Sx-780 no sound on right side

Discussion in 'Pioneer Audio' started by c.vandamme, Apr 14, 2018.

  1. c.vandamme

    c.vandamme New Member

    Messages:
    1
    Just picked up an sx780 and the thing sounds amazing. I noticed that when the FM tuner is playing and the switch is on stereo I get no sound from the right side. Flip to mono and both sides work . I checked the tt and the aux with my phone(spotify) and it works perfectly fine,the only problem is the fm tuner.
     

     

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  2. paul_ral

    paul_ral Active Member

    Messages:
    258
    check preamp transistors
     
  3. Watthour

    Watthour Electron Rancher - JS3600

    ...And the muting FETs, and the function switch, and the stereo/mono switch, Q7/Q8 on thw tuner board, and numerous other possibilities. The easiest, and possibly most important is to verify the switches are clean and passing signal.
     
  4. dr*audio

    dr*audio Fish fingers and custard!

    Messages:
    18,651
    Location:
    Cleveland, Ohio
    If you have an oscilloscope you can trace the signal through the circuits and see where it is being lost. Start at the output of the MPX decoder IC.
     
  5. paul_ral

    paul_ral Active Member

    Messages:
    258
    you can switch left with right FM outputs
     
  6. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

    Messages:
    4,788
    Location:
    Las Vegas Nevada
    You can use a good meter like a Fluke set to ACmV's as a signal tracer. Its takes some experimentation if you never tried it.
    Use it set up on a known good line level signal. Like the AUX input with a CD playing. Probe the back of the RCA jack. Get use to the meter fluctuation as the music is playing.
    Select FM and tune to a strong station probing the points the experts mentioned above.
    Black probe to ground and the red probe sheathed, leaving a small exposed point at the tip, thus, preventing a slip of the probe causing a short between two components/pins. Only a small amount of metal probe is needed to contact the IC legs/pins.
    A nicely sized diameter of Heatshink is perfect for the probe sheath but tape is as good.
    Please heed my advice when working around the IC's. A slip will will make you wished you never started. Take your time.
    The Transistors Watthour mentions are the pesky 2SA726 and could well be the culprit(s).
    If unsure of your skills: Cleaning and lubing the controls is always the first line of troubleshooting.
     
    Last edited: Apr 15, 2018

     

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  7. Watthour

    Watthour Electron Rancher - JS3600

    Zeb is correct about the transistors in the tuner board output - Those are the troublesome 2SA726 (little bastards). The two FETs in the FM muting circuit are on the AF board, and are 2SK34s. They're about as easy to locate as hen's teeth. Although their failure is pretty rare, I've had two units with these FETs failed. In both cases a BF256B worked out as a replacement.
     

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