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-   -   Need help with Nad 3020A (https://www.diyaudio.com/forums/solid-state/195608-help-nad-3020a.html)

Nomdecode47 30th August 2011 01:11 PM

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... :bawling:

Anyone got any ideas ?

Thanks !

jaycee 30th August 2011 01:50 PM

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?

Nomdecode47 30th August 2011 02:03 PM

Quote:

Originally Posted by jaycee (https://www.diyaudio.com/forums/solid-state/195608-help-nad-3020a-post2691192.html#post2691192)
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...

planet10 30th August 2011 06:07 PM

This might help if you don't have it alredy.

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

dave

jaycee 30th August 2011 09:39 PM

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.

Nomdecode47 31st August 2011 04:26 PM

Quote:

Originally Posted by jaycee (https://www.diyaudio.com/forums/solid-state/195608-help-nad-3020a-post2691776.html#post2691776)
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...

DF96 31st August 2011 04:40 PM

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.

Nomdecode47 3rd September 2011 06:53 AM

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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