CR-420 with left channel distortion

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

  1. zaibatsu

    zaibatsu Well-Known Member

    Messages:
    569
    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.
     

     

    Please register to disable this ad.

  2. skidmarks70

    skidmarks70 New Member

    Messages:
    32
    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

    Messages:
    40,048
    Location:
    Fort Dodge, Iowa
    Mouser --- search " metal oxide"
     
    Last edited: Feb 8, 2018
  4. avionic

    avionic " Black Knights " Subscriber

    Messages:
    40,048
    Location:
    Fort Dodge, Iowa
  5. avionic

    avionic " Black Knights " Subscriber

    Messages:
    40,048
    Location:
    Fort Dodge, Iowa
    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

    Messages:
    32
    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?
     

     

    Please register to disable this ad.

  7. avionic

    avionic " Black Knights " Subscriber

    Messages:
    40,048
    Location:
    Fort Dodge, Iowa
    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

    Messages:
    40,048
    Location:
    Fort Dodge, Iowa
    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
  9. avionic

    avionic " Black Knights " Subscriber

    Messages:
    40,048
    Location:
    Fort Dodge, Iowa
    Most common handheld multimeters I've used are not very accurate below 1 Ω.
     
  10. skidmarks70

    skidmarks70 New Member

    Messages:
    32
    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.
     
  11. skidmarks70

    skidmarks70 New Member

    Messages:
    32
    OK, so I just found that the emitter leg on TR419 was completely detached from the transistor. Obviously this needs to be replaced. Any suggestions for an A817?

    Would it be reasonable to assume that the damage would have stopped there? I didn't see any other signs of damage, but if I'm going to be ordering some more parts anyways, I'd rather spend a couple extra bucks.
     

     

    Please register to disable this ad.

  12. avionic

    avionic " Black Knights " Subscriber

    Messages:
    40,048
    Location:
    Fort Dodge, Iowa
  13. skidmarks70

    skidmarks70 New Member

    Messages:
    32
    I finally have some progress to report. After testing a bunch of components, I found 5 wrecked transistors and a fried diode. I replaced all of that and built a dim bulb tester. When I powered up, there was no bright light! No smoke! I have sound!

    But (always a but), when there's no signal (or have the volume all the way down) I get some noise in the left channel. The amount of noise is only affected by the loudness setting. Also, I checked the idle current setting and I'm reading about 1 mV between TP1 and TP2 and that doesn't change when I adjust VR401. So something is still broken.
     
  14. avionic

    avionic " Black Knights " Subscriber

    Messages:
    40,048
    Location:
    Fort Dodge, Iowa
    Check over your work for cold solder connection or solder bridges. Also check for damaged solder pads. And solder connections of circuit board connections to ground (E).
     

Share This Page