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Old 18th January 2008, 06:00 PM   #1
diyAudio Member
Join Date: Oct 2007
Default Need help with NAD amp! Goes in protection

Hi! I may be of topic but i didnt now where to put this,

I recently got my hands on a NAD C350 integrated amp.
(home stereo).

But the amp goes in protection after 2-3 seconds.

I have checked the output transistors and they seeme to
be OK.

And i checked the drivers, and the seeme to be ok.
A litle hard to measure because the have mirrored
the pins on that transistor modell.

I removed all the drivers and started the amp.
And i got green light!

But what the hell? I measured them and there is no
short from what i can se. they all measure about 0.6 in
diod check, and same for the outputs.

Anyone have a clue what could be the problem?

I saw inside the case that signal earth had been repaired
for a couple of years ago. And i saw a fixed print. But cant
find anything that looks broken.

But you can se that the board is a bit darker around
the drivers and then there is 3 other transistors.
But none of them seemes to have a short.
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Old 18th January 2008, 07:51 PM   #2
shagone is offline shagone  United States
diyAudio Member
Join Date: Apr 2007
did you check to see if the resistors near the drivers and outputs were in tolerance? maybe the drivers or outputs are leaky or other parts in the circuit are out of tolerance. also try completely removing the old solder and applying fresh solder. sometimes when there is too much heat the solder gets oxidized and creates a high resistance connection.

this should be in the "solid state" section by the way but let us know how it goes anyhow. afterall we are all DIY'ers here anyhow. i have an NAD integrated amp too. i think it may be a C350 as well and i'm using it as a pre-amp in my garage because the amp section is out. Maybe i'll take another look at mine if you get yours going.
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Old 19th January 2008, 07:45 AM   #3
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Join Date: Oct 2007
Ok how do i know if the drivers are leaky?
Dont show up on my DMM, but then its
a real mess with the transistors since
they swapen the "pin order".

Dont like it.

What is a acceptable value on the "driver"
resistors? The big square ones, 0.4 ohms?

I also want to check the "turn resistors??"
to se if the idling is ok, but i dont know
where to measure.

Anyone else who have an idea?

Is it an idea replacing all the drivers just in case?
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Old 19th January 2008, 09:25 AM   #4
diyAudio Member
Join Date: Oct 2007
I noticed that if i connect a speaker to the amp
and turn it on i get a "bop"sound and then it
goes in protection.

I also noticed that when i removed the outputs
it still got in protection, i put them back and

just removed the drivers, and then i got green
light, does this mean that its the drivers wich
creates the fault? or some component around
them wich is out of tolerance.
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Old 19th January 2008, 10:10 AM   #5
cyteen is offline cyteen  England
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Location: S.E
If you attach you multimeter to the speaker outputs to measure DC offset you may find that it starts low and then rises until the protection circuit kicks in. Test both channels.
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Old 19th January 2008, 10:46 AM   #6
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Join Date: Oct 2007
hi i measured on the output "connections" at the back
of the amp. And i got around 1,2-1,4 on both channels.

The amp goes in protection after just 1-2 seconds.

I have a storage scope, can i use it for easier troubleshooting?

I also noticed that the drivers get varm, not much but you
can feel it, isnt this strange since the protection
circuit goes on? Or am i wrong, they all cet equal varm
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Old 19th January 2008, 11:51 AM   #7
cyteen is offline cyteen  England
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Location: S.E
I though you might have the same issue as a machine I am currently looking at, It too goes into protection mode, but after 30 seconds or so. I had missed the rising DC offset because at turn on it seemed fine. The odd thing was on both channels even after replacing the output transistors on one side.

I've reached pretty much the same position of not knowing the next move.
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Old 19th January 2008, 12:02 PM   #8
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ok, its strange when you cant find anything
that looks suspisius, but it still wont work.

But something is wrong, thats for sure
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Old 19th January 2008, 03:43 PM   #9
diyAudio Member
Join Date: Nov 2003
Location: Louisiana
The pop indicates that you have DC on the output of the amplifier. If you measure the DC voltage on the collector of the output transistors, you'll probably see that it's near rail voltage.

If the drivers have run hot, they're probably leaking or otherwise defective. If you swap them into the other channel (one at a time) you will find the defective one. It would be better to replace them with new transistors but the fault isn't causing excessive current draw so swapping them should be safe.

This amp doesn't remove power to the amplifier in protection mode like a car audio amp. That's why the drivers continue to run hot when it's in protect. The protection circuit simply breaks the connection to the speaker terminals.

This is a diagram for a c320. Maybe it will help.
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Old 19th January 2008, 04:05 PM   #10
burbeck is offline burbeck  United Kingdom
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Location: Ratae Coritanorum
strange place to put this thread.

i have had a few of these amps for repair. a good few of them had the same fault, this was a faulty capacitor in the protection circuit value 330uf 16v, c143 which is located near ic 105

i would replace this component with a 330uf or 470uf 35v as a matter of course, might just be the problem here.

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