NAD T760 right channel problem, help! - diyAudio
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Old 10th July 2014, 05:44 PM   #1
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Red face NAD T760 right channel problem, help!

Hello,

I'm new here and I hope you can help me with this surround receiver.
The problem is that I can't adjust the idle current (2mV +/- 0.2mV) of the right channel. This channel was blown up, there was a leg gone from the 2SA1941 transistor and it was on fire! I replaced almost all parts of this right channel, but still having problems. All other channels works 100%.

I'm all out of options.
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Old 10th July 2014, 06:49 PM   #2
Mooly is offline Mooly  United Kingdom
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If there is a just a problem with the idle current then the amp should basically work OK.

Does this faulty channel have sound ?

What is the voltage across C507 and does this voltage vary as you alter the bias pot ?

What is the voltage on Q519 base and Q521 base as measured with respect to ground. You should read around + 0.7 and - 0.7 respectively.
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Old 10th July 2014, 07:06 PM   #3
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Thanks!
I will measure these voltages.

If I have them, I will let you know.

Kind regards
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Old 10th July 2014, 07:09 PM   #4
Mooly is offline Mooly  United Kingdom
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OK

And be careful measuring. Its the voltage across C507 that is important, the other voltages are measured from ground.
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Old 14th July 2014, 07:40 PM   #5
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Hi Mooly,

I have the voltages. Across C507 = 2.14V, now P501 gives a voltage of 2 mV.
What I did: I replaced Q523, Q525, Q521, Q519, Q513, Q515 and Q517 in one time.
Is the voltage across C507 correct?

Thanks
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Old 15th July 2014, 06:21 AM   #6
Mooly is offline Mooly  United Kingdom
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The voltage across C507 is adjustable with the bias (idle) preset but the absolute value isn't critical or easily defined. I asked you to measure it initially because you mentioned the bias wasn't adjusting... so we needed to see what was there. If there were say 5 volts present then that would have pointed to an open circuit base-emitter junction somewhere in the driver or output stage.

When the voltage across C507 is high enough to overcome the four base-emitter volt drops of the drivers and output transistors combined then those transistors conduct and you get the idle current flowing in the output transistors. From that point, a very tiny increase in voltage would give a massive increase in idle current.

If you can adjust the idle current correctly having replaced all those transistors then its OK. I assume the amp is working now ?
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