1. Time for some upgrades in server hardware and software to enhance security and take AK to the next level. Please contribute what you can to sales@audiokarma.org at PayPal.com - Thanks from the AK Team
    Dismiss Notice

Pioneer SX-780 Stereo LED doesn't work (LED is good?)

Discussion in 'Pioneer Audio' started by Raccoon1400, Oct 26, 2018.

  1. Raccoon1400

    Raccoon1400 Super Member

    Messages:
    2,615
    Location:
    Ottawa, Canada
    I'm working on a pioneer 780, and I can't get the FM stereo light to come on. I can get stations in stereo.
    The "source" light comes on but it is dimmer than it should be. The source and tape monitor lights get about 2V across them, and the FM stereo light gets about 1.2V. But the voltage going to it look correct according to the schematic. 7.4V on one side of it and 6.5V on pin 6 of the tuner IC that drives it. When I test the LEDs with my meter, I get a double diode drop worth of voltage across each one.

    What could be going on here?

    EDIT: I'm getting the 6.3V at the chip whether it is in stereo or not. So the chip isn't turning the light on?
     
    Last edited: Oct 26, 2018

     

    Please register to disable this ad.

  2. Watthour

    Watthour Electron Rancher - JS3600

    Review the schematic. The STEREO LED (D21) receives constant power. The ground side of that LED is switched by the PA1001 IC.

    The chances are statistically very low that the PA1001 has a problem, and it is more likely that the tuner is not decoding the MPX signal properly. It may help to turn the FM MUTING to off in case there is a something pulling that signal up at Pin 16

    Are you certain that the audio is in stereo? If so, the VCO for the 19KHz quad detector might be adjusted. If VR1 in the tuner section tests O.K. (4.7KΩ), carefully note/photo document the position of VR1 and attempt to adjust it using a strong local stereo broadcast as a source. If no progress is made, return VR1 to its original position.

    The method is described here:

     
  3. Raccoon1400

    Raccoon1400 Super Member

    Messages:
    2,615
    Location:
    Ottawa, Canada
    It is definitely in stereo. I tried that adjustment and I could hear it go in and out of stereo but no light still.
     
  4. Raccoon1400

    Raccoon1400 Super Member

    Messages:
    2,615
    Location:
    Ottawa, Canada
    How do I check the 19kHz? I can't find it.
     
  5. Watthour

    Watthour Electron Rancher - JS3600

    If you have an oscilloscope you can check the oscillator frequency (and amplitude). The X-Y plot method is in the service manual, or you can monitor Pin 15 on the PA1001 for a 76KHz rate.

    If you don't have a 'scope handy, you can attempt to adjust as shown above.
     
  6. Raccoon1400

    Raccoon1400 Super Member

    Messages:
    2,615
    Location:
    Ottawa, Canada
    The act of touching the probe to 15 throws off the adjustment.

    Vr2 (4.7k) looks like it is the pilot adjustment but it has no effect that I can notice.

    In your first post you said Vr1 is 4.7k but it is 22k. Vr2 is 4.7k
     

     

    Please register to disable this ad.

  7. Watthour

    Watthour Electron Rancher - JS3600

    The capacitance of the scope probe may be affecting the circuit. It may be better to try to pick the signal from one end of C40.

    VR2 is the pilot / subcarrier cancelling adjustment, and probably should be left alone unless using a siggen to set the input and and modulation levels.

    VR1 is the adjustment for the VCO (Voltage Controlled Oscillator) and it determines the frequency. The PA1001 divides that 76kHz signal (twice) to derive a 19 kHz pilot source. Checking both the frequency and the P-P level could be helpful.
     
  8. Raccoon1400

    Raccoon1400 Super Member

    Messages:
    2,615
    Location:
    Ottawa, Canada
    Here's what the scope shows me at C40

    Can I try shorting IC pin 6 to ground to try turning on the lamp?

    I'm not convinced it isn't an issue with that part of the chip, since the FM stereo seems to be working fine.
    How can I verify it is getting good signal strength, since there is no signal strength meter?

    I do have an fm signal generator. (sencore)
     

    Attached Files:

  9. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,806
    Location:
    Fort Dodge, Iowa
    Raccoon
    Do you have a freq counter?
     
  10. Raccoon1400

    Raccoon1400 Super Member

    Messages:
    2,615
    Location:
    Ottawa, Canada
    My fluke 115 has one I've used successfully for pilot signals before.
     
  11. Raccoon1400

    Raccoon1400 Super Member

    Messages:
    2,615
    Location:
    Ottawa, Canada
    Any more ideas? Can anyone answer the questions in post 8?
     

     

    Please register to disable this ad.

  12. Watthour

    Watthour Electron Rancher - JS3600

    Sorry for the confusion earlier, but I didn't consider the exchange factor. I think 76kHz here might be about 90kHz Canadian. Then again, that market has been all over the map lately.

    76kHz should be well within the range of your 2215. Is that the cleanest, most stable wave you could get? Did you try with different trigger modes? If that doesn't clean it up, maybe connecting to Pins 10/11 will give you a clearer 19kHz image.

    Yes, Pin 6 of PA1001can be grounded to verify the rest of the circuit is intact (just in case a solder joint or resistor is open). It may be safer to ground this at R38, or connect a chip clip and mini-grabber BEFORE powering on the unit to avoid shorting anything else.

    If yours has a multiplex capability, follow the service manual alignment instructions, you should be able to see the sync pattern in X-Y mode on the scope.
     
  13. Watthour

    Watthour Electron Rancher - JS3600

    O.K. For the sake of "just because" I pulled a spare 780 out of the storage rack and made some room on my bench.

    [​IMG]


    Then connected the check-a-ma-doodles:

    [​IMG]

    The faster scope showed this at Pin 15:

    [​IMG]


    The slower scope showed this at Pin 15:

    [​IMG]


    And the counter showed this at TP23:

    [​IMG]
     

    Attached Files:

  14. Raccoon1400

    Raccoon1400 Super Member

    Messages:
    2,615
    Location:
    Ottawa, Canada
    Here's another picture but with different triggering settings. I get the same thing at C40 as I do at pins 10-11.

    I can't quite figure out how to set up the MPX alignment with the scope and XY mode.

    If the receiver gets stereo, then I don't understand what could prevent the light from coming on except a problem with the part of the chip that turns the lamp on.

    EDIT: just saw your last reply. Your waveform at pin 15 looks about the same as what I get there. I can't get a good frequency reading at tp23 though, it shows about 60Hz.
     

    Attached Files:

  15. Raccoon1400

    Raccoon1400 Super Member

    Messages:
    2,615
    Location:
    Ottawa, Canada
    Oh and shorting pin 6 to ground does turn on the stereo light.
     
  16. Watthour

    Watthour Electron Rancher - JS3600

    It could be a case of a failed PA1001. There are supposedly NOS pieces in China, or at least they are being advertised that way.
     

     

    Please register to disable this ad.

  17. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,806
    Location:
    Fort Dodge, Iowa
    DIP clip..I like chack-a-na-doodles..:D
     
  18. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    27,887
    Location:
    uk.. the middle bit
    is r38 good ?
    am sure i remember reading something about shorting pin 6 to ground as not being a good plan . ?
     
  19. Watthour

    Watthour Electron Rancher - JS3600

    It's probably not the best method, but PRESUMING (since the PA1001 internal schematic isn't published) that it's a typical sinking open collector output, grounding it shouldn't necessarily cause failure. I know that's a BIG presumption, but it's a common TTL and CMOS scheme.
     
  20. Watthour

    Watthour Electron Rancher - JS3600

    I just about had to. Typing in cap-lock usually gives me a my-brain headache. Besides, it fits pretty well alongside some of the slobber joints in the repair work.

    Coincidentally, and if I'm correct, the very first dim-bulb test in history was my ASVAB.
     

Share This Page