P2100 bias issue

Discussion in 'Yamaha' started by two.dogs, Jan 2, 2019.

  1. two.dogs

    two.dogs Active Member

    Messages:
    370
    Location:
    Denver, CO
    The service manual says to set idle current to get 13mV just after turn on or 22mV stabilized, One channel works great, but the other wont go below 26mV hot or cold. The weird thing is that the bias circuit is working on both channels. On the good channel I can adjust the b1 and b2 voltages to +.6v and -.6v and the idle current is bang on. I can turn down the adjusting pot and the bias voltages decrease down to about +/-.2v and the idle current pretty much shuts off. But I can do the same on the bad channel. Even when I turn down to +/-.2v the idle current is too high at 26mV. DC offset is less then .05v so that isn't a problem. I pulled the output transistors and they tested good on my Atlas. The .47ohm resistors also measure OK. With a bias of .2v the output transistors should be in cut off. I'm confused.

    John

    input side.JPG output side.JPG idle.JPG
     

     

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  2. avionic

    avionic " Black Knights " Subscriber

    Messages:
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    On the bad channel. What is the +B and -B voltage on the output collectors?
     
    Last edited: Jan 2, 2019
  3. two.dogs

    two.dogs Active Member

    Messages:
    370
    Location:
    Denver, CO
    Measured directly at the collector (after the molex connector)
    +51.35
    -51.45
     
    Last edited: Jan 2, 2019
  4. avionic

    avionic " Black Knights " Subscriber

    Messages:
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    Location:
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    Check all fusible resistors in the bias circuit.Transistors as well.
     
    Last edited: Jan 2, 2019
  5. two.dogs

    two.dogs Active Member

    Messages:
    370
    Location:
    Denver, CO
    OK. Looking at the schematic I see 8 of these per channel - all small value 330ohm or less. Resistor symbol with small open circles on either side. Must I pull them out or can I measure in circuit comparing one side to the other? Also count 8 transistors in the bias circuit - or do you mean the phase splitter as well?

    legend.JPG i_spy.JPG
     
    Last edited: Jan 2, 2019
  6. avionic

    avionic " Black Knights " Subscriber

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    Nice avatar btw.
     

     

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  7. two.dogs

    two.dogs Active Member

    Messages:
    370
    Location:
    Denver, CO
    Transistors TR8-15 check good on the Atlas. Fuse resistors also check good.
    223 (220),
    224 (220),
    475 (470),
    68 (68),
    22 (22),
    331, (330),
    335 (330),
    22 (22),
    122 (120)
    IMG_3887.JPG IMG_3889.JPG
     
  8. two.dogs

    two.dogs Active Member

    Messages:
    370
    Location:
    Denver, CO
    Measuring from PE to E, I can adjust the bias voltage up but then as I turn the pot ccw it loses its effect on the bias voltage at about the 11 o'clock position. However the pot still continues to lower the base voltage (b1 and b2) well into what should be the cutoff range for the outputs. Perhaps P1 is being held high. Maybe remove the output transistors and test voltages?
     
  9. two.dogs

    two.dogs Active Member

    Messages:
    370
    Location:
    Denver, CO
    Got it mostly reassembled and removed the output transistors. This shows voltage between PE and E on both sides. Hmmm.


    IMG_3899.JPG IMG_3900.JPG
     
  10. avionic

    avionic " Black Knights " Subscriber

    Messages:
    45,470
    Location:
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    Is there a test point for O (output). I'm not seeing how test point E can be used for bias adjustment. PE and O would put you across one of the emitter resistors. Ohms law --- 21.1 mv across an .47Ω resistor would put your bias at 45ma.

    I think there is an error in the published adjustment procedure.

    Test point E is an electrical ground.
     
    Last edited: Jan 11, 2019 at 10:17 PM
  11. two.dogs

    two.dogs Active Member

    Messages:
    370
    Location:
    Denver, CO
    Yep, there is a mistake in the service manual. Idle current should be set by measuring between PE and CT, not PE and E. If you do what the service manual says, then you'll get idle current plus DC offset. My DC offset is 26mv. Point "O" and "CT" are the same potential. My amp is fine.
     
    Last edited: Jan 12, 2019 at 1:26 PM

     

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  12. avionic

    avionic " Black Knights " Subscriber

    Messages:
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    :beerchug:
     

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