Pioneer SX-828 Idle current won't go below 25mv

Discussion in 'Pioneer Audio' started by grindfix, Sep 8, 2018.

  1. grindfix

    grindfix Full time hobby Subscriber

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    Location:
    New Albany, OH
    This receiver had typical Power Supply Transistor / Resistor overheat situation. Everything still was functioning so I decided to renew PS board components per Mark's part list. While I was waiting on parts to arrive I cleaned all controls and switches along with cleaning of Bias and DC offset trimmers to prevent them from opening up on me during adjustment. This happened to me before and caused pretty bad situation on SX-929.
    Now receiver is back together and I'm taking amp measurements. DC can be adjusted close to zero with very minimal fluctuation. Idle current is 20.5mv on one channel and 25mv on another with trimmers at min. I took pictures of trimpot positions before I moved them for cleaning. Original position of the trimmers was somewhere in the middle. That tells me that bias was way too high, I wish I had actually measured it before doing any work to it. I just didn't want to keep receiver ON with PS board looking so scary.
    Idle current being so close to Mark's specified 20mv I suppose my situation is not really bad but probably has a tendency to get out of hand at some point. What would you guys recommend to do?
     

     

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  2. inductor

    inductor Super Member

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    quiescent doesnt have to be exact, if there are no iffy components i would leave it
     
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  3. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

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    Replace them?
    It should be able to set to zero.
    If you removed them to clean, check the solder connections.
     
  4. grindfix

    grindfix Full time hobby Subscriber

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    Location:
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    I did not remove trimmers for cleaning. Soaked with deox-it for a while followed with about 20 end to end rotations. They respond to movement with gradual bias increase, so wiper is in contact. Will try to clean some more and order replacements if no result. Shouldn't be anything else causing this, right?
     
  5. john stumpf

    john stumpf AK Subscriber Subscriber

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    not necessarily.there are lots of noisy 45+ years old transistors in that and you can bet the e caps have seen better days theres two zeners that should be replaced as well. if you only want to do the vr,s do use multi turn bornes units you wont be sorry.
     
  6. rcs16

    rcs16 Super Member

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    I am not sure about the SX-828 as i'd have to look but simple things like the socket contact resistance for the TO-3 metal can o/p bjts can affect the bias current settings. clean them up and find out. :)
     

     

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  7. markthefixer

    markthefixer On Hiatus, dealing with Dad's estate full time Subscriber

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    Did you replace any transistors in the power amps?

    That area is pretty sensitive, they run a 100 ohm pot there...
     
  8. grindfix

    grindfix Full time hobby Subscriber

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    Location:
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    No, all original. The only board that I worked on is PS board.
     
  9. grindfix

    grindfix Full time hobby Subscriber

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    Location:
    New Albany, OH
    I desoldered one of the Idle current VRs and tested it out of circuit. It measures at 100.5 ohms. Wiper is in contact and measures at 0.3-100.2 ohms. I don't believe replacing trimmers will help here.
     
  10. Watthour

    Watthour Electron Rancher - JS3600

  11. grindfix

    grindfix Full time hobby Subscriber

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    Location:
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    47.3v/-47.3v
    13.3v/-13.3v
     

     

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  12. Watthour

    Watthour Electron Rancher - JS3600

    Those measurements seem very close to what they should be. Some resistance measurements around the drivers and pre-drivers might reveal if something has drifted more than 10-15% off its tolerance.
     
  13. grindfix

    grindfix Full time hobby Subscriber

    Messages:
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    Location:
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    Tested all resistors on one channel. Had to desolder several to eliminate parallel bleed through. All very close to expected.
    Can protection board have any effect on this condition? When VR3/4 are at lowest idle current position on the start-up idle current is around 5mv, then it slowly creeps up-to 20mv on one channel and 25mv on another. It does not go over. All measurements are without any load. No dummies or speakers. No input. Selector in Aux with volume at "0". Tested with headphones. Signal goes through. Sounds good and very dynamic. No hum, hiss or crackle at idle.
     
  14. inductor

    inductor Super Member

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    for a 40yr old circuit lots will have changed, unless you spend weeks finding what the problem is, if it sounds ok ..........................
     
  15. grindfix

    grindfix Full time hobby Subscriber

    Messages:
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    Location:
    New Albany, OH
    That approach will work for a while. Short while, most likely, before current will get out of hand and burns up the amp when someone decides to crank it up a notch.
    I'm just hoping someone ran into this and have a suggestion. I may pull amp module out and try cleaning output transistor sockets today. New thermal compound won't hurt either.
     

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