Pioneer SX-650 - troubleshooting power amp board

Discussion in 'Pioneer Audio' started by ViperZ, Feb 9, 2019.

  1. ViperZ

    ViperZ AK Subscriber Subscriber

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    Hi all, I finally found some time to play with another receiver. This will be Pioneer SX-650.

    Original symptom was - relay clicking on and off.

    - measured all power supply voltages - all correct
    - measured all voltages to the power amp board - all correct

    Right channel DC offset = 0.41V stable. Left channel DC offset is jumping up and down between 0 and ~1V. I can hear something shorting on the power amp board when relay clicks.

    Measured voltages on 4 pins of the power pack in left channel:
    * Pins 1/5 = +/- 36V
    * Pins 2 and 6 - instead of +/- 1.2V , I have the same weird jumping DC on both pins (right channel is solid +/- 1.2V)

    Would next step be breaking path(s) between 2SA798 and the SP-40W power pack?

    Thank you.

    IMG_20190209_152202.jpg
     

     

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  2. ViperZ

    ViperZ AK Subscriber Subscriber

    Messages:
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    Location:
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    Pulled out dual-PNP transistor - put it on my transistor tester - both halves are ok. Turned on the receiver - all power pack pins are +/- 32V - this power pack is done. Now I just need to make sure it didn't take any resistors, C9 electrolytic or especially Q3 transistor with it.

    These receiver are such a pain in the butt to disassemble to get the power pack out... Lucky me I have another SX-650 that was physically damaged and can offer a fresh power pack.
     
  3. ViperZ

    ViperZ AK Subscriber Subscriber

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    Can anyone tell me how to replace that sp-40w module? I can't seem to get to the scress? Do I have to desolder BOTH modules first???
     
  4. ViperZ

    ViperZ AK Subscriber Subscriber

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    Might as well replace the whole thing...

    IMG_20190209_195627.jpg
     
  5. avionic

    avionic " Black Knights " F-15 Eagle Keeper Subscriber

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    Those scress's are a pain.
     
  6. ViperZ

    ViperZ AK Subscriber Subscriber

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    Scress screws. What happens when you need that damn autocorrect :)

    I couldn't get to the screws that hold Darlington packs so had to desolder both modules.. on both receivers. Will swap the whole thing in, reinstall double PNP, and hopefully it will work. Q3 junctions ring ok in circuit. C9 is not blown.

    Funny fact - this receiver is from 1976, my parts receiver is from 1968.
     

     

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  7. avionic

    avionic " Black Knights " F-15 Eagle Keeper Subscriber

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    Parts is parts..
     
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  8. ViperZ

    ViperZ AK Subscriber Subscriber

    Messages:
    315
    Location:
    Toronto, ON, Canada
    Replaced the Darlington, put everything together - same problem. Should have gone with my gut instinct. Replaced double-PNP transistor - everything works like a charm. The damn thing showed fine on low voltage test on the transidtor tester.
    Now I have two working Darlingtons available for future fixes lol.
     
  9. zebulon1

    zebulon1 Building a new bench. Finally! Subscriber

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    Darlingtons are finicky. They can test fine but not work in operation. Test bad and their bad.
    Not practical for most but I have a stash of known good common ones and use them when troubleshooting becomes illogical.
     
  10. ViperZ

    ViperZ AK Subscriber Subscriber

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    Location:
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    In this case it was only 2SA798. I just have never seen such weird behavior from a pair of transistors.

    IMG_20190210_172540.jpg
     

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