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Pioneer SX-780 Powers On, No Sound

Discussion in 'Pioneer Audio' started by Thewierd1, May 11, 2018.

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  1. Thewierd1

    Thewierd1 New Member

    Messages:
    28
    Good day Audiokarma,

    Recently bought an SX-780 for little to nothing knowing it was dead and was wondering if I could get it working, however I don't know where to start.

    The symptoms I see so far are:

    - After power on there is no relay click at all from the protection circuit
    - The lights Power on.
    - When the tuning dial is moved the signal meter appears to do nothing / no response.
    - There is no sound ouput (duh)

    I've done some reading around on the forum but nothing seems to fit this description, any ideas where to start?
     

     

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  2. Watthour

    Watthour Electron Rancher - JS3600

    Test the power supply voltages at the regulator transistors, Q17, Q19, and Q25.
     
  3. Thewierd1

    Thewierd1 New Member

    Messages:
    28
    Thanks for your reply!

    Sorry I couldnt get back I was out of town.

    I have a schematic for the unit but am still not quite sure which ones I should be looking for, any specific description I should be looking for?
     
  4. Watthour

    Watthour Electron Rancher - JS3600

    Those with the heat sinks added to the collector tabs, near the left side of the chassis.

    [​IMG]
     
  5. dr*audio

    dr*audio Fish fingers and custard! Subscriber

    Messages:
    18,680
    Location:
    Cleveland, Ohio
    Actually, don't bother to test the voltages, examine the solder joints on those 3 transistors on the left. Look for a dark circular crack in the solder joint. If you see that or if the joints are grainy, not shiny, re-solder them all. Should fix the problem. If not, then you should check the voltages. This happens so often with Pioneer equipment that it's the first thing to check. They ran the transistors hot with too small heat sinks on them, and they desolder themselves.
     
    merlynski likes this.
  6. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,791
    Location:
    Glen Burnie Md.
    I've also seen the diodes de-solder themselves, so it might be a good idea to check them also.
     

     

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  7. rcs16

    rcs16 Super Member

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    Bloody amazing how those hot bjts last as long as they do considering how hot they run them at. It would have cost them very little to make bigger heatsinks. A simple large piece of aluminum is all = pennies. Got to blame those bean counters :)
     
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  8. Watthour

    Watthour Electron Rancher - JS3600

  9. merlynski

    merlynski Curmudgeon Electronicist Subscriber

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    Location:
    High Plains, Eastern Colorado
  10. Watthour

    Watthour Electron Rancher - JS3600

    They are helpful to elevate diodes and resistors from the board for cooling, and insulate the leads to prevent shorting to adjacent components. That can also be done simply by keeping the leads on replacement devices longer and forming them to suit, but without the electrical insulation benefit.
     
  11. Thewierd1

    Thewierd1 New Member

    Messages:
    28
    Hey thanks for all the replies guys! Really appreciate it.
    I've been super busy with my job that I haven't measured the voltages(still not quite sure how to do that, forgive me)

    However these are the 3 transistor joints from below
    How do they look?
     

    Attached Files:

     

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  12. Watthour

    Watthour Electron Rancher - JS3600

    I've seen far worse that that, still operating, too.

    [​IMG]

    Q25 output can be tested at the PA3004, Pin 1.

    Check these points as well:

    [​IMG]
     
  13. Thewierd1

    Thewierd1 New Member

    Messages:
    28
    Alright. Just on the way home now will check when I get there.

    Should I resolder or just leave those for now
     
  14. Thewierd1

    Thewierd1 New Member

    Messages:
    28
    Also when you say test the voltages are you referring to one lead on the case and one at the test point? (Learning as we go )
     
  15. Watthour

    Watthour Electron Rancher - JS3600

    One lead (usually negative) on the chassis or clipped to a good chassis grounding point, and the other for probing.

    Take the advice of many others, and insulate the test lead probe so that only the tip is exposed. Heat shrink, PCV electrical tape, or about anything will work. We just don't want to have anything adjacent get inadvertently shorted.
     
    Thewierd1 likes this.
  16. Thewierd1

    Thewierd1 New Member

    Messages:
    28
    Alrighty,
    So arrived home, and tested those points you pictured:

    the +39V point == +39.8V
    -39V point == -40.3V
    +13.5V point == ~0.10V

    Havent tackled the transistors (Nor do I know what I'm looking for there)

    Thanks again for your help, really appreciate it!
     

     

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  17. Watthour

    Watthour Electron Rancher - JS3600

    The second photo in Post 12 shows the voltage readings. Just above the "+39VDC" label is a ceramic/cement resistor, mounted to the main heat sink. Check the DC voltages at both ends of that. One end should be about 51V, and the other about 26V.
     
  18. Thewierd1

    Thewierd1 New Member

    Messages:
    28
    So:

    Left side: +44.4V
    Right side: +53.1V
     
  19. Watthour

    Watthour Electron Rancher - JS3600

    That would most likely indicate that there is no load applied.

    Dr. Audio advised earlier that the solder joints should be refreshed. That's even more important now, since the connections at Q25 have either failed, or there is a problem with Q25 itself. Disconnect power and re-flow the joints using a 30-50W iron, some clean solder, and a bit of flux.
     
  20. Thewierd1

    Thewierd1 New Member

    Messages:
    28
    Will do,
    I'll report back tomorrow.
     

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