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Yamaha CR-800 bias all over

Discussion in 'Yamaha' started by sssboa, Nov 6, 2018.

  1. sssboa

    sssboa Well-Known Member

    Messages:
    657
    So Yamaha CR-800
    I had it with dead left channel. Problem on the main amp board.
    I reflowed all soldering on the left half, and boom, I had the left channel back and clear.
    So I put it all together. Checked bias/middle point as they call it in the manual. For the left channel it was oscillating between - 30/+30mV, very unstable. Suddenly it was all over - 300/+300mV with OL open circuit now and then. It stayed like that. I have heavy crackle in the left channel now. The other channel is all ok. The relay clicks ok. Idle currents set ok.
    Any ideas?
     
    Last edited: Nov 6, 2018

     

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  2. avionic

    avionic " Black Knights " Subscriber

    Messages:
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    Location:
    Fort Dodge, Iowa
    Idle current = bias
    middle point = DC offset
     
  3. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,685
    Location:
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    If its DC offset that's erratic. Look for issues in the input differential ie long tailed pair.
     
  4. sssboa

    sssboa Well-Known Member

    Messages:
    657
    I just had a little accident.
    I turned on the receiver with the TR717 output touching the receiver's case/ground.
    Some elements burst into flames.
    I checked and the outputs including TR717 are ok.
    The driver (?) TR715 is shorted which burnt R733; D705 and C727 got scorched just by being physically close to the resistor R733, I guess.
    You think touching the metal case/collector of TR717 to ground caused 715 to short or just a coincidence? I haven't checked other transistors.

    cr800trtr.jpg
     
  5. sssboa

    sssboa Well-Known Member

    Messages:
    657
    Yeah, DC offset, the thing that should be close to zero.
     
  6. avionic

    avionic " Black Knights " Subscriber

    Messages:
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    Location:
    Fort Dodge, Iowa
     

     

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  7. sssboa

    sssboa Well-Known Member

    Messages:
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    OK, so I will replace the whole pair TR713 and TR715. I have replacements, just need some rearrangement of legs. Maybe the dying TR715 was the cause of my initial problem?
    So is this pair the usual reason for DC offset problems?

    What about the pair TR709 and TR711? Would this one be a problem for the idle current thing? :)

    What do you think about preventive replacement of transistors in general? Actually I have replacement for all transistors and diodes on this board :)
     
    Last edited: Nov 7, 2018
  8. avionic

    avionic " Black Knights " Subscriber

    Messages:
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    Location:
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    Could be.Give it a shot.
     
  9. sssboa

    sssboa Well-Known Member

    Messages:
    657
    So I replaced the burnt resistor R733 and the shorted transistor TR715 (I replaced its counterpart on the other /right/ channel too). I switched on, same crackling from left channel, DC offset all over, impossible to set, showing open loop quite often. I switched off and on again and the new resistor got on fire like the original one. The original, even when burnt, maintained the specs, the new one fell into pieces. I used carbon 1/4W, a bit thicker than original.
    I checked the TR715 but this time it's not shorted (or maybe it doesn't read yet?). I checked TR713 and it's not shorted either. So what burns the resistor?
     
    Last edited: Nov 14, 2018 at 5:30 PM
  10. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,685
    Location:
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    Check TR 707,709 and 711 and all the diodes. Something is holding TR715 at rail and burning up your resistor.
     
  11. sssboa

    sssboa Well-Known Member

    Messages:
    657
    They're all good.
     

     

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  12. avionic

    avionic " Black Knights " Subscriber

    Messages:
    43,685
    Location:
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    Maybe.Maybe not. What did you test them with? Possibly failing under load.
     
  13. sssboa

    sssboa Well-Known Member

    Messages:
    657
    Know what you mean. I will replace all that I have replacements for.
     
  14. sssboa

    sssboa Well-Known Member

    Messages:
    657
    I tested with peak tester and some Chinese one.
     
  15. sssboa

    sssboa Well-Known Member

    Messages:
    657
    So everything on the left side I replaced with new components or swapped with the right side counterparts.
    The said resistor still burns on switch-on. I switched on through a dimming bulb which was shimmering.

    Reminds me my problem with CA-1000. I gave that one to a shop. They said that the problem was lack of ground.
     
    Last edited: Nov 15, 2018 at 2:01 AM

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