CR-420 with left channel distortion

Discussion in 'Yamaha' started by skidmarks70, Nov 15, 2017.

  1. zaibatsu

    zaibatsu Active Member

    Messages:
    473
    I don't think you need to turn it on if that short was the cause of the failure.

    You should pull the complementary output and see if that's failed as well, and then check the emitter resistors, and driver transistors, plus check for any visually burnt flameproof resistors. Basically when an output shorts, it usually takes out some other parts with it - your goal now is to find which parts those were, hopefully without blowing up more stuff.

    You'll want new output/s, shoulder washers, and whatever else is needed to make sure the same thing doesn't happen again - and it's probably worth re-doing with fresh isolators on the other output/s as well if you suspect this could happen on those.

    It's probably also a good time to build a dim bulb tester if you haven't already, so that when you fire it up with fresh parts, it doesn't blow another set of outputs if you missed something.
     
  2. skidmarks70

    skidmarks70 New Member

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    30
    Measuring in-circuit, it seems like everything is in good shape. The emitter resistors measured 0.8 ohms vs 0.47, but I was surprised my meter measured anything. I couldn't find any flameproof resistors on the schematic. There's a few big red resistors that are marked as 'metalized oxidation' on the schematic and those also are good.

    What do you mean by isolators?

    Thanks!
     
  3. avionic

    avionic " Black Knights " Subscriber

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    Mouser --- search " metal oxide"
     
    Last edited: Feb 8, 2018
  4. avionic

    avionic " Black Knights " Subscriber

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  5. avionic

    avionic " Black Knights " Subscriber

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    Subtract your meter lead resistance from .8 Ω. Short your two meter leads together to get a ballpark meter lead resistance.
     
  6. skidmarks70

    skidmarks70 New Member

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    Ah, so isolators are the little plastic insulators between the back of the transistors and the metal bracket? Those looked to be in good shape, but I suppose it's smart to replace them. I put some cpu thermal paste on both sides of the plastic when I replaced the outputs.

    When I short my leads I get 0.0 ohms for resistance. Below the threshold maybe?
     
  7. avionic

    avionic " Black Knights " Subscriber

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    Not plastic - but mica (muscovite) ... I pretty much always try to use the original insulators as long as they are not broken. Just clean off the old heatsink compound and apply a fresh thin layer of new compound. As for the plastic bushing. If they are cracked or deformed they get replaced.
     
    Last edited: Feb 9, 2018
  8. avionic

    avionic " Black Knights " Subscriber

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    What kind of CPU paste. Some of that CPU compound is electrically conductive. And is not compatible with your application.
     
    Last edited: Feb 21, 2018 at 11:27 PM
  9. avionic

    avionic " Black Knights " Subscriber

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    Most common handheld multimeters I've used are not very accurate below 1 Ω.
     
  10. skidmarks70

    skidmarks70 New Member

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    Well, I'm an idiot. The thermal paste I grabbed from my box was silver based. Oops. I'm still hopeful that the damage is limited to the one transistor. I need to get some new washers and clean off that paste.

    I'll report back when I make some progress.
     

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