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.
**edit: According to Stu in the comments below, some amps have 63V caps, so I guess slightly larger caps might fit. Either way I wouldn’t recommend replacing your capacitors with new ones of a lower value than stock**
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.
October 23rd, 2008 at 7:37 am
I am going to try this, as my amp just died and my protection board looks just like this.
Have you done the mod where you solder wire between the pre-amp and main in to replace the jumpers?
I would like to do this mod too, but I’m not exactly sure what I am looking for to do this…help!
Thanks
Shawn
October 31st, 2008 at 10:40 am
Hi, thanks for the very informative article. My C370 has these exact symptoms described. I’ve had a look at the protection circuit and it is in fact already running 100v caps and they are 10uF, 100uF and 4.7uF, which is different from what you suggest.
When I bought it off my friend he told me it had been repaired previously so I assume these were changed then.
Also that diode close to the caps has a copper plate on either side, maybe installed at the same time to try and act as a heat shield?
Do you suggest I replace these back to the original ones as stated here? I really would like to get this great sounding amp working again!
Thanks
October 31st, 2008 at 10:10 pm
To be honest I’m not too sure how other revisions of the amp might have differed. But if I were to hazard a guess, I’d say you’re probably right Gareth. That copper plate was probably added after, and the guy was probably out of 47uF caps. Especially considering they have a higher voltage rating than the rest of the circuit.
As a course of action, I’d first move the diode to the other side of the board like I did. For the caps, hard call. They’re probably supposed to be the same as mine, but there could have been some change in the circuit. I don’t have enough information to say, really, so I don’t want to recommend an action for the caps
. I’d probably change them to the values I found, but I also like messing with things that would otherwise work.
Either way, I’m interested to hear how you fare.
November 4th, 2008 at 2:44 pm
Thanks for the pointers Ian.
I swapped the diode round as recommended and replaced the caps the yesterday in accordance with the spec on the circuit diagram. While it is behaving better i.e you can run it for a bit longer at low-ish volumes and it’s fine, but if you crank it up a bit, it spits the dummy out and does the same old trick.
It will run for at least 30 mins at ‘8 o clock’ on the dial, but only lasted 5 mins at ‘10 o clock’, this is with easy to drive missions.
I’m at a bit of a loss about what to do next.
Do you think this is still a problem with the protection circuit or could it be any number of things?
Thanks again
Gareth
November 8th, 2008 at 2:27 pm
Hi,
I had the same problem and this has been fixed by a repair shop a few months ago. Now I have another problem with my C370. If I turn it on, nothing happens. The device is not getting any power, I think. Any ideas what the problem might be and how I can repair it?
Thx.
November 24th, 2008 at 2:04 am
Hi,
My C370 had the same problem. It has been fixed by a repair shop. After a few months of normal functioning the amplifier died. if you turn it on, nothing happens. I called the repair shop and they told me that I may try to change the fuse. As the repair shop is two hours drive, I will give a try to repair. The question is where is the fuse? I removed the cover and didn’t see anything which looks like a fuse. PLEASE HELP!
Yucel
y.blackcloud@gmail.com
December 9th, 2008 at 12:41 pm
I performed the fix this afternoon. After turning it back on armed with safety goggles and a broom, it works perfectly! Big thanks!
Just an fyi for anyone else considering this fix: My caps weren’t discoloured, but replacing them did fix the problem. It did require removing the back panel (which wasn’t clear from the pics above), and remember to check the cap polarity before inserting. I believe all the caps have their striped or negative side facing to the right (with the amp facing you).
January 10th, 2009 at 9:53 am
I owe you a beer, replacement capacitors cost me a total of 33p and a night of sobriety last night to ensure my hands were not shaky today. The 47uF capacitor removed turned out to be 63v not 50v (the unit was bought new so im pretty certain this mod has not already been done) but all were replaced with 50v ones as suggested and since putting it all back together it seems to be working great - much better than forking out £65 to get someone else to do the same job.
Many thanks fella
January 10th, 2009 at 10:11 am
Good to hear that Stu. Hopefully NAD was just overpseccing that capacitor and thats why yours was 63V. I edited the page above to recommend people try to find the same value cap as their amp has as i wouldn’t want to cause anyone problems, but i can’t imagine they changed the circuit much.
And now you can spend a nice saturday evening with beer and excellent audio instead of that sobriety crap
Ian
January 18th, 2009 at 9:16 am
I just got my nad amp. 316 back from repair, plugged it all in and was listening to cd. Not really loud, heard click and amp went into protect mode. What to do, anyone tell me what the fix is…thanks..John
January 20th, 2009 at 10:42 am
Need Help with my 316av, Surround Sound Amp, nad…..Hooked it up and it went into the protect mode, shortly after, wont work now. I think the speaker wire was reversed when I hooked it up, Think…. I just paid 150. to fix and repair company wants to charge me again to look at it. Not sure if it will be worth another 100 to fix. Let me know if you any suggestions…chefjhb@aol.com…John..tks.
March 12th, 2009 at 8:33 am
Hi there,
The problem with my c370 is that when i power on it goes into protection mode for about 5-10 mins then goes back to normal and works fine. Do you think this fix will work on mine?
any help appreciated
March 14th, 2009 at 2:23 pm
Hi,
I was lucky enough to acquire two of these amps with what i thought was the same problem as the above. Applying this fix to the first worked perfectly, brilliant - thanks! The second now powers on and stays “green” but i’m not getting any output from the speaker outputs or the headphone output.
The history of the still defective amp is that it’s been used everyday for about 8 hours for the past 5 years so perhaps it’s time for its retirement anyway but does anyone know of, how shall i say, the next weakness in line to fail on this otherwise quality bit of kit? And a way to fix it…?
Any help greatly appreciated.
March 24th, 2009 at 4:52 am
Made the repair exactly as detailed and it fixed the problem completely. Well done, sir!
I want to second what a poster said above. Upon close inspection, none of the offending caps were discolored, but clearly replacing them did the trick. So even if nothing appears cooked inside, the cap values may have changed and will still need replacement.
Worked like a charm — I am in your debt sir….
April 28th, 2009 at 12:38 am
I have an old NAD 319 with the protect problem - can I fix it the same way as this guide, or can someone explain what I need to replace?
May 5th, 2009 at 9:44 pm
THankyou my friend. tried this modification, and it worked perfect. my old C370 is up an’ running again
June 16th, 2009 at 8:38 am
J’ai appliqué votre méthode et l’ampli Nad C370 remarche ! Bravo et Merci pour vos si bonnes explications.
June 17th, 2009 at 9:57 am
De rien!
Je suis content que ça fonctionne pour tout le monde ici.
Peut-être je devrais faire un translation français de ces instructions.