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SX-880 in Protection

Discussion in 'Pioneer Audio' started by Motnick, Oct 22, 2018.

  1. Motnick

    Motnick Active Member

    Messages:
    338
    Location:
    Wyandotte Michigan
    I picked up this SX-880 over the summer and overall it’s in nice shape. Unfortunetly it won’t come out of protection. I have been pushing myself to troubleshoot this one on my own and I have made some good progress, but it seems like I’m going to need some advice on this one if I ever hope to listen to it.

    When I first got this unit I powered it up on the DBT and the bulb stayed dim but no relay click. I checked the offset and bias, and discovered that the bias could not be adjusted up from zero. I also discovered that Q36 in the protection circuit was shorted. I replaced it with KSC1845, but the unit stayed in protection.

    Next, based on lots of previous threads on this site about the SX-880, I reflowed the power supply transistors which are known to unsolder themselves. Still no luck.

    I did some more measuring of the voltages throughout the unit and I discovered that the regulated supply voltages (+41.5/-41.5) were missing. I was able to track the missing regulated supplies to a shorted Q28 which I replaced with KSC2383. After changing Q28 the supplies returned and I was able to adjust the bias in both channels to 30mv. I adjusted the offset in one channel down to 0mv, but I could not get the other channel below 200mv. I tried cleaning all of the controls with D5 to see if that would help bring the offset down, but I was still unable to get it below 200mv until I pulled one end of the “P” jumper wire.

    With the “P” jumper pulled, I now have the bias and offset adjusted properly in both channels. Unfortunetly the protection issue remains and I cant seem to track it down on my own. Time to ask AK for some help here.

    I’m wondering if Q35 might be bad?

    Q35 (PA3004 on DBT while unit in protection)
    1 = 13.25v
    3 = .548v
    4 = 95mv
    5 = 0v
    6 = -12.6v
    7 = 9vAC
    8 = 1.15v

    [​IMG]
     
    Last edited: Oct 23, 2018

     

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  2. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
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    pin 3 looks a bit low .. check q27 to rule out it out .
     
  3. Motnick

    Motnick Active Member

    Messages:
    338
    Location:
    Wyandotte Michigan
    I pulled Q27 2SB536 and it tests fine with a 6-way diode check.
     
  4. petehall347

    petehall347 the brandy coffee man Subscriber

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    pin 8 according to blurry schematic i have says 7.4v .. i do not know if this is a clue or not .
     
  5. Motnick

    Motnick Active Member

    Messages:
    338
    Location:
    Wyandotte Michigan
    Maybe an isssue with C417?
     
  6. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
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    possibly yes .. worth a look .
     

     

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  7. Motnick

    Motnick Active Member

    Messages:
    338
    Location:
    Wyandotte Michigan
    The tester on my meter only goes up to 20uf and I don’t have a 33uf cap in my parts stash to try. I pulled the cap and it does rail my meter though.
     
  8. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
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    explain rail your meter ?
     
  9. Motnick

    Motnick Active Member

    Messages:
    338
    Location:
    Wyandotte Michigan
    The capacitance tester on my meter only goes up to 20uf. If you put a larger cap onto the meter the reading will climb up until it goes above 20uf and then it reads nothing, meaning the capacitance is above 20uf. It’s a cheap meter.
     
  10. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
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  11. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
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    Location:
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    not sure what c417 is for ..
    there are recent threads about the pa3004 and i do know some voltages are marked incorrectly on some schematics .. no idea if they are right or wrong here .
     

     

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  12. Motnick

    Motnick Active Member

    Messages:
    338
    Location:
    Wyandotte Michigan
    Looking at the schematic more, and keeping in mind that your schematic is blurry, you did mean to have me check Q27 2SB536 in the power supply and not Q37 2SC1885 in the protection circuit?
     
  13. petehall347

    petehall347 the brandy coffee man Subscriber

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    relay driver transistor . was thinking base emitter leakage
     
  14. Motnick

    Motnick Active Member

    Messages:
    338
    Location:
    Wyandotte Michigan
    Let me check it. Im pretty sure that I pulled it and checked it previously, but I’ll check it again.
     
  15. Watthour

    Watthour Electron Rancher - JS3600

    By George, I think he's got it:

    [​IMG]
     
  16. Motnick

    Motnick Active Member

    Messages:
    338
    Location:
    Wyandotte Michigan
    I removed Q37 (2SC1885) and tested it with the 6-way test, and it passes the test. I went ahead and replaced it with a KSC2383 anyway. I have the same results even with Q37 replaced.

    (PA3004 - on DBT)
    1 - 13.3v
    3 - .514v
    4 - 94mv
    5 - 0v
    6 - -12.6v
    7 - 9.0v AC
    8 - 1.15v
     

     

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  17. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
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    try it off DBT
     
  18. Motnick

    Motnick Active Member

    Messages:
    338
    Location:
    Wyandotte Michigan
    PA3004
    1 - 13.25v
    3 - .521v
    4 - 92mv
    5 - 0v
    6 - -12.6v
    7 - 9.25v AC
    8 - 1.16v
     
  19. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
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    voltages at 65 and 68 ?
     
  20. Motnick

    Motnick Active Member

    Messages:
    338
    Location:
    Wyandotte Michigan
    65 - -35mv
    68 - -10mv

    I notice that voltage at 65 starts around -35mv and falls to close to zero after the unit is left on for a few minutes. 68 remains stable the entire time.
     

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