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Pioneer SX-780 need help signal tracing. Powers on no sound. Darlington packs are good.

Discussion in 'Pioneer Audio' started by Kingbleu, Oct 2, 2018.

  1. merlynski

    merlynski Curmudgeon Electronicist Subscriber

    Messages:
    432
    Location:
    High Plains, Eastern Colorado
    FWIW pin 7 is NOT used on the STK0050 . The pins used are 1,2,3 & 8,9,0. Inputs are 0 and 1; Power supply pins are 9 (+) and 2 (-); Outputs are 8 and 3.
    Pioneer SX-780 amp.jpg

    If you remove the STK0050 and put 1K resistors in you can get the DC loop closed for troubleshooting without the STK.
    Pioneer SX-780 dc-test.jpg
    This worked like a charm for me.

    Also did you replace Q17 with new? I did not see it so stated. Post 19 lists the BE junction as 0.4 volts, not enough to be a well biased on PN junction.

    EDIT: more text
     
    Last edited: Oct 6, 2018
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  2. rcs16

    rcs16 Super Member

    Messages:
    3,893
    Location:
    Baldwin, Ontario, Canada
    Looks okay to me, possible low Vbe due to low collect current being draw.
    OP did not state if he changed Q17
    measured out of circuit or one leg lifted, it should measure the same as R308

    and you have proper V at R301-304?

    are both channels the same? if not use the good channels parts for & as comparison, use to swap, figure out what is faulty.
    do a ohmmeter test on the parts in the bad channel and compare against the good channel. even swap the stk if one is good.

    Meryl's method is good in order to close the DC feedback loop and test the input and voltage amp gain stages.
     
    Last edited: Oct 6, 2018
  3. Watthour

    Watthour Electron Rancher - JS3600

    Q16 is in the right channel, as a driver for the STK module. It is the functional equivalent of Q15 in the left channel.

    Q17 is a power supply regulator.

    Are we certain of the component number designations? Both Q5 and 16 should have close to the regulated voltage of Q19, or 39V.
     
  4. rcs16

    rcs16 Super Member

    Messages:
    3,893
    Location:
    Baldwin, Ontario, Canada
    I do not think we even know what channel is bad :)
    I am using the big jpeg file of the schematic as it is the easiest one to read.
     
  5. Watthour

    Watthour Electron Rancher - JS3600

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  6. Kingbleu

    Kingbleu Active Member

    Messages:
    182
    Location:
    Southern California
    No I did not replace Q17 with new. I did test it on a Peak Atlas DCA55 semiconductor tester. I can replace it with a known working one if need be.
     

     

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  7. Watthour

    Watthour Electron Rancher - JS3600

    Q17 appears to be producing the correct regulated voltage, despite the somewhat low collector input.
     
  8. Kingbleu

    Kingbleu Active Member

    Messages:
    182
    Location:
    Southern California
    Do you have your own site? I would like to check it out. That is a nice schematic how did you make it. Do you use Photo Shop?
     
    Last edited: Oct 6, 2018
  9. Kingbleu

    Kingbleu Active Member

    Messages:
    182
    Location:
    Southern California
    Thats right I messed up the pin numbers. I will go back through the thread and make corrections to the previous posts. Merlynski thanks for the breakdown on what pin does what for the STK, I think now that I know what pins are power supply and what pins are Inputs/Outputs I can start tracing the signal and I should be able to figure out what channel is good and which is bad.
     
    Last edited: Oct 7, 2018
  10. Kingbleu

    Kingbleu Active Member

    Messages:
    182
    Location:
    Southern California
    .
    :rflmao:
     
    Last edited: Oct 6, 2018
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  11. Kingbleu

    Kingbleu Active Member

    Messages:
    182
    Location:
    Southern California
    I went back and took one leg out of circuit now resistors R307, R308, R309, R10 measure good.
    I checked Voltage and have proper voltage at R301-R304
    Both channels are the same for this section.
     

     

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  12. Kingbleu

    Kingbleu Active Member

    Messages:
    182
    Location:
    Southern California
    After testing Q15 it appears that the left channel is the one at fault.

    Q15 is not testing good
    E 39.3v
    C 35.2v
    B 38.6v

    Q13 not testing good
    E 1.0v
    C 33.7v
    B 1.4v

    Q11
    E 1.4v
    C 33.6v
    B 1.9v



    Are you referring to the misnumbered pins on the Darlington packs?
     
  13. Kingbleu

    Kingbleu Active Member

    Messages:
    182
    Location:
    Southern California
    Relay is clicking R259 200ohm was blown. I put in a 220 ohm resistor (for testing thats all I had) and now the relay is clicking. Here are the readings now.

    Q15
    E 38.5v
    C 4.1v
    B 37.7v

    Q13
    E -37.7v
    C -1.0v
    B -37.1

    Q11
    E -37.1v
    C -1.0v
    B -36.6v

    Left channel Power Pack pins 3 and 8 seem to be outputting to many millivolts.
    pin1 -1.16v
    pin2 -32.3v
    pin3 64mv
    pin8 78.4mv
    pin9 32.2v
    pin0 1.2v
     
  14. Watthour

    Watthour Electron Rancher - JS3600

    I was asking about the voltages shown in post #22. The voltages shown for Q16 seem to be more appropriate for Q17.
     
  15. rcs16

    rcs16 Super Member

    Messages:
    3,893
    Location:
    Baldwin, Ontario, Canada
    I am not sure if it is your sx-780 schematic, if it is you did a great job. 1200dpi

    Good job on finding R259, as you discovered in this case most troubleshooting can be done with power off and the ohmmeter

    Left channel Power Pack pins 3 and 8 seem to be outputting to many millivolts.

    It looks okay, measure the DC offset of both channels. probe L1 & L2 to ground?
    measure the bias current, DMM mV across stk pins 3-8, divide by 0.44 to get bias I. should be around ~30mA
     
    Last edited: Oct 7, 2018
  16. Kingbleu

    Kingbleu Active Member

    Messages:
    182
    Location:
    Southern California
    Pop Kabooie
    While measuring the bias current my probe slipped. :yikes: Now both STK-0050's are toast.
    Before I blew it up I set DC balance to zereo and bias current was around 32ma in left channel. In the process of measuring bias current for the right channel the probe slipped.
    I'm gonna take the rebuilt packs out of my working SX-780 and install them. I didn't get a chance to text AUX but the FM tuner was still dead no power no needle moving.
     

     

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  17. merlynski

    merlynski Curmudgeon Electronicist Subscriber

    Messages:
    432
    Location:
    High Plains, Eastern Colorado
    Suggestion: Don't mess with your good SX-780, use it for a reference. Insulate all but the very tips of your meter probes (guess how I know). Put 1K resistors in place of bad STKs (see post #21 again). Re-verify all power supplies' voltages off DBT, you cannot trust they will all be true and correct until off DBT. Verify DC levels in power amp circuits. Once this is done remove 1K resistors, put in a new set of the STK replacements (like you built for your other SX-780). Check DC offset and bias adjust functionality ON DBT, set bias to minimum, go off DBT, recheck DC offset, set bias correctly.
     
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  18. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

    Messages:
    2,705
    Location:
    Calgary, Alberta
    Get yourself a pair of the mini-grabbers! Worth it...... Makes testing safer and easier!!
     
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  19. sregor

    sregor AK Subscriber Subscriber

    Messages:
    3,496
    Location:
    Mass
    DC off set should be half way between pins 3 and 8, so just means your DC is about 71 Mv. Can be adjusted some.... Doubt seriously it has anything to do with output modules. Can usually be improved by changing the differential pair if out of adjustable range. Really not that bad....
     
  20. Kingbleu

    Kingbleu Active Member

    Messages:
    182
    Location:
    Southern California
    Well boys I got some sound :bowdown: all be it dirty, gritty,with a nice humming in the background, sound it is. I put in the newly rebuilt STK modules. Bias is set at 7mv and DC balance is set to zero.
    AUX will not produce sound
    TAPE 1, 2 produces sound
    Flipping the switches loud pop then it goes into protection comes back out of protection 60 seconds later.
    Flipping the switches with the volume all the way down to zero does not put it in protection.

    Im going to give it a deoxit bath and see what happens.
    Thanks everyone, we are getting closer.
     
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