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Nad 705 repair

Discussion in 'Solid State' started by Eastham, Jan 23, 2018.

  1. NAD80

    NAD80 Super Member

    Messages:
    2,230
    Location:
    Carlisle PA
    Talked to the EE. He thinks the transistors might not be turning on, so the 32vdc stays high on the collector. Check that the Emitters are tied to ground. The base voltage is not high enough to turn on the transistors. Maybe try to turn on the transistor 409 by putting a 10K resistor between Base and Ground I think he said. Just trying to get into thinking the other way as to why they are not turning on.
     

     

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  2. Eastham

    Eastham More Class-A than ever!

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    Emitter of Q409 is to ground, Q410 isn't. Tried the resistor, voltage is still high.
     
  3. NAD80

    NAD80 Super Member

    Messages:
    2,230
    Location:
    Carlisle PA
    Did you try to turn on 410? I think 410 turns on first which then turns on 409. If you can get 1v to base of 410 and see if it turns on. And hopefully 409 will then follow and turn on.
     
  4. Eastham

    Eastham More Class-A than ever!

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    Ok so I tapped off pin 20 of IC403 1.4v and sent it to TR410 base, now I have 660mv on collectors.
     
  5. Eastham

    Eastham More Class-A than ever!

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    Something that may be worth noting I get 1v from pin 18 of IC403 for a brief second then it goes and that's when the voltage seems to float upto 32v.
     
  6. NAD80

    NAD80 Super Member

    Messages:
    2,230
    Location:
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    Sounds like that is what is happening. Wonder if IC403 is working correctly. The 1 volt for 410 comes from pin 18 of IC403. Have used a trick to see if a bad crystal in the reason a unit is not working. Put another crystal (from other tuner 705 PCB) across the other one PCB connections and see if it start working. Remove it from the other tuner gently with as little heat as possible to melt the solder. The 403 IC is trying to start but not staying in operation.
     

     

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  7. Eastham

    Eastham More Class-A than ever!

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    Doing that actually got it running for a little bit then It went away again, now I don't see the 1v blip anymore, just nothing. I also notice pin 20's voltage has dropped to about 1.25v. While it was working I was getting 660mv ish at both collectors
     
  8. NAD80

    NAD80 Super Member

    Messages:
    2,230
    Location:
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    Did you replace the crystal? If so try replacing the 403IC. I have to look up the datasheet for that IC and check some things.
     
  9. Eastham

    Eastham More Class-A than ever!

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    Will do tomorrow if that's ok, got a bit of a head cold and I'm feeling pretty drowsey right now, gonna go get some sleep. Another thing though, pin 8 of IC403 isn't getting the 11.9v it supposed to on FM, maybe that has something to do with this?

    Edit: Pin 8 is actually getting voltage, just me bein a numpty...
     
  10. NAD80

    NAD80 Super Member

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    Get some rest. At least we are going in the right direction. Glad to help. Did find the datasheet.
     
  11. Eastham

    Eastham More Class-A than ever!

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    Indeed we are! I can't thank you and your EE friend enough, without you or him I'd have probably gave up a while ago. I'm kinda dubious about using IC403 from the other board seeing as it showed similar symptoms, a new one from Little diode is about £5 so I may grab one from there as well just in case.
     

     

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  12. NAD80

    NAD80 Super Member

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    OK. We will pick this back up when the IC gets in. Get some rest and get better.
     
  13. Eastham

    Eastham More Class-A than ever!

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    Thanks, I'll get the chip on order now and report back when it gets here. I should hopefully be better by then and less of a zombie...
     
  14. NAD80

    NAD80 Super Member

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    All right. Good night.
     
  15. Eastham

    Eastham More Class-A than ever!

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    Good night.
     
  16. Alain L

    Alain L New Member

    Messages:
    4
    Sorry , i'm new not his site and i need help but i dont know how to start a new post. Anyway i need help with a Nad 200
    Good day,

    One channel with 3 out of 4 output shorted, driver shorted, and one small transistor on output board equally shorted. Replace every thing and use light bulb and was sounding on that channel very bad. Adjust bias and was playing better. I let it play for a while and then removed light bulb. Was cranking the amp slowly then blow output again. These output come from eBay and i' m afraid they might not be what there's suppose do be... 2sb 557 and 2sd427.

    What replacement should i use and does anyone have a schematic for this amp and know what is the procedure to adjust bias and current limit.

    Thanks Alain
     

     

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  17. Eastham

    Eastham More Class-A than ever!

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    Yeah, those ones from eBay where probably fakes, get a matched set of MJ21193 MJ21194's from Mouser or Digikey, those should substitute fine. Get your service manual here: https://www.hifiengine.com/manual_library/nad/200.shtml you have to sign up but it's free to use and they have SM's for most audio devices. Make sure you check some of the components down stream of the outputs too and more importantly check your voltages before going straight to AC.
     
  18. Alain L

    Alain L New Member

    Messages:
    4
    Thanks for you're reply,

    Already went to hifiengine but this is not the service manual and there's no schematic. Voltage was looking ok prior to going ac comparing with the other channel but i am a little surprise that the protection circuit did not is job and i dont like too much taking mesure on the good channel to compare because it is easy to touch something and create other problems. Curiously, the only output that went bad were the new probably chinese fakes... Was not doing repairs for years so i'm a little bit rusted... If i remember well we can remove the output and still test the circuit to see if there is more problems without damaging these more expensive output. I'm i right on this?

    Thanks for you're help
     
  19. Eastham

    Eastham More Class-A than ever!

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    Yes, you can remove the outputs and check the Emitter, Collector, Base voltages to make sure they're not to high.
     
  20. Eastham

    Eastham More Class-A than ever!

    Messages:
    2,165
    Location:
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    And as for the bias, best I can suggest is to find the bias voltage across the emitter resistor of the good channel and set it to that on the bad channel, that should put you in the ball park of "in spec" for now until we can find a service manual.
     

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