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Old 30th August 2011, 02:11 PM   #1
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Default Need help with Nad 3020A

Hi everyone,

I just got a Nad 3020A amplifier which does not work properly.

At first, it exhibited fairly large amounts of noise at both outputs. I replaced every electrolytic capacitors and cleaned every potentiometers. I am now left with these issues (without any input signal) :

- I have -9V DC at the left output of the preamplifier, 0V at the right
- I have 10V DC at the output of the power amplifier on both channels.
- Huge amounts of noise coming out of both channels of the power amp (crackles)
- I hear a component making audible noise, but I haven't been able to figure out which one (somewhere around the transformer I think)

I checked the power supply, and it seems OK. Voltages are approximately as indicated in the schematics and seem stable.

I checked almost every transistor or diode in the power amp and the preamp section and they all seem fine.

Now, I'm kind of lost...

Anyone got any ideas ?

Thanks !
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Old 30th August 2011, 02:50 PM   #2
jaycee is offline jaycee  United Kingdom
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As the pre and poweramp stages can be seperated - test them seperately. Does the power amp behave normally when you connect it's inputs to ground, or feed it with an external signal?
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Old 30th August 2011, 03:03 PM   #3
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Quote:
Originally Posted by jaycee View Post
As the pre and poweramp stages can be seperated - test them seperately. Does the power amp behave normally when you connect it's inputs to ground, or feed it with an external signal?
I have done all the tests with the preamp and the power amp separated. They never behave normally, whichever the input is...
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Old 30th August 2011, 07:07 PM   #4
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This might help if you don't have it alredy.

http://p10hifi.net/planet10/TLS/down...viceManual.pdf

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Old 30th August 2011, 10:39 PM   #5
jaycee is offline jaycee  United Kingdom
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Only common component is the power supply really. It might be worth taking the transformer out and testing it separately. The regulated section is used by both the pre and power amp so my guess is the fault is there somewhere.
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Old 31st August 2011, 05:26 PM   #6
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Originally Posted by jaycee View Post
Only common component is the power supply really. It might be worth taking the transformer out and testing it separately. The regulated section is used by both the pre and power amp so my guess is the fault is there somewhere.
I have just retested the power supply with an oscilloscope and the voltages are OK and stable.
I am currently reflowing every solder joint, there might be an issue there...
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Old 31st August 2011, 05:40 PM   #7
DF96 is offline DF96  England
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Quote:
Anyone got any ideas ?
Have you tried fault tracing? Simple replacing things or 'resoldering' is as likely to introduce new faults as fix old ones.

Check voltages. Check where the noise starts from. Compare the two channels. Find the faults which are upsetting the DC voltages.
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Old 3rd September 2011, 07:53 AM   #8
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Finally found it !

For the power amp, it was a fried BC549C transistor (the first one on the signal path). For the preamp, it was a BC559B and a BC549C (the first two on the signal path).

What seems odd is that they looked OK when tested with my multimeter. This might explain why I had some trouble tracking them...
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