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QUESTION: FM Stereo Sensitivity

Discussion in 'Electronic Repair' started by EADGBE, Nov 6, 2007.

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    EADGBE Guest

    I went to a garage sale recently and couldn't pass up an old Technics
    SA series receiver in great condition. (A Technics SA series receiver
    was my first bit of halfway decent stereo equipment.)

    The receiver's FM tuner works great and sounds very good, but I can
    only get the "FM Stereo" indicator light to light up on one station,
    even though the signal strength meter indicates VERY strong FM signals
    on almost every station all across the FM band.

    I only have one of those little clear-coated two-conductor wire FM
    antennas hooked up to it right now, but if I recall correctly, that
    was all that was needed to get FM Stereo on virtually every station in
    my area.

    Could something be weak in the FM tuner circuitry, or do I need a
    stronger antenna?

    And whilst I'm at it, what is the functional difference between the
    two 300 ohm FM antenna terminals and the grounded 75 ohm FM antenna
    terminal, besides the obvious difference in resistance?

    Any help would be much appreciated.
  2. Scott Dorsey

    Scott Dorsey Guest

    Your notion of "halfway" is sort of optimistic.
    Odds are the stereo detector stage needs alignment or else it has some
    other issue with it. Take it to a tech. The audio output of the
    FM detector goes through a coupling cap into a second discriminator
    stage. All kinds of things can go wrong with it.
    No, the stereo demodulation circuit is bad.
    If you have a 300 ohm source, you use the 300 ohm input. If you have
    a 75 ohm source, you use the 75 ohm input. Usually there is a balun
    inside the case which transforms one to the other, and the balun is
    a little lossy, so one of the inputs will have slightly better sensitivity.
    Which is basically academic since you will need the correct one for your
    source anyway.
  3. To add to Scott's reply:- The 300 ohm input will be balanced twin feeder.
    This is the cable with two parallel wires, spaced about 12mm (1/2") apart.
    The 75 ohm input is unbalanced and uses coaxial (screened) cable. In the UK,
    we prefer to use the 75 ohm input as screened cable protects the antenna
    signal from interference, especially from mains wiring and/or metal objects
    which could change the impedance of the balanced unscreened cable.

    Having said that, Scott is right in that if you have a 75 ohm antenna, use
    coax, if you have a 300 ohm antenna use balanced twin.

  4. jakdedert

    jakdedert Guest

    The SA designation is still used for their receiver line, and has been
    since Matsushita formed the Technics brand. How old is it?
    Depends very strongly on where you are, geographically speaking...and
    where the receiver is physically located.
    Those work acceptably in urban settings. If it's hooked up right, and
    there are reasonably strong stations within 15-30 miles, you should get
    Depends. Obviously if the gear is old, it could be defective.
    It's not resistance, but impedance...probably beyond a concise
    explanation. Suffice to say: hook twinlead (what you have) to the 300
    ohm connection and coax (like cable TV lead) otherwise.


    EADGBE Guest


    I am fully aware of the Technics' questionable "hi-fi" status. I
    simply bought this as a nostalgia trip. Believe me, the Technics SA
    receiver I had as a kid was AUDIOPHILE stuff compared to what I had
    been listening to before I got it.

    For the record, the receiver I just got is an SA-400, made from 1978
    to 1979.
  6. Mr. Land

    Mr. Land Guest

    Stereo notwithstanding, can you get decent sensitivity at both ends of
    FM band?

    I used to work on a lot of these types of receivers. Our shop's
    "tune-up" for these included FM sensitivity adjustments (i.e. RF
    at several points along the FM band, and IF and stereo demux
    For the RF alignment, you could probably approximate the optimal
    if you can find relatively weak stations at both ends of the band.

    For the IF and stereo demux, though, we used a combination of a
    dedicated signal generator and a 'scope. Not sure if or how you could
    accomplish that w/o some specialized test equipment.

    I was always a little surprised at how far off these old technology
    receivers could be, even when they weren't that old.

    If you're getting decent signal meter deflection, I agree with the
    poster(s) - you probably need your IF/Stereo demux aligned.

    Good Luck!

    EeAaDdGgBbEe (a 12-string)
  7. jakdedert

    jakdedert Guest

    You probably have this info, but: 45wpc at 0.04 THD and 0.04 IMD rated
    from 20-20khz bandwidth with a 90 dB s/n ratio (phono). 10.8 fm mono IHF
    sensitivity with a 1.2 dB fm capture ratio...cost $360 in 1979 dollars
    (over $1000 today, by the consumer price index).

    By some measures, I'd day that qualified as "HiFi".

  8. Mark

    Mark Guest

    see if you can locate a schematic or serivce manual and look for a
    muting or stereo threshold adjustment...

    these receivers are designed to switch to mono when the signal is weak
    and sometimes there is an adjustment for the threshold...

    or the tuner could be out of alignment or broke such that it thinks
    the signal is weak when its not weak...

  9. Meat Plow

    Meat Plow Guest

    Need to align the stereo section of the tuner which requires some
    equipment that generates specific signals.
    75 ohm is coax type connection, 300 ohm is twin lead. The often have a
    common input terminal.
  10. David Farber

    David Farber Guest

    VR301 is the VCO adjustment. It's located near the tuning gang. This would
    be the easiest thing to adjust that might correct the problem. First take
    note of where it's set so that if the adjustment doesn't work, you can
    return it to its prior state. Then turn it slightly one way or the other and
    see if your stereo signal light comes on for more stations. If not, go the
    other direction. Without a frequency counter, you'll need to trial and error
    this adjustment.

    Good luck.
  11. G

    G Guest

    75 is commonly unbalanced, or uses ground reference. I do have some 72 ohm
    balanced lead next to me, and there is no shield. You can buy balanced
    shielded however. I used to use some 300 ohm shielded lead for a TV antenna
    way back. One of my tuners I started to use, which I bought of Ebay, is slightly off
    tune, and you have to hit mono to get good reception. It can also affect the stereo
    and muting thresholds.

  12. It's highly likely this is the problem. This setting can drift to the point
    where the decoder can fail to switch to stereo. I've seen the same issue in
    other stereo receivers.
  13. EADGBE

    EADGBE Guest

    Oh, yes. In fact, the tuner pulls in most stations quite clearly even
    when there is NO antenna connected at all.

    With an antenna, the signal strength meter and FM fine-tuning meter
    both respond VERY strongly, throughout the entire FM band.

    The signal strength meter typically registers between 3.5 and 4.5, on
    a scale of 1 to 5, and the fine-tuning meter always allows me to
    precisely "center" the tuning onto a station.

    LOL!!! Good to see a fellow guitar player who understands my
  14. EADGBE

    EADGBE Guest


    Thanks so much for the information. I work on analog stuff (tape
    decks, turntables), but I'm not too well-versed in tuner circuitry.
    This will be a big help.

    FWIW, I do have a frequency counter, but it's not in hand just yet.
    (Just ordered it.)
  15. It's the VCO adjustment...

    Mark Z.
  16. The problem with these in an urban area is they do little to sort
    multipath problems - which are all too common with high buildings etc
    around. And multipath can cause very audible distortion.
  17. jakdedert

    jakdedert Guest

    True, but irrelevant to the OP's issue.

  18. EADGBE

    EADGBE Guest

    OK, I accessed VR301 - plainly labeled "VCO" by the way - and turned
    it clockwise a very tiny amount.

    That did the trick!

    I now have FM Stereo indication on all stations!

    Thanks so much to David, William, Mark, and everyone else who offered

    My garage sale find is now 100% functional!
  19. David Farber

    David Farber Guest

    I looked at the service manual again and the alternative method of alignment
    says to adjust VR301 until the stereo light comes on. When you get your
    counter, hook it up to TP301 via 100k ohm resistor and adjust VR301 for
    19KHz. It might help if you move the tuning dial so there is no station
    being received during the test.
  20. But .. is the output stereo? The indicator could be bad ... it is
    on my old (and very excellent) Sony tuner.

    This is supposedly a pro group. Record the output and look at the
    waveforms in your favorite editor. Are they completely different?
    Are there places where they are 90 degrees out of phase? Then
    it is working OK in stereo on that station. Or just listen!

    Doug McDonald
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