I destroyed my NAD 3020i, help guys

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misfits73

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Joined 2012
#1
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
 
#4
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.
 
#6
misfits73 said:
Jumpers removed, no hum but no sound at all from cd player
Click to expand...

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
 
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#9
misfits73 said:
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?
Click to expand...

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.
 
#11
RS232 said:
You can't measure the capacitor in the board.
You need a capacitance meter and remove the caps from the board.
Click to expand...
He only needs an identifiable place to measure the rail voltages, not the capacitor quality. If he can't access the main cap solder pads, then other points are possible.

There are only 2 terminals on a cap and 2 probes to measure with - what's the problem, misfits?
 
#14
morinix said:
How old is this amp?
Click to expand...


Could easily be 30yrs, and even in their first decade were well known for living on the edge of reliability of the "bean counter approved" parts used throughout

There was at one time a cottage industry of DIY mods and tweaks that among other things included replacing all EL caps in the PS filters and elsewhere, and hardwiring the pre-power jumpers

This could get me into some trouble, and this is coming from a guy who owned many of the early 3000 and 7000 series amps/receivers, but as much of a giant killer as this was "in the day" (circa early 80s), unless it's a very simple repair, or you're looking to retain the surprisingly decent phono stage, or some of the other features, it could very likely be surpassed in performance and reliability by any of a number of current digital amps - either kit or prebuilt
 
#15
Ok since the amp is that old my crack guess is the reservoir caps c508 and c509 are going south. They were in the beginning, that is why the hum started. Now that they had to deliver a big transient when you shorted to ground they are near total death. Need to replace them. You will notice new found detail after those are new. A dc measurement may not really tell you the whole story. Still though the 8-4 switch may be at least damaged too but I would look really hard at those caps and address them first then worry about the switch.
 
#16
morinix said:
Ok since the amp is that old my crack guess is the reservoir caps c508 and c509 are going south. They were in the beginning, that is why the hum started. Now that they had to deliver a big transient when you shorted to ground they are near total death. Need to replace them. You will notice new found detail after those are new. A dc measurement may not really tell you the whole story. Still though the 8-4 switch may be at least damaged too but I would look really hard at those caps and address them first then worry about the switch.
Click to expand...


just be very careful when desoldering from the PCB - traces were notorious for lifting at eyelets - solderwick and suckers and careful prying of any folded over leads


if you do venture on this, it probably wouldn't hurt to find a schematic for the particular model - and be aware that over the life span of the 3020 family there were many revisions, there's a chance that any schematic you find might not exactly match your unit
that made for fun for the warranty departments, let me tell you
 
#18
I just finished repairing a Cambridge Audio with a toasted transformer (I could see a spark in the transformer) and is now playing Freddy Mercury.
Going to have a look at my 3020i that have a small noise on power up, not the normal thump you have on power up but a noise like the discharge of a capacitor that goes from one speaker to the other.
Another thing in my model 3020i does not have the jumpers from pre to power amp.
About your short it depends how you shorted it
I personally don't see how this can cause any problem to the transformer if it was just a momentary short or else you would have blown the protection fuse
 
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#19
I removed the PCB, the capacitors are ok
I noticed that Q508 gets overheated and it was touching the C523 capacitor which had a small mark on it (maybe from the heat).This capacitor is 80μF (and discharging) instead of 22. The soldering was not quite good, i'm cleaning the PCB now and i'll let you know

It can't be such a big damage while i was playing with the meter, very strange
 
#20
Actually i found the problem...
When i first smelled some burn it was close to the volume knob area.
After taking some closer look i noticed this:

An externally hosted image should be here but it was not working when we last tested it.


I just touched the route (sorry for not knowing the word) and simply came off the pcb, it had also some small burn sign.

Now what?

I studied electrical engineer but don't know a thing of printing circuit boards ,lol
 
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