NAD 3120 one channel loud buzz

[broth10000]

Hello all,

I've been trying to locate the source of a loud buzz on the right channel of a NAD 3120. It starts right away when I turn on the amp. It's not in the speakers as they work fine with my Technics amp. The left channel works correctly on the NAD3120 and sounds amazing with my dynaudio speakers.

I can't visually spot defective capacitors, although I can spot some glue which they have been attached with.

I have a multimeter but my only experience with it is to check for continuity, which I did for the ground connections (all seems fine) and I also checked for loose screws.

Any other suggestions? Thanks kindly for your ideas.

Best
Bert

 

[broth10000]

ps. when I remove the pre-amp jumpers the buzz remains.
when I plugin headphone the buzz is on the right headphone also
could it be something with the quirky volume knob?

 

[broth10000]

I have also tested swapping the channels with a pair of RCA cables on the back instead of jumpers. The hum remains on the right speaker... Any ideas?

 

[poundy]

did you resolve this?

 

[broth10000]

unfortunately not!

 

[poundy]

I'll have a think

 

[broth10000]

thanks so much!

 

[Mooly]

Does the channel that buzzes actually play music correctly (albeit with buzz) or not? If its just buzzing all the time we need to do basic voltage checks around the output stage.

The fact it does still buzz with the links removed eliminates the preamp (and volume and all that stuff)

 

[broth10000]

ah okay that is good to know...

it buzzes all the time, no music, just a really loud low buzz

 

[Mooly]

So this sounds like a failed output stage somewhere.

First thing to do is measure the DC voltage across the speaker terminals. Do NOT connect speakers though. Just measure the voltage with the amp on and no load.

 

[broth10000]

Not sure if i did it correctly. I used a velleman DVM831 digital multimeter using the V~ 500 setting (red in VΩmA, black in COM) and played a 750hz tone through aux input on half volume. The right channel gives and output of 33, the left of 1.

 

[broth10000]

I re-read your advice, and now measured without any amp inputs and volume on zero: R is 33, L is 0

 

[maudio]

You should measure dc, not ac.
The 500V~ range is ac

 

[broth10000]

Thanks!
R=around 15.10
and L=0.05

 

[Mooly]

So a DC fault on the right channel by the sounds of it.

Most failures usually come down to output transistors and their drivers but it would need more detailed fault finding now to see what has actually happened. That would mean careful measurement and being able to identify the correct parts on the board and also being able to isolate them as necessary for testing if needed.

It is up to you how you feel about doing that 🙂

 

[poundy]

def output and the 68ohm will be gone as well(check both), they are under rated-change to 1/2w ones when you do as only 1/4's are fitted along with the driver/s and sometimes the pre driver
normaly the + rail is affected on these faults, i rarly find the PNP gone

 

[poundy]

ah okay that is good to know...

it buzzes all the time, no music, just a really loud low buzz

Is the buzz a high or low tone

 

[poundy]

So a DC fault on the right channel by the sounds of it.

Most failures usually come down to output transistors and their drivers but it would need more detailed fault finding now to see what has actually happened. That would mean careful measurement and being able to identify the correct parts on the board and also being able to isolate them as necessary for testing if needed.

It is up to you how you feel about doing that 🙂

Mooly is right here. If you can, or able to remove both the output transistors and short out the C/E of the multiplier see if the centre voltage at the terminals comes down. It is difficult without the unit in front of you to look at.

 

[poundy]

C626 is a common point of failure on these
I always replace this cap regardless with either a 35v or 50v

are you using a lamp with the amp? ie not plugged direct in the mains