Fri 12 Oct 2007
NAD C370 protection circuit repair (amp fails, goes into protection mode)
Posted by Ian under Uncategorized , howto , audioSo, youre sitting there listening to music and all of a sudden your amp clicks out, the power light goes red, and theres no music. ‘Shit. Its broken’, you think. Power off/on? Same thing.. Once it cools down, it powers up, but it clicks out again after 10 or 15 minutes of use.
Well if you found me by the google because precisely this happened to you, you’re in luck. I just repaired mine for the wonderful sum of 63 cents. Anyway, let’s move on.
To perform this fix you need to be somewhat comfortable with a soldering iron, and be willing to take your amp apart. If you aren’t very good with an iron, you’ll want to read through some how-to pages elsewhere until you’re comfortable with replacing capacitors on a printed circuit board.
My C370 ripped apart on my workbench
So it turns out the problem with the design is that NAD decided to put a hot-running diode right beside (touching!) some electrolytic capacitors. This dries out the caps, which changes their values. When their values change, the protection circuit goes all wonkey.
The protection circuit
First of all, locate the protection circuit pictured above. As you can see from the picture, mine had some obvious heat problems. Nothing looks broken per se, but it looks nicely toasted, especially on the right side and top. To get it working again, I’d reccomend replacing the three topmost capacitors. Their values are 4.7uF, 47uF and 10uF all rated at 50V. I wouldn’t get anything larger than 50V as I’m not sure if it would fit. For those who are interested, here is the schematic for the protection circuit:
NAD C370 Protection Circuit Schematic
With those three caps replaced, the amp should work fine. But in a couple years, those caps will die too. I found a great solution on a forum: move the diode to the other side of the board. Watch the polarity!
Diode on the back of the board
Now apply power, hope there aren’t any sparks, and enjoy the music!
January 19th, 2008 at 11:26 am
thanks for tip on my c370,not had it apart yet,but sounds like the same problem.
January 21st, 2008 at 4:58 pm
Thanks !!!!!
What a dumb design … I love this amp so I guess I’ll go about trying to make the repair.
May 29th, 2008 at 2:44 am
Well, my symptoms were slightly different, but not unique - the Amp would work fine until powered off, and then on power on it would go into protection for a few minutes before eventually powering up properly. It was getting worse too, so I decided to give this fix a go. A trip to RS for the caps, and a really awful soldering job later, and my amp now powers on normally and sounds like it always did! I also moved the diode to the back of the board to prevent it happening in future. Thanks for the the tip!
Piete
June 11th, 2008 at 6:32 am
Hello. I would like to know how to increase the bass inside the Nad C370 amp. I see there is grey dials on the right and left front of the amp. Im not sure what they are but it sais Bias on one of them (Circuit board). Can you tell me if this can be turned to increase more bass on the amp so it sounds less flat sounding. My email is c.a.p.superstar@hotmail.co.uk
August 13th, 2008 at 6:44 am
Hi
Tried repair but same problem
Any further ideas?
Cheers
August 13th, 2008 at 6:58 am
Tibor,
Hm. were the symptoms identical to the ones described? If so, it is probably still a problem with that protection circuit.
Did replacing the capacitors change the symptoms at all?
September 21st, 2008 at 4:44 pm
Hello Root.
Thank you very much for your crystal clear instructions. I did what you said and my C370 is now working again.
Lloyd.