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SX-780 Q20 concern with no sound or relay click

Discussion in 'Pioneer Audio' started by dfarrington, Dec 8, 2018 at 1:59 PM.

  1. dfarrington

    dfarrington New Member

    Messages:
    4
    Hello, I am new to working on the SX-780 and just getting back into electronics after 30 years. My SX-780 at first had a bad STK so I replaced both. Everything was fine and I hooked it up, turned it up nearly half way with 4 speakers and the protection circuit kicked out. I opened it back up and found the right channel STK shorted. I removed the right channel STK and protection okay and left channel worked. Before I stuck another STK in, I pulled the .22ohm resisters on the right channel and replaced with new. When I went to test to see if I still had the left channel, the relay did not click. I am now lost. I have pulled both STK's and here are the voltage reading across q25, 19 and 20 (3 transistors with heat sinks)

    Q25
    13.6
    14.1
    27.3

    Q19
    39.9
    40.4
    51.4

    Q20
    1v
    0v
    -54.3v

    Left Darlington pins with no STK in place
    1 1v
    2 -41.5v
    3 0v
    4 0v
    5 0v
    6 0v
    7 41.5v
    8 0v
    9 41.5v
    0 39.5v


    Right Darlington pins with no STK in place
    1 1v
    2 -41.7v
    3 0v
    4 0v
    5 0v
    6 0v
    7 40.8v
    8 0v
    9 40.8v
    0 39.6v
     

     

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  2. Watthour

    Watthour Electron Rancher - JS3600

    Q20 is unhappy for some reason.
     
  3. dfarrington

    dfarrington New Member

    Messages:
    4
    I replaced it and got the same readings. I did find what I believe is R260 (numbers on schematic are not clear) blown so I replaced both 200 ohm resistors. Do you think it could be Q24 causing bad readings on Q20?
     
  4. sregor

    sregor AK Subscriber Subscriber

    Messages:
    3,574
    Location:
    Mass
    To test amp without stk0050, you need to add 1k resistors between where pins 0 and 8 would go, and between pins 1 and 3. There is no path for feedback without a module or a resistor, and the output floats towards the rails, which causes protection circuit to not turn on.
    Q24 is a likely candidate for no voltage out of Q20. It is an FET used as a current source. Search current source in pioneer forums.
    There is also this thread where @kevzep bread boarded a two component solution for a current source. You may be able to get by with a resistor about 5k to 10k (my guesstimate - not fact) or search for other solutions. YMMV
     
  5. dfarrington

    dfarrington New Member

    Messages:
    4
    Thank you. I will get some 1k's and perform more research as you suggest and go from there. Thanks again.
     
  6. Watthour

    Watthour Electron Rancher - JS3600

     

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  7. dfarrington

    dfarrington New Member

    Messages:
    4
    Crystal clear. Thank you!
     

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