Please help fix a receiver

Discussion in 'General Electronics Chat' started by iamAlex, May 30, 2009.

  1. iamAlex

    Thread Starter New Member

    May 30, 2009
    3
    0
    Hi all,

    I am struggling trying to troubleshoot the amplification circuit of a Yamaha rx-v870 receiver.

    I have checked all transistors using basic diode check and resistors and they seem ok. I am getting wildly fluctuating voltage at the output transistor's emitter.

    I am trying to isolate the amplification stage to verify that the output transistors are not leaking. Below is the circuit in question:
    [​IMG]
    To isolate the amplification stage, I have removed Q127 and Q129.
    The way I see it, if none of the 6 output transistors are leaking the voltage at Q154 and Q156 emitters should be 0v.
    Is this correct?
    Or is there something else I should do to find any problems with the output transistors?

    Thanks,
    Alex
     
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    You can't remove Q127 and Q129 and measure the output voltages as they are needed to bias the entire output stage.

    If you had crackling etc as the original fault, it was probably the PNP transistors Q117 and Q128 (2SA970) these little PNPs are known to become unstable and I've replaced a lot in many old '70s amps. A squirt with freezer spray will usually stabilise them to test. If you dont have freezer spray, you can hold a soldering iron 1/2" from them to warm them and listen to them go beserk.

    Also heat test the 3 signal diodes in the biasing, and it might be good measure to replace the few electro caps in an amp of that age. :)
    (edited typos)
     
  3. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    The 5-10 mV oscillation is probably just ripple voltage from your supply. What exactly are the symptoms of your broken radio?
     
  4. radiohead

    Active Member

    May 28, 2009
    474
    31
    A heat sink on your tabbed transistors (and maybe a CPU fan) will help dissipate heat
     
  5. iamAlex

    Thread Starter New Member

    May 30, 2009
    3
    0
    That 5-10mV is what the output should read. Not what I am seeing.

    I know that if I remove them I cant expect the voltages at the output transistors to be as indicated on the circuit diagram.

    My question was: I want to ensure that the 6 output transistors are not leaking (because I am getting high dc at the output). To do so I am thinking of decoupling the amplification stage by removing Q127 and Q129.
    When I do that, I am expecting to see an output of 0v at Q154,Q156 emitters.

    Is this a reasonable assumption?

    Thanks,
    Alex
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    With those transistors removed, there is no predicting what the drivers will do. The signal drive comes from Q117. The feedback goes to be base of Q116 from the line that originates between Q154 and Q156.

    You would be ahead to remove and meter check all your semiconductors. If you are concerned about it, put a couple of resistors in the power supply rails so the amp can't go to full power.
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Have you replaced PNP transistors Q117 and Q128 (2SA970)?? Do that straight away, then see if you still have DC issues at the output. :)
     
  8. iamAlex

    Thread Starter New Member

    May 30, 2009
    3
    0
    I cant find a Q116 in this diagram. Do you mean Q128

    QUOTE=beenthere;146929]
    You would be ahead to remove and meter check all your semiconductors. [/QUOTE]

    If you mean doing a simple diode conductance test on the transistors, I already did that

    Yes I have. still issues tho.

    Im not sure why no one can confirm that the output at Q154 and Q156 should be 0v with Q127 and Q129 removed (ei. no base voltage applied to Q141 Q144)?
     
  9. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    It seems to me that if you remove Q127 and Q120 the voltage at the bases of Q141 and Q144 will be floating and without a base current path. This means no current flowing between emitter and collector of the latter transistors with a rather meaningless zero volts at their emitters.

    It seems to me that a better approach would be to disconnect the output transistors and see if the drivers are fluctuating (causing the fluctuation at the output). Another check would be the voltage of the power rails at the collectors of the output xistors. This could indicate a supply problem.
     
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