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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

    Messages:
    540
    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

    Messages:
    1,572
    Location:
    uk
    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

    Messages:
    4,846
    Location:
    Las Vegas Nevada
    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

    Messages:
    540
    Location:
    New Albany, OH
    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

    Messages:
    660
    Location:
    philippines
    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

    Messages:
    3,933
    Location:
    Baldwin, Ontario, Canada
    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

    Messages:
    21,085
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    Bensenville,Illinois
    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

    Messages:
    540
    Location:
    New Albany, OH
    No, all original. The only board that I worked on is PS board.
     
  9. grindfix

    grindfix Full time hobby Subscriber

    Messages:
    540
    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

    Messages:
    540
    Location:
    New Albany, OH
    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:
    540
    Location:
    New Albany, OH
    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

    Messages:
    1,572
    Location:
    uk
    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:
    540
    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.
     
  16. grindfix

    grindfix Full time hobby Subscriber

    Messages:
    540
    Location:
    New Albany, OH
    I connected 8 ohm dummy loads and turned it on with DBT. No issues. Connected to outlet directly and re-adjusted DC offset. Idle current starts from around 8mv and settles at around 20mv after 20 minutes. Then I connected CD player to Aux and played into dummies at approximately 50% of volume. Heatsinks got really hot. Can't keep finger on them for more then 2 seconds. After source was turned off idle current was around 32mv and within a minute or so dropped to around 10mv, then came back to 20mv as heatsinks cooled down. So, I think amp is operating properly and able to back current down. Its very possible that with high CD player output voltage I ran amp at the max capability into clipping.
    I will repeat that with the scope tomorrow and measure output power. How hot is too hot for heatsinks though?
     

     

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  17. Watthour

    Watthour Electron Rancher - JS3600

    The typical threshold of pain for human skin is around 112°F (just under 44°C). Permanent skin damage (3rd degree burns) can occur as low as 140°F (60°C). A conscious person typically cannot voluntarily hold skin on a surface at 125°F for more than a few seconds.

    For electronic components, degradation normally does not occur until reaching a minimum of 185°F (85°C). You will have to actually measure the temperature of the heat sink, calculate the thermal resistance of the component, and determine whether the actual semiconductor junction temperatures are above a danger threshold, usually 100°C (212°F) at a bare minimum, and more often 125° or 150°C. As a conservative number, if the heat sink is near 160°F (70°C), something may be approaching a danger level.
     
  18. grindfix

    grindfix Full time hobby Subscriber

    Messages:
    540
    Location:
    New Albany, OH
    I took measurements. Injected 1kHz sine wave into both channels. Current in series with speaker measured at 2.9A just before clipping into 8ohm dummy loads. 2.9x2.9x8=67wpc.
    Heatsink temp is at 74C. If I back it down to 2.5A heatsink temperature drops to 70C or below.
    This receiver is rated at 54wpc music into 8ohm loads. I think it all adds up. Max power output = max heatsink temperature.
    Sounds like I'm safe, correct?
     
  19. Watthour

    Watthour Electron Rancher - JS3600

    It would seem that at its rated output, the temperatures would be acceptable. Your supposition that the output level of the CD player is well above the 200mV input rating for AUX and TAPE inputs is probably correct. Many CDs output at five times that level, or more.
     
  20. grindfix

    grindfix Full time hobby Subscriber

    Messages:
    540
    Location:
    New Albany, OH
    I can see clipping lines on the scope while playing music from CD player well before output reaches 1.5A mark. Sounds like clipping occurs in preamp section. Possible?
     

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