identifying cause of burnt out resistors

Discussion in 'Technical Repair' started by pjcfraley, Sep 13, 2018.

  1. pjcfraley

    Thread Starter New Member

    Sep 13, 2018
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    I am working to repair a Pioneer SX950 receiver. The left amplifier channel is not working and frys the voice coil of speaker woofer. I discovered two burnt resistors on the amplifier board (R37, R41, pictured). The transistor right next to one of the resistors had burn marks on it, but i think it was just residual from the resistor. Now need to identify the cause. The circuit protection is not working, but also thinking there may be an issue with the power supply board. Attached is schematic with problem resistors and transistor marked. If anyone could give me an idea of where to look that would be great
     
  2. danadak

    Well-Known Member

    Mar 10, 2018
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    Do you have a schematic of the amp ?

    Regards, Dana.
     
  3. abrsvc

    New Member

    Jun 16, 2018
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    have you checked the output transistors yet? I suspect that one or more are shorted which is what started this mess.
    I will look further in the service manual, but I would first check the outputs. The transistor that is circled appears to be part of the driver pair for the outputs. Since one channel is working, the easiest at this point is to A/B the 2 circuits. I would suspect a few more low value resistors are open as well.
    Dan
     
  4. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Can you post the circuit diagram from the manual also?
     
  5. abrsvc

    New Member

    Jun 16, 2018
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    I don't have the manual in electronic form. I have the paper one. Sorry.
     
  6. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Ok is there a circuit diagram on it, if so can you take a picture with your mobile/camera and post.it..
     
  7. abrsvc

    New Member

    Jun 16, 2018
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    Member of the old school, still have a flip-phone...

    I do have access to a copier though. PM me an address and I'll send out a copy of the schematic.
     
  8. Ylli

    Member

    Nov 13, 2015
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  9. abrsvc

    New Member

    Jun 16, 2018
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    The service manual can be found at the link below:

    https://www.hifiengine.com/

    Sign up and you can get access to a number of older unit service manuals there. (no cost to sign up!!)

    Dan
     
  10. pjcfraley

    Thread Starter New Member

    Sep 13, 2018
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    are you referring to the power supply? Attached. A copy of the Amp board was on my OP
     
  11. pjcfraley

    Thread Starter New Member

    Sep 13, 2018
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