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-   -   NAD 3020 hum (http://www.diyaudio.com/forums/solid-state/35061-nad-3020-hum.html)

rigadig 30th May 2004 04:36 PM

NAD 3020 hum
 
Hi, I have an old 3020, it worked fine, then one day, it just started making a low humming sound through the speakers. It started quiet, but increased in volume over time untill it go quite loud, the stayed at this level. If i turned the amp off for a couple of seconds, then back on again, the noise dissapeared for a minute or two, then started again (increasing in volume as before). I know a little about basic electronics (but not much in depth transistor theory or power electronics), and have no idea what is causing this, because one day the amp was fine, and the next this strange problem.

An electrical engineering proffessor at my university (Newcastle) told me that he thought it could be the regulator (but he was not sure because he never saw the amp) and told me to check using an oscilloscope (which i don't have!!). I opened the amp up, but I have no idea what a regulator looks like. I had taken the amp to a shop to get it fixed, but they charged me 25 to look, and said they could not fix it. When I opened it there is a very thick layer of brown dust over the whole board (could this be causing the problem), so i don't think that the shop ever really tried to fix it.

Basically, if anyone can help I would be very greatful because i don't want to throw it away.
thanks

pinkmouse 31st May 2004 01:53 PM

It sounds like the big electrolytic power supply capacitors may have got a bit old and dried out. They should be relatively easy to replace. Have a look for two big cylindrical objects connected near the transformer, and tell us what it says on them.

As for the dust, a good clean out wouldn't hurt! :)

rigadig 31st May 2004 02:59 PM

I am away at the moment, so I will have to wait until tuesday evening to check what they say on them. I will buy a can of CO2 as well to blow some dust off. Is replacing them just a case of unsoldiering them and replacing them with any capacitor from an electronics catalogue of the same capacitance value?
Do all capacitors die of old age like this, or just the big power ones?
Thanks

pinkmouse 31st May 2004 03:26 PM

Yes, the caps need to have the same capacitance value and working voltages. Make sure when replacing them that you connect them up with the correct polarity, or they will go bang and make a real mess. Oh, and being roughly the same size helps as well!

Electros do tend to give up after a while, due to evaporation out past the seals. If you really wanted to go for it, and have the spare cash, then it wouldn't hurt to replace all the electrolytics in the amp, (but watch out for non polarised versions)

As for cleaning, a hoover with a soft furnishing attachment works well, and switch cleaner for all the pots, switches, and connectors you can get to.

Compressed air in cans is way too expensive for something as robust as an amp, save it for CD mechanisms and cameras. :)

Geoff 31st May 2004 03:36 PM

Please bear in mind that the NAD2030 has separate supplies for the pre-amp section and the power amp (the schematic can be found on my website) and it would be wise to change both sets of capacitors unless you can determine which are causing the problem (if indeed it is a capacitor problem rather than something else).

The reservoir capacitors for the pre-amp section (C905, C906) are 470uF 35V and those for the power amp (C901, C902, C915, C916) are 2200uF 35V.

pinkmouse 31st May 2004 03:48 PM

Thanks Geoff.

BTW, did you know your site is down at the moment? I went there earlier to see if you had a schematic for the NAD, and couldn't get in, and ditto just now...:scratch:

jaycee 31st May 2004 04:11 PM

I see that the amp has the ability to seperate the preamp and power amp stages via what looks like a jumper cable. If there is a set of terminals marked "Pre Out" and "Power In" on the back that are coupled together you may want to try disconnecting them and connecting eg a CD player directly to the Power In terminals.

This will allow you to determine if the hum is coming from the power amp or preamp stage. It is most likely though to be the power amp stage, as other posters have said the electrolyte in the power supply capacitors will have dried up and they will need replacing. As long as they are the same dimensions, the same or greater voltage, and the same or greater capacitance they will work well. It may be worth using 105oC rated capacitors if you can as these will last longer.

Geoff 31st May 2004 04:50 PM

Quote:

Originally posted by pinkmouse
Thanks Geoff.

BTW, did you know your site is down at the moment? I went there earlier to see if you had a schematic for the NAD, and couldn't get in, and ditto just now...:scratch:

It's a Bank Holiday, what did you expect! :)

Seriously though, I seldom visit my own site (for obvious reasons) but there was no problem when I tried a few minutes ago. You perhaps tried during a busy period. One of the disadvantages of the 'free' webspace is that there is a limit of 1500 people at any one time accessing all of the BTInternet personal websites.

rigadig 31st May 2004 05:01 PM

will it matter what type of capacitors i use then. Could I use for example some type other than electrolytic, film capacitors perhaps?, would these last longer? also, what is a 105oC? Can anyone suggest a good uk supplier of these components, so far i only know of maplin or RS
Thanks

Geoff 31st May 2004 05:50 PM

For the values in question, you will need to use electrolytics. 105degC is the temperature at which the design life span is determined. Other electrolytics will have a temperature rating of 85degC. The 105degC types will generally last longer, though this can vary from one manufacturer to another.

Forget Maplin (poor stock range) and RS (expensive). Try instead:

Rapid: http://www.rapidelectronics.co.uk/

or

CPC: http://custom1.farnell.com/cpc/default.asp

email me if you need some more specific advice.


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