NAD 3020i Hum

otherside · 25th July 2006, 11:41 PM

Hey guys,

Besides trying to build my own chipamp i have a NAD 3020i, i'm quite sure many of you know that amp it's a very common model. It's quite an old one around 15 years of age. It was on storage for a while and about 3 years ago i started using it again. It's no fantastic sound but also not bad.

Since the "revival" of it it has a constant hum on the speakers, this is quite static, it doesn't change with volume change or input, it's always there, not extremely loud but enough to annoy.

Today i got an oscilloscope from work to check out the frequency and shape of this noise. Frequency is around 100-110Hz and shape of a sine (well not exactly but close) amplitude is around 60mV p-p with another 60mV DC offset. As i suspected it's possibly from the psu since fully rectified 50Hz beomes double if not smoothed correctly (very very ballpark). I tried changing the large caps which are 4700u/50V with some brand new ones 10000u/63V and not much changed. I also tried bypassing them with smaller values (1u, 10u, 1n, 100n) but nothing changed.

Any ideas how can i reduce this huuuum?

If you need more details let me know.

(As far as i can tell the output stage is a pair of MJ2955/MJ3055)

Thank you very much

Gold_xyz · 26th July 2006, 04:34 PM

if you have already replaced all the cap on psu you can be tried
to short-circuit the inputs and to see from where noise produces him.

and then with the oscilloscope to go up again step by step to
the input that produces the noise...

also check all ground connection points.

(sorry for my terrific english...

)

bye

ash_dac · 26th July 2006, 05:57 PM

Hi,

I had a nad 3130 before I gutted it. These things really don't last.

1. dry solder joints absolutely everywhere on mine

2. corroded rca jacks, and rusty transformer.

3. bulging capacitors everywhere!

Have you tried connecting source equipment to different rca sockets ?

I would look for bulging caps.

AKN · 26th July 2006, 08:57 PM

otherside,

How is things progressing for you?

You may check +43V and -28V voltages that comes from regulators built around Q507 and Q508. These regulators have feed from separate winding on transformer and powers pre amp and voltage amplifying stages in power amp.
Caps C516 and C517 are smoothing caps after rectifier and should be candidates to check.

otherside · 26th July 2006, 09:52 PM

Thanks a lot guys for the replies

I'll look into it on more detail,

does anyone know if/where i can find the schematics of this model ?

I tried to follow the noise and see where it's generated, it quite a wild guess without the schematics but from what i can see it's not comming from the pre-amp section, most likely the decoupling near to the output stage is damaged or just the components past their lifetime. Anyway a schematics or component ref would help a lot.

AKN · 26th July 2006, 10:05 PM

Mail me and I send schematic in return.

Gold_xyz · 27th July 2006, 06:56 PM

i have found a schematics here

otherside · 27th July 2006, 07:08 PM

thanks a lot guys, the schematics help a lot, i'll let you know how it goes.

25th July 2006, 11:41 PM	#1
otherside diyAudio Member Join Date: Jul 2006 Location: Edinburgh	NAD 3020i Hum Hey guys, Besides trying to build my own chipamp i have a NAD 3020i, i'm quite sure many of you know that amp it's a very common model. It's quite an old one around 15 years of age. It was on storage for a while and about 3 years ago i started using it again. It's no fantastic sound but also not bad. Since the "revival" of it it has a constant hum on the speakers, this is quite static, it doesn't change with volume change or input, it's always there, not extremely loud but enough to annoy. Today i got an oscilloscope from work to check out the frequency and shape of this noise. Frequency is around 100-110Hz and shape of a sine (well not exactly but close) amplitude is around 60mV p-p with another 60mV DC offset. As i suspected it's possibly from the psu since fully rectified 50Hz beomes double if not smoothed correctly (very very ballpark). I tried changing the large caps which are 4700u/50V with some brand new ones 10000u/63V and not much changed. I also tried bypassing them with smaller values (1u, 10u, 1n, 100n) but nothing changed. Any ideas how can i reduce this huuuum? If you need more details let me know. (As far as i can tell the output stage is a pair of MJ2955/MJ3055) Thank you very much

26th July 2006, 08:57 PM	#4
AKN Enjoy good sound diyAudio Member Join Date: Aug 2005 Location: In the middle of Sweden	otherside, How is things progressing for you? You may check +43V and -28V voltages that comes from regulators built around Q507 and Q508. These regulators have feed from separate winding on transformer and powers pre amp and voltage amplifying stages in power amp. Caps C516 and C517 are smoothing caps after rectifier and should be candidates to check. __________________ / Anders

26th July 2006, 10:05 PM	#6
AKN Enjoy good sound diyAudio Member Join Date: Aug 2005 Location: In the middle of Sweden	Mail me and I send schematic in return. __________________ / Anders

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26th July 2006, 04:34 PM	#2
Gold_xyz diyAudio Member Join Date: Dec 2005 Location: Italy	if you have already replaced all the cap on psu you can be tried to short-circuit the inputs and to see from where noise produces him. and then with the oscilloscope to go up again step by step to the input that produces the noise... also check all ground connection points. (sorry for my terrific english... ) bye

26th July 2006, 05:57 PM	#3
ash_dac diyAudio Member Join Date: Jan 2005 Location: UK	Hi, I had a nad 3130 before I gutted it. These things really don't last. 1. dry solder joints absolutely everywhere on mine 2. corroded rca jacks, and rusty transformer. 3. bulging capacitors everywhere! Have you tried connecting source equipment to different rca sockets ? I would look for bulging caps.

26th July 2006, 09:52 PM	#5
otherside diyAudio Member Join Date: Jul 2006 Location: Edinburgh	Thanks a lot guys for the replies I'll look into it on more detail, does anyone know if/where i can find the schematics of this model ? I tried to follow the noise and see where it's generated, it quite a wild guess without the schematics but from what i can see it's not comming from the pre-amp section, most likely the decoupling near to the output stage is damaged or just the components past their lifetime. Anyway a schematics or component ref would help a lot.

27th July 2006, 06:56 PM	#7
Gold_xyz diyAudio Member Join Date: Dec 2005 Location: Italy	i have found a schematics here

27th July 2006, 07:08 PM	#8
otherside diyAudio Member Join Date: Jul 2006 Location: Edinburgh	thanks a lot guys, the schematics help a lot, i'll let you know how it goes.

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