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I have rebuilt one the amp boards of a Pioneer spec-4 amplifier. The bias is good but I'm getting to much drift(+ to -) on the DC offset and it is to high. The new trim pot does work but the Dc offset voltage is just to unstable.I have replace all the questionable transistors including the outputs (mje15093/94 ). Do i have to have all match pairs (hFe) of transistors for the amp to be stable ? And what else can check . All the supply voltages to the board are good and stable. What do you think

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  • calman46 Sep 28, 2009

    Hi , I'm going to replace those varistors D2,D3,D4,D5 I think their leaking too .The transistors are new replacements but have the same or better specs than the(blown) originals . The protection circuit is working and I have the bias turned down a little so I should be safe. I'll recheck the resistors .I figured and open resistor would give no Dc offset at all. Thanks

  • calman46 Oct 02, 2009

    I'll keep you updated , thanks

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  • Master
  • 5,807 Answers

Hi,

you may need to replace the transistors connected to it.

same with the other transistor(s) on VAS circuits.

should have at least 60 mA , 80 mA max





more than 80mA will damaged your output transistor.

you may need also to check on the resistors

Posted on Sep 28, 2009

  • 5 more comments 
  • Rommel Cana
    Rommel Cana Sep 28, 2009

    try setting your bias to lower setting like 10 ohms or max is 100 ohms


    here is your guide





    there might also be leaky diode ( 3 diode in series)

    if still having problems , if the transistors on top and bottom is OK , you may need to temporary

    shunt 18 ohms resistor on either of the two 3 diode in series

    note: you might see only 1 pc of the 3 diode in series, the one i encircled


  • Rommel Cana
    Rommel Cana Sep 28, 2009

    if the drifting is too high, ie more than 4V

    then you might need to extract the output transistors to be safe.

    continue tracing, setting the initial 60 mA

    till the output is zero, only then you connect the output transistors


  • Rommel Cana
    Rommel Cana Sep 28, 2009

    your amp will work with small speakers just for testing before connecting the output

    transistor.


    please note , that it will make the driver transistor hot,

    but it wont be destroyed.


    if it is destroyed you have other problem, finished with amp working to a little speaker

    then if it is stable you can connect the output.



    when measuring output voltage on the amp, don't connect speaker.to be very sure of the output of the amp, and not damaged your small speaker


  • Rommel Cana
    Rommel Cana Sep 28, 2009

    good luck , and keep me posted, remember 60 mA no more than 80 mA

  • Rommel Cana
    Rommel Cana Sep 28, 2009

    the collector on the upper should have at least 1,1 V

    the collector on the lower should have at least -1.1v


  • Rommel Cana
    Rommel Cana Sep 30, 2009

    a good amplifier never produce more than 0.7V on its output

  • Rommel Cana
    Rommel Cana Oct 01, 2009

    try also to check the differential inputs, sometimes it also causes the problem.

    the transistors and the resistors connected to it, assuming that the final output transistors

    are not connected.


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