SA-8100 protection circuit issue

Discussion in 'Pioneer Audio' started by KeithD, Jul 2, 2017.

  1. KeithD

    KeithD Active Member

    Messages:
    228
    Location:
    La Crosse, WI
    Went back and measured voltages on the power amp board:

    Pin Volts
    3 25
    23 1

    1-2 0.7
    24-25 14.5

    Clearly this is way off from what they were before replacing the transistors. What does this indicate? Did something get fried when I was un-soldering/soldering?
     
  2. KeithD

    KeithD Active Member

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    228
    Location:
    La Crosse, WI
    I see on the schematic that pins 3 and 23 connect to the base of Q3 and Q4, respectively, which are the 2SA726 resistors I replaced with KSA992FBU.
     
  3. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

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    The replacement transistors should be reversed from the originals when in position. BCE vs ECB.
     
  4. KeithD

    KeithD Active Member

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    228
    Location:
    La Crosse, WI
    I guess I don't understand that. The data sheet shows the assignment of each pin, which for the 992 is ECB looking at the flat side. The circuit board in the amp has the diagram indicating what each pin is supposed to be: the pin with the arrow is E, pin on opposite side of plate is B, and pin without arrow, but on the same side of the plate as E, is C. This is shown in the diagram Pete posted in post 31. Am I missing something?
     
  5. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

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    No sorry it sounds like you did it correctly. If the voltages have only changed since you replaced transistors then it has to be something here.
     
  6. KeithD

    KeithD Active Member

    Messages:
    228
    Location:
    La Crosse, WI
    OK, well not really sure what to do from here. What should my next step be to troubleshoot the problem? Should I take out the 992s in Q3 and Q4 and put the 726s back to see if things go back to where I started? Are there other tests to pinpoint the problem?

    Appreciate any help at this point!

    Keith
     
  7. mattsd

    mattsd AK Subscriber Subscriber

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    It sounds like you put them in properly, if we could see a photo of the board with the new transistors on it, it might help in diagnosis.
     
  8. KeithD

    KeithD Active Member

    Messages:
    228
    Location:
    La Crosse, WI
    OK, here are pictures of the left and right half of the power amp board. File Jul 14, 5 24 36 AM.jpeg File Jul 14, 5 25 02 AM.jpeg

    One of the things I am concerned about is the fact that the revision A and D power amp and protection boards are not the same. The layout is different as are many of the connections. The service manual available on hifiengine has schematics for revision A. It also has a document called "Schematics", which just has schematics of each board, but they are all revision D. Comparing the two sets, there are many differences on those two boards.
     
  9. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

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    Yes you definitely have them in the correct way. Did you just tilt board over without disconnecting any of the wires? Make sure all of the wires are on correctly and secure.
     
  10. Goldie99

    Goldie99 Active Member

    Messages:
    488
    Location:
    The original Boston, UK
    You might want to check R24 (?) as well (bottom left of your first photo, just above terminal 2) - looks burnt or damaged in the photo.
     
  11. mattsd

    mattsd AK Subscriber Subscriber

    Messages:
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    Yep, they are in the correct way. However, R23 (the resistor directly in front of pin 2) looks like it has been toasted and if so is probably bad. Currently getting ready to head out the door to work so I can't diagnose the cause right now, but desolder and lift one leg of that resistor and check it with a multimeter to see if it still reads around 10 ohms.

    Edit: Goldie beat me to it, R24 is for the right channel though.
     
  12. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

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    Well spotted guys, it is late here and obviously my eyes aren't working well enough for me to have seen that burnt resistor :rolleyes:.
     
  13. KeithD

    KeithD Active Member

    Messages:
    228
    Location:
    La Crosse, WI
    OK, thanks guys. Will check that this afternoon when I get home. I was able to scrounge a new 10 ohm 1/2 W replacement here at work, so I've got one to put in there if it is bad, though I assume I should try to find the cause before putting in a new one.
     
  14. KeithD

    KeithD Active Member

    Messages:
    228
    Location:
    La Crosse, WI
    Pulled one end of R23 and measured resistance, and it is open, so that one is clearly bad. Now, I'll need next steps to determine the cause...
     
  15. zebulon1

    zebulon1 This summer heat slowed me down. Subscriber

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    Check everything.
    The ceramic caps should be good.
    So resistors, diodes and transistors get checked.
    I'll check the resistors in situ, and only lift one leg if the value is off.
    The metal can transistors are always suspect.
     
  16. KeithD

    KeithD Active Member

    Messages:
    228
    Location:
    La Crosse, WI
    OK, I can check resistors, and I think I know how to check if a diode is ok, but I do not know how to check a transistor. What do I measure on which leads?
    Thanks!
     
  17. zebulon1

    zebulon1 This summer heat slowed me down. Subscriber

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    I'm away from my data but I would replace them (transistors) anyway. There should be a list somewhere in this forum.
     
  18. KeithD

    KeithD Active Member

    Messages:
    228
    Location:
    La Crosse, WI
    Checked all the resistors on the power amp board and all register pretty close to the correct value. Checked diodes and got a reading for each one. Did not check the transistors. Four are the new ones anyway. Replaced R23. Still getting -35V on pin 3 and about 1V on pin 23.
     
  19. petehall347

    petehall347 the brandy coffee man

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    uk.. the middle bit
    disconnect q5 base and see if offset drops to zero .
    if it does drop check the differential
     
  20. KeithD

    KeithD Active Member

    Messages:
    228
    Location:
    La Crosse, WI
    OK, please explain what you mean by "drop check the differential."

    Thanks

    Edit: disconnected base on Q5, voltage on pin 3 was 31V.
     
    Last edited: Jul 15, 2017

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