Pioneer SX-828 Bias is negative

Discussion in 'Pioneer Audio' started by grindfix, Mar 19, 2017.

  1. grindfix

    grindfix Member

    Messages:
    91
    Following previous posts I've replaced all e-caps on PS, Prot, Head, AF and all transistors and E-caps on Main amp board along with new trim pots. Turned it on without load or dummy resistors through DBT and took measurements. Bias is within -0.030mv on both sides and I do not have enough trim pot adjustment to go positive. DC offset is able to set 0. Turned it on without DBT and Bias starts climbing to over -0.060mv as unit warms up and keep climbing so I turned it off.
    Few questions:
    How critical is for Bias to be on positive vs. negative mv in my case?
    Does Bias climb indicate a problem or because of no load at speaker terminals?
    Is it a good idea to attempt Bias and DC adjustment with dummy 10 ohm resistors?
    Thanks
     
  2. markthefixer

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

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    Bias - meaning IDLE CURRENT only has one polarity, it will not adjust through zero to the opposite polarity.
    What / where is your procedure you are following, I may take exception to it.
    dummy resistors (10 ohm etc) are not needed.

    Idle current climb is undesirable. it's called thermal runaway. and 30 mV may be too high DEPENDING UPON WHERE YOU ARE MEASURING IT.
    If you are measuring across just one 0.5 ohm emitter resistor, your current is twice as high as you want.
    The directions seem to at least have your meter leads reversed.

    In the past I have even discovered and confirmed errors in Pioneer's adjustment procedures contained in the service manuals.
     
  3. grindfix

    grindfix Member

    Messages:
    91

    Mark,
    Thank you for helping out!
    The SX-1280 you helped me with sounds like new now.

    I'm using Bias / DC offset recommendations from your 2007 post
    http://www.audiokarma.org/forums/index.php?threads/offset-bias-voltage-on-sx-828.94933/#post-963551
    I did make sure my negative leads are as you specified on 11 & 12 and positive on 17 & 18.
    Are those possibly posted reversed in error?
    Thanks
     
  4. grindfix

    grindfix Member

    Messages:
    91
  5. markthefixer

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

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    Location:
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    Yes, my post from 2007 was incorrect. Ten years ago it's been waiting to bite me. I fixed it.

    Start out a no idle current, turn it up a bit and see what happens, if it stays stable or climbs and check how hot the output transistors and the q9 - q12 drivers get. let it sit for a few minutes watching for any increases in temp or current. It may be that after a certain point above the 20mA it starts to run away.

    it is possible that a reversed transistor could do this. the feedback system will minimize many "gotcha's".

    The STV-4 MUST be in contact with the heat sink that holds the output transistors, it is SUPPOSED to turn DOWN the idle current as the heated output transistors naturally try to climb in current, a warm transistor's turn on voltage drops - so a fixed base current tends to increase and must be turned down.
     
  6. grindfix

    grindfix Member

    Messages:
    91
    Thank you, Mark!
    Having very close measurements on both channels I doubt there is a reversed transistor somewhere. I was using spec sheets from Mouser for all transistors and paid attention to all PCB markings, double-checking myself. I did not replace D4 and D5. They are still mounted on output heatsinks. Initial problem with receiver was that Bias measurement was taken with meter in the mAmp mode which smoked 10 ohm R42 and VR4 and, of course the outputs. After realizing what happened I've decided to replace all transistors and e-caps on main amp board.
    I'll be back with results soon.
    Thanks
     
  7. petehall347

    petehall347 the brandy coffee man Subscriber

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    did you renew the outputs emitter resistors ?
     
  8. grindfix

    grindfix Member

    Messages:
    91
    No. I did not replace emitter resistors. All good though. Adjusted Bias to 20mv (it was climbing for about 10-15 minutes and I kept going between channels hoping not to run out of adjustment room) Played music with 12" JVC junk floor speakers and have to agree with many of you guys, this receiver sounds very nice. Heat sinks are not even warm after about 40 minutes. Will be putting it back together tomorrow.
    Mark, as always, thank you!
     

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