SX-780 Q20 concern with no sound or relay click

Discussion in 'Pioneer Audio' started by dfarrington, Dec 8, 2018.

  1. dfarrington

    dfarrington Member

    Messages:
    55
    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 Member

    Messages:
    55
    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:
    3,683
    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 Member

    Messages:
    55
    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 Member

    Messages:
    55
    Crystal clear. Thank you!
     
  8. dfarrington

    dfarrington Member

    Messages:
    55
    Okay. Time to start over. I still have no relay click. New STK's are installed. I had resistors in place of STK's, but went ahead and installed new STK's. I have kept it on auxillary , with no load, balance, treble and base neutral and volume all the way down. I've thrown several new parts at it only due to the receiver was mint and sounded awesome prior to relay shutting it down so here are all of the readings as of tonight. What should I do now?

    Left stk
    1. -41v
    2. -41v
    3. -39.5v
    8. -39.8v
    9. 41.3v
    0. -39.5v

    Right Stk
    1. -1.06v
    2. -41v
    3. 17mv
    8. 17mv
    9. 41.3v
    0. .563v

    Q25
    13.3v
    13.9v
    26.24v

    Q19
    39.6v
    40.3v
    49.8v

    Q20
    -24.7v
    -17.5v
    -54.5v

    PA3004
    1. 13.47v
    3. 0v
    4. -.8v
    5. 0v
    6. -5.2v
    7. 0v
    8. .47v

    Q20
    -24.7v
    -17.5v
    -54.5v

    Q23
    49.9v
    40.3v
    40.3v

    Q21
    14.1v
    40.3v
    14.7v

    Q24
    -16.95v
    -54.6v
    -54.6v

    Q22
    0v
    -16.9v
    -16.9v

    Q15
    39.2v
    3.5v
    38.6v

    Q13
    -23.7v
    -1.04v
    -22.9v

    Q11
    -23.1v
    -1.04v
    -22.4v

    Q17
    25v
    33.3v
    24.4v

    Q18
    -15.2v
    -18.6v
    -14.6v

    Q1
    1. 0v
    3. 0v
    4. -13.1v
    5. -11.8v
    6. 0v
    7. 0v
    8. 22.8v

    Q2
    Same readings as Q1

    Q3
    1. 20.4v
    2. 20.4v
    3. 0v
    4. -9.5v
    5. -.58v
    6. 0v
    7. 21.1v

    Q4
    Same readings as Q3
     
  9. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    28,345
    Location:
    uk.. the middle bit
    pins 3 and 8 are the main problem .. did you test the 0.22 ohm resistors ? then again 0 and 1 are wacky so needs looking at first . can you please edit in transistor readings which are b c e ?
     
  10. dfarrington

    dfarrington Member

    Messages:
    55
    Yes, I ordered new and tested them prior to install and they measured .5ohm so I put them in and no difference. I tested the old ones and they also measured .5 so I put them back in as they fit better than new.
     
  11. dfarrington

    dfarrington Member

    Messages:
    55
    Q25, 19 and 20 are listed looking at solder joints with silver face of SX-780 toward you. So Q25 is farthest away. Listed as left pin, right pin then middle pin as listed top to bottom. I haven't checked pin outs to know E, C,B. In addition, all other transistors are listed farthest away pin to nearest for each with SX-780 in the same orientation as noted for Q25.
     

     

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  12. merlynski

    merlynski Curmudgeon Electronicist Subscriber

    Messages:
    782
    Location:
    High Plains, Eastern Colorado
    You still have a problem with the power supply voltages around Q20. Until the power supply voltages are correct you put your new STKs at risk.
    The voltages on the left STK are out spec. I would put the 1k resistors back in place of the STKs and then fix the power supplies.
    Fix this problem. Once you get the power supplies fixed (still with 1k resistors) the relay should pull in, as DC offset will be close to 0vdc. If it is not then there are other thing to check before putting the STKs back in.
     
  13. Watthour

    Watthour Electron Rancher - JS3600

    Something is up with connections or components. Q20 base and Q22 collector should not be over half a volt apart, and Q22 B-C should not have the same voltages.

    EDIT: merlynski beat me to it. Since Q20 is among the always-suspect regulators for heating, it might be good to heat,re-flux, and resolder its joints. If that doesn't help, it may be reasonable to pull it and test thoroughly out of circuit, then check the solder pads, circuit foil traces, and surrounding components for faults and poor connections.
     
    Last edited: Jan 1, 2019
    merlynski likes this.
  14. dfarrington

    dfarrington Member

    Messages:
    55
    I will put the new .22 in tomorrow and see if it changes readings.
     
  15. dfarrington

    dfarrington Member

    Messages:
    55
    1k's going back in tomorrow night. I'll send update. Thanks all
     
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  16. dfarrington

    dfarrington Member

    Messages:
    55
    Q20 is an NTE292
    Q22 is an A904 from a working receiver
    Q24 is a K34 from a working receiver.
    Looking down at heat sinks on component side with silver face of sx-780 facing me Q23 and Q24 are installed with flat sides away from Q20.
    Q21 is mounted with flat side toward Q20. Q22 is mounted with flat side away from Q20. I should note that R318, Q20 and Q24 are currently jumpered on the -53.4v trace due to grommet came off at Q20.
     

     

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  17. merlynski

    merlynski Curmudgeon Electronicist Subscriber

    Messages:
    782
    Location:
    High Plains, Eastern Colorado
    Page 24 of the SX-780 Service Manual from HiFiengine.com has the pinouts for the original transistors (and ICs) so you can ID and verify the proper pins are in the proper holes in the pcb. If you have a non-factory component you will need to verify pins from the matching datasheet.
     
  18. Watthour

    Watthour Electron Rancher - JS3600

    I would be very suspicious of that NTE292 in the spot for Q20. In the first place, it is not original, so someone has been down this path before, and it's probably not a coincidence. Second, a better replacement is probably going to be a KSB596Y or similar device.

    Apparently, this circuit has seen high current in the past if someone had to perform repairs with grommets and foil. Again, the root cause may not have been rectified the first time around.
     
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  19. dfarrington

    dfarrington Member

    Messages:
    55
    Thank you all. Stk's pulled and 1K's put in their place. Q20 is back to what I tested as a good 2sc536a. Still no relay click, but I am retaking readings.

    Q20
    B = -18.2v
    C = -54.1v
    E = -26.6v

    More to follow.
     
  20. Watthour

    Watthour Electron Rancher - JS3600

    What is the voltage across C310?

    It would also be helpful to know if R311, 312, and 313 are close to specs.
     

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