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NAD 7225 PE STOPPED WORKING, FUSE BLOWS WHEN REPLACED, REPLACED OUTPUT TRANSISTORS, FRIED RESISTORS, ONLY ONE CHANNEL WORKING

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Hello sir

you can go throughh this
www.Fixya.com/Nad+Manual

Posted on Jun 19, 2008

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NAD 7225 PE  STOPPED - 9ff2474.png
Keep us posted and thank you for using our service.
FixYa for all your troubleshooting needs.

Posted on Jun 19, 2008

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Hi,

A couple of things to check:
1. check the speakers;
2. since one channel is working, with no signal input and volume down, compare DC voltages of the left and right channels using the operational channel as basis;
3. doubt if problem involves power supply since as you posted the other half is working.

Hope this be of initial help/idea. Pls post back how things turned up or should you need additional information.

Good luck and kind regards.

Thank you for using FixYa.

Posted on Jun 19, 2008

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I know these amps well. The power envelope circuit has probably failed as well.

Posted on Jun 19, 2008

  • 3 more comments 
  • Graeme Ross
    Graeme Ross Jun 19, 2008

    I also change out any bd139 transistors for 2sd669 also, as the bd transistors often caused them to self destruct. I also reccommend replacing all the driver transistors as well as the output, and checking EVERY transistor in the DC coupled chain. Any transistor in the output stage could be causing it to malfunction. you also need to setup the bias correctly after any work to the output stages.

  • Graeme Ross
    Graeme Ross Jun 19, 2008

    can you tell me what parts you have replaced?

  • Graeme Ross
    Graeme Ross Jun 19, 2008

    The power envelope circuit supplies a "boost" voltage to the output stages. When output transistors fail, these transistors also go shorted, usually apparant by strange differing voltages on the collectors ot the outputs. I strongly reccommend that you softstart the amp with a variac if you have one at about 30% mains voltage to allow you to fault find without blowing fuses or damaging the new parts you have just fitted.

  • Graeme Ross
    Graeme Ross Jun 19, 2008

    OH, use a 2sb649 for the complementary transistor to the 2sd669. Change the bias control transistor(if its a bd139) mounted to the heatsink as well.

  • Graeme Ross
    Graeme Ross Jun 30, 2008

    How did you go with the amp repairs?

    robotek


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