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Nad 705 repair

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

  1. Eastham

    Eastham More Class-A than ever!

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    One other thing I forgot to mention, I'm getting 32v on the collectors of Q409 and Q410 and I'm getting 17v on the emitter of Q401.
     

     

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  2. NAD80

    NAD80 Super Member

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    Sorry about the delay. Are the voltages at Q401 on the base and collector close to the schematic. What are Q403,Q404,Q402, Q405, Q409, Q410 base, collector, emitter voltages?. And what are the voltages on IC 403? Been trying to understand how the circuit works.
     
  3. Eastham

    Eastham More Class-A than ever!

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    Voltages for the transistors. Going E,C,B

    Q401: 6.86v 120mv (fluctuates) 6.86v
    Q402: -15.86v 7v -15.4v
    Q403: 5.10v 11.86v 5.8v
    Q404: 0v 8v 7.6v
    Q405: 5.01v 11.6v 5.5v
    Q409: 0v 31.5v 0v
    Q410: 6mv 31.5v 10mv

    Going to meaure all the voltages on IC403 now.
     
  4. Eastham

    Eastham More Class-A than ever!

    Messages:
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    Voltages for IC403

    1: 1.737v
    2: 0
    3: 0
    4: 0
    5: 0
    6: 5v
    7: 0
    8: 0
    9: 10.77v
    10: 0
    11: 66.7mv
    12: 71.8mv
    13: 71.9mv
    14: 2.79v
    15: 5.04v
    16: 5.05v
    17: 0
    18: 0
    19: 0
    20: 1.37v
     
    Last edited: Feb 11, 2018
  5. NAD80

    NAD80 Super Member

    Messages:
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    Location:
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    Check the R452,R453,R454, R456 resistors, Caps C445,C446,C450. Hope I got the Parts right. The schematic is not too clear when magnified. Wondering if the 32vdc is coming from the 32vdc power supply line. 7.6vdc on base of Q404. The collectors of Q409,Q410 at 31vdc.
     
  6. Eastham

    Eastham More Class-A than ever!

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    All the resistors check fine and I've replaced all three of those caps. I think that is where the 32v is coming from.
     

     

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  7. NAD80

    NAD80 Super Member

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    Desolder one side of R452 from the PCB. If the 32v goes away then we know the source.
     
  8. Eastham

    Eastham More Class-A than ever!

    Messages:
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    Yep, it's gone. But interestingly I'm reading 590mV on the collector of Q409 now.
     
  9. NAD80

    NAD80 Super Member

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    Wonder if there is a solder wick/ bridge getting the 32 v to the collectors of 409,410. R453 fails under load? Since the 10k should drop the voltage from 32v.
     
  10. Eastham

    Eastham More Class-A than ever!

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    I tried replacing R453 with a part from the other Nad with no luck sadly, checking for a short between the connector and the collector of Q409 shows roughly 9.8k. it seems impossible that I could have 32v on these components but somehow I do..
     
  11. NAD80

    NAD80 Super Member

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    It will be something unexpected at this rate, you already done the obvious fixes.
     

     

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  12. Eastham

    Eastham More Class-A than ever!

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    It's certainly a puzzle.
     
  13. NAD80

    NAD80 Super Member

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    You have replaced the 10K resistor with an identical resistor. Have seen resistors fail under load and measure good with a DMM.
     
  14. Eastham

    Eastham More Class-A than ever!

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    I used R308 from the main PCB of the other receiver which I assume should be good.
     
  15. NAD80

    NAD80 Super Member

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    Location:
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    1/4W vs a 1/6 W part better wattage part.
     
  16. Eastham

    Eastham More Class-A than ever!

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    I figured it would suffice and being from a known working part of the receiver would help me troubleshoot.
     

     

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  17. NAD80

    NAD80 Super Member

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    The 1/4 R308 watt is better than the 1/6W part. Thinking about running this by a EE at work tomorrow.
     
  18. Eastham

    Eastham More Class-A than ever!

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    It's a really curious problem isn't it?
     
  19. NAD80

    NAD80 Super Member

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    Yes it is. Will let you know if I get further ideas on the cause of this. And the fix.
     
  20. Eastham

    Eastham More Class-A than ever!

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    Thanks I really appreciate it.
     

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