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Old 5th June 2007, 07:46 AM   #1
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Default NAD C370 power-on problem

I am having problems with power-on cycle on my NAD C370 amp. The delay between power-on and deactivation of protection (the moment when the amp unmutes sound and amber LED turns green) increased to 3-5 minutes. After that, amp seems to be working fine. The problem does not affect the sound.

My questions are:

1. Where can I get schematics for protection circuitry? I saw power amp schematics on this board, but it did not include protection curcuits.

2. Any recommendations on where to look for problem first? I must admit I am not very experienced in DIY electronics, but I have basic skills and know how to use multimeter. I feel I would be able to repair the amp... if only I knew what exactly is wrong with it.

Thanks in advance for any help.

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Old 5th June 2007, 08:57 AM   #2
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Location: Berlin
Default Nad C370

look at this

...and search for it at:

you will find a schem "with" the protection circuit.

greetings from Berlin Michael
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Old 28th June 2007, 10:48 AM   #3
cyteen is offline cyteen  England
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Location: S.E
A bit late but may help someone:
Attached Images
File Type: jpg nad-c370-upc1237ha-protection_module.jpg (91.0 KB, 583 views)
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Old 28th June 2007, 04:19 PM   #4
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Thanks for the post, this is very helpful!

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Old 29th June 2007, 08:11 AM   #5
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Location: Whakatane, North Island
Default Nad C370

As I do work for a NAD Dealer, this might help:

Your problem is probably due to The Electrolytic Capacitors on the Speaker Protect Board - (you are in the right place)!

These Electros get overheated by heat from the Zener diode, which is mounted at the top of the Pcb, and they then dry up, losing their value.
My fix: To replace all these electros, and remove the Zener Diode from the front/top of the Pcb, then resolder it back onto the back/
bottom of the board, so letting it's heat go elsewhere.
(make sure of it's correct polarity).
In our Opinion, this is a VERY bad design point of the C370, as it
has the effect of a built-in 'time bomb', and the Amp will eventually fail no go. I can't see why NAD would have done this.
I have now repaired several the same, and this works really well.
Hope this helps You!
Regards, Telnet.
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Old 20th September 2007, 06:43 AM   #6
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Hi Telnet -

Thanks for the tip, I have been able to repair the amp successfully. You were right on point - the problem was caused by fried capacitors on protection board. Replacing caps C783, C784, C785 worked like charm.

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Old 27th August 2010, 11:58 AM   #7
jwh is offline jwh  United Kingdom
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Location: Clitheroe, UK
Thanks to all above. Have just replaced the 3 capacitors described, which has cured my C370 start up delay problem. I didn't move the Zener, should have really, but it was at least 12mm above the capacitors.

I was disappointed that when I rang the local Nad dealer, they said they could cure the problem, which they had not heard of and they didn't have any service manuals or schematics. Estmated repair cost, 80-100!!

Thanks again.

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Old 7th August 2011, 11:25 AM   #8
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Hello, I have the same problem with my NAD c370, but I must be blind becuse I can't find the Speaker Protection Board in my reciver. Can anybody please help me where to find it.
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Old 5th June 2013, 03:13 PM   #9
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Originally Posted by turbonezze View Post
I can't find the Speaker Protection Board in my reciver. Can anybody please help me where to find it.
Old post, but this may help someone

Click the image to open in full size.

Also, remove the black diode (Zener) above the caps and solder it on the other side of the board - so it does not heat up the new caps again. Watch proper polarity!

Last edited by Gary Indiana; 5th June 2013 at 03:17 PM.
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Old 6th June 2013, 03:35 AM   #10
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Join Date: Sep 2011
I got replacement caps at local Frys for about a dollar each. After the replacement amp runs as new. No more long delays, only a normal couple of seconds.

Practical hands-on suggestions

1. Clean the inside from old dust with vacuum and a clean dry paintbrush. The unit is much nicer to work on when clean.

2. You must remove the Speaker Protection Board in order to replace the three bad caps and to turn around the Zener diode. First unscrew the section of bottom panel to see the solder side of the PCB. Use a good solder sucker pump, or solder wick, to free all pins one-by-one and the four corners of the steel surround.

3. When finished cap replacement, do make sure there are no solder bridges between solder points. They are so close to one another that it's easy to make unintentionally a tiny solder bridge.
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