog. Does this sound like the malfunction of a
    certain part that I could replace? Could it be the MPX chip? Or maybe
    the mode switch is bad?

    Thanks,

    Michael Salmons
    salmonsm @ missouri . edu
     
  2. Asimov

    Asimov Guest

    "Michael Salmons" bravely wrote to "All" (16 Jun 04 11:02:47)
    --- on the heady topic of "stereo tuner only does mono"

    MS> From: (Michael Salmons)

    MS> Hello,

    MS> I've gotten my hands on a very nice older Sony FM tuner (ST-A6B). It
    MS> has a few problems but the most troubling is it will only do mono, not
    MS> stereo. It has a mode switch with three positions: mono, hi-blend and
    MS> stereo. They are all identically mono EXCEPT in hi-blend, imagining it
    MS> as a stereo image, the sound is moved over a bit to the left, but
    MS> still with no channel separation.

    MS> audio quality is excellent in all other respects.

    MS> This, as I stated before, is an older tuner, probably made in the
    MS> mid-seventies, all analog. Does this sound like the malfunction of a
    MS> certain part that I could replace? Could it be the MPX chip? Or maybe
    MS> the mode switch is bad?

    MS> Thanks,

    MS> Michael Salmons
    MS> salmonsm @ missouri . edu

    You don't say if the stereo lamp lights when tuning in a station. If it
    does this rules out the tuner, detector, and possibly the stereo
    decoder circuits.

    If there is a tuning meter and the signal level goes above 1/2 scale
    it should be enough signal to turn on the stereo light. If it doesn't
    then you will have to look at the MPX signal from the detector
    (quadrature or other) for the presence of sufficient 19KHz (38KHz)
    pilot to enable the stereo demodulator IC to do its job. It is
    possible the detector was misaligned by curious hands.

    If there is a defective stage, one trick is to use a small value
    capacitor (100pF to 1nF) to bypass that stage thus injecting signal
    into the processing chain even if some part is defective. It
    introduces distortion but it is only for go/no-go testing of the stage
    function.

    The other important point is to check the ESR of the supply filter
    capacitors especially on the tuner board. It is not so unusual that a
    30 year old electro is dry or needs reforming. There is a trend for
    small 1uF caps to go bad more often. Be wary of electros near heat
    sources.

    One thing about 70's receivers is that add 3 decades and it is possible
    the front end is a lot less sensitive than when new. It may require a
    new rf input amp if it got zapped a bit during 30yrs. A lack of RF
    sensitivity symptom requires a generator to diagnose and adjust the
    noise figure but in a pinch can be done using a switched noise source.

    A*s*i*m*o*v

    .... This message transmitted on 100% recycled photons.
     
  3. Or he can just try adjusting the VCO slightly. Common problem.

    Mark Z.
     
  4. Chuck

    Chuck Guest


    If the adjustment doesn't do the trick, try replaceing the 470 pf
    polystyene capacitor in the vco circuit and then adjust the vco pot.
    The capacitor usually has to be polystyrene or a silver mica to work.
    I believe if the stereo lamp on this model is open that the circuit
    won't work. Chuck
     
  5. I just wanted to follow up. After staring at the board I realized
    after a while that there are a number of adjustments possible with
    this tuner. I cleaned them all with tiny drops of contact cleaner and
    voila, after adjusting one called 19k sep, the stereo meter lit up
    and the signal turned to stereo. i just hadn't cleaned enough! Thanks
    so much for you input, this question elicited either stupefaction or
    apathy in a lot of people I asked, and you guys came up with some
    interesting ideas about what might be going on. I appreciate it. Great
    group!!!

    Michael Salmons
     
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