Question about NAD C320 2-Channel Amplifier

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Amp : sansui au11000 blown transistors on driver board

I recently replaced my amps output transistors. i couldnt souce the originals so i used mj 15024 & mj 15025 .i checked for any shorts with 40 watt bulb in series with power input . checked dc offset ok!.i then took it to repair shop to have bias set (i dont own an occiloscope) the repair guy used a steel screw driver & subsequently burnt out 8 transistors on the side of the driver board he was adjusting.(all around the trimpot) here is my question! he sez that the amp shorted out because of a pre-existing condition i know its a stupid question to ask but what do you think happened? regards ross

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  • redrooster Jan 16, 2009

    hey thats uncanny that is exactly what i ended up finding what was wrong! there was another transistor on board f-2568 tr06 that was a bit wonky that i replaced, it made it go into protection.

  • aircraften Mar 18, 2009

    denon pma630 neg.output tr.brakedown.i tried to repl.o/p tr.driver tr.but still valid

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Sometimes just replacing the outputs that have shorted are not enough to complete the repair of a main amp...this is because sometimes there is an intermittant or full failure in the bias circuit. Typically the Bias POT is the culpret.....when these pots get dirty or fail, the bias current ramps up way too high and then the Output transistors blow.......always replace the POT as part of the repair.....also check the bias varistor (Diode Stack) as these can open up after the first failure. Always pay attention to the bias circuit as this is critical to completing the repair properly....bias current adjustments can be made by measuring the voltage accross the Emitter Resitors connected to the Power Transistors...should read about 10 millivolts. The MJ15024 and MJ15025 Outputs are perfect replacements for Your Power Amp...I use them as well. Your tech probably just touched the bias pot, and that was all it took to make it fail (Open up)..If he was savvy, he would have replaced that POT before Touching it.

Hope this info helps....

Eric

Posted on Jan 16, 2009

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May i ask you did you put silicone grease in your transistor and apply a good mica insulator in all of your transistor---------look for any burnt resistor or explosive capacitor -----check for scew tie your supply voltage to earth by wrong.

Posted on Aug 03, 2008

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