Yamaha CR-800 bias all over

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

  1. sssboa

    sssboa Well-Known Member

    Messages:
    854
    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 " F-15 Eagle Keeper Subscriber

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    47,832
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    Idle current = bias
    middle point = DC offset
     
  3. avionic

    avionic " Black Knights " F-15 Eagle Keeper Subscriber

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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:
    854
    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:
    854
    Yeah, DC offset, the thing that should be close to zero.
     
  6. avionic

    avionic " Black Knights " F-15 Eagle Keeper Subscriber

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  7. sssboa

    sssboa Well-Known Member

    Messages:
    854
    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 " F-15 Eagle Keeper Subscriber

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    Could be.Give it a shot.
     
  9. sssboa

    sssboa Well-Known Member

    Messages:
    854
    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
  10. avionic

    avionic " Black Knights " F-15 Eagle Keeper Subscriber

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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:
    854
    They're all good.
     

     

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  12. avionic

    avionic " Black Knights " F-15 Eagle Keeper Subscriber

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    Maybe.Maybe not. What did you test them with? Possibly failing under load.
     
  13. sssboa

    sssboa Well-Known Member

    Messages:
    854
    Know what you mean. I will replace all that I have replacements for.
     
  14. sssboa

    sssboa Well-Known Member

    Messages:
    854
    I tested with peak tester and some Chinese one.
     
  15. sssboa

    sssboa Well-Known Member

    Messages:
    854
    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
  16. sssboa

    sssboa Well-Known Member

    Messages:
    854
    I replaced the trimmers for bias and it works ok, nothing burns, bias sets alright. Strangely the original trimmers tested ok, but had a feeling from the start that trimmers could be the cause.
    The new Alps trimmers (dated 2002) are even more difficult to set than the vintage ones, have a shorter turn so are less precise I could say, but once set they're good.

    trimmers2.jpg
     

     

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  17. mbz

    mbz Super Member

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    2,583
    Location:
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    I stopped using single turn trimmers, maybe ok for dc offset but too flaky for bias.
    Prefer something like
    652-3296P-1-502LF, the 5k is within tolerance of the OEM 4.7k
    Certainly more expensive but any slight "knock" won't throw the bias too far out of adjustment.
     
  18. sssboa

    sssboa Well-Known Member

    Messages:
    854
    Yeah.
    I have these, cheaper than yours :smoke:
    https://www.digikey.co.uk/product-detail/en/bourns-inc/PV36Y502C01B00/490-2926-ND/666553
    but they are waiting for more important projects :)
     
  19. sssboa

    sssboa Well-Known Member

    Messages:
    854
    In one of my CR-800s the idle current / bias goes down from 25mV to around 13mV after a few months in both channels. DC offset does not change significantly in the same time.
    I wonder if the trimmers are to blame or transistors? I think I will never know unless I replace the trimmers with multiturn ones. :cool:
     

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