I destroyed my NAD 3020i, help guys - diyAudio
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Old 26th September 2012, 06:21 PM   #1
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Default I destroyed my NAD 3020i, help guys

Hi guys ,
first post and i need your help regarding something stupid i made.

I have the NAD3020i amp connected to the Mission 701 speakers and the last days i was hearing a hum (zzzzzzz) when i was powering up the amp.
It wasn't loud and it wasn't from the speakers,it was directly from the power supply of the amp.not loud at all.
I removed the cover,everything was crystal clear inside,in perfect condition all the components.
As i was checking with my multimeter probes the cables where there is the selector for 4-8 ohms, one my probes touched the cover chassis and there was a short circuit along with a loud buzz.By that time i'm getting hum from the speakers which is getting louder as i turn on the volume knob.I smelled something and i turned it off immediately.
I don't know what happened, i powered it up again,played some cds,no smell that time, the speakers sound ok,but i have that constand hum which gets louder in higher volumes.

Do you have any ideas what's going wrong?
Is it an amps or speakers problem?
I have the service manual if someone can help.
I triggered the problem when my prob touched the metallica cover

Please help
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Old 26th September 2012, 08:26 PM   #2
RS232 is offline RS232  United Kingdom
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Just one idea...
Take the jumpers in the back out and check if the hum is from the preamp or the power stage.
Let us know how it comes out.

Ric
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Old 26th September 2012, 08:39 PM   #3
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Jumpers removed, no hum but no sound at all from cd player
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Old 26th September 2012, 08:42 PM   #4
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It seems you shorted one of the power transformer windings to earth. There is a basic 4R power winding and an extension with further turns and voltage for 8R only use. It will work at the lower supply, but lack some 8R power. Perhaps it was the switch that burned as it's quite flimsy. If you resolve not to be careless with the probes again, you can measure voltage of the power rails (across the main electrolytic capacitors) to verify that they change when the 4/8R switch is changed.

If you have hum in the speakers, it normally indicates old and dried out main capacitors but here, with the sudden change, it could mean other problems too, A NAD 3020 is a good sounding but very cheaply engineered product. It is easy to wreck them and very difficult to fault find and repair for newbs. My suggestion, unless you are prepared to consign it to the cupboard if your abilities are not what they might be, is don't play about, take it to a licensed repairer because this will take more than just time and following suggestions.
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Old 26th September 2012, 08:55 PM   #5
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I shorted something on the back of the 4Ω-8Ω switch
Can you tell where are the main electrolytic capacitors?
Any other measurements to take on the attached image?

Click the image to open in full size.
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Old 26th September 2012, 09:06 PM   #6
RS232 is offline RS232  United Kingdom
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Quote:
Originally Posted by misfits73 View Post
Jumpers removed, no hum but no sound at all from cd player
If your CD player has a variable output or volume control you can plug it in strait to the power amp input (the jumper you removed is marked IN/OUT) IN power amp input, OUT preamp output.
But if you don't have hum with the preamp disconnected the problem is in the preamp stage.
To test the preamp you need a good working amp to connect it and see if the hum is back in the speakers.
The main capacitors are the 2 black cylinders just above the big metal transistors.
Slide the 4/8 ohm selector to 4Ohms and measure the supply voltage
Change it back to 8 Ohms and measure again
You should have a lower voltage on the 4Ohms setting
You can measure Ac before the bridge rectifier or DC after

Last edited by RS232; 26th September 2012 at 09:16 PM.
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Old 26th September 2012, 09:11 PM   #7
morinix is offline morinix  United States
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Schematic here http://p10hifi.net/planet10/TLS/down..._schematic.gif
The 4-8 ohm switch is directly connected to taps on the power supply transformer. I'll bet you either blew the switch and/or the the fuse(s) F503, F502.
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Old 26th September 2012, 09:43 PM   #8
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All the fuses are ok.
I cannot measure the capacitors, can you guide me without having to remove all the PCB?In which points to place my probes for C508 & C509?
If i damaged the switch is it easy to replace it?
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Old 26th September 2012, 09:58 PM   #9
RS232 is offline RS232  United Kingdom
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Quote:
Originally Posted by misfits73 View Post
All the fuses are ok.
I cannot measure the capacitors, can you guide me without having to remove all the PCB?In which points to place my probes for C508 & C509?
If i damaged the switch is it easy to replace it?
You can't measure the capacitor in the board.
You need a capacitance meter and remove the caps from the board.
About the switch (I don't know were you are located) but i think Maplin have similar ones or you can salvage one from a computer power supply that has a voltage selector switch (it will have 110/220 marked on it) but is the same switch.
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Old 26th September 2012, 09:59 PM   #10
morinix is offline morinix  United States
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How old is this amp?
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