• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

NAD c370 Protection mode

C

Coolfx

New Member
Hi everyone and happy new year,

My C370 started recently to enter in protection mode, first I just had to shut it on and of and it was ok for weeks (no h24 powered on of course :p) but it became worse to finally stuck on protection.

I tried to disconnect each channel and found the right channel board solve the protection mode.
Nothing look really bad but there are some hot markings but resistors are measured in specs in those area.
I checked voltage on each pin of the protection board:
pin 1 -4.31v
pin 2 -9.43v
pin 3 -12.5mv
pin 4 37.5v AC
pin 5 4.1mv
pin 6 21.3v
pin 7 26mv
pin 8 11.79v
pin 9 42.1v
pin 10 3.38v

It seems overload and DC-Offset left are bad. What can I check to find the source of the problem ?

I'm pretty capable in (de)soldering, got an ESR meter, a thermal camera and an osciloscope (osciloscope that I don't know yet how to use)
 
Last edited:
Register to hide this ad
Are the pin voltages for connector M702 for the protection circuit?

Is there any DC on the rights channel output at the input to the speaker relay?
What is the bias measurement?
 
Yes these measurements are from M702 aka protection board.

Where can I measure the input of the output relay ? on the coil just in front ?
I cannot find the bias measure points. (I'm a complete noob in audio equipment repair, I'm only use to modify computer boards for extreme overclocking without any formal training)

BTW I found the main problem, hotpluging CZ404 connectort I saw some sparks on the solder joints, after a reflow of each connector joints and replacing every 25v and 50v caps (I didn't had the 100v 10uf in stock and 6.3v 2200uf is perfect ESR) on the board it doesn't trigger the protection.

I'm planning to test and recap everything in need and do the adjustment but for those I will need some guidance as I'm not even capable of finding the measuring points even after finding info through the web...
 
You should know that on the C370 the protection board itself has a bad design with a large zener diode directly underneath a couple of capacitors that get baked and fail. This will prevent the unit from coming out of protection - at least it did on the 2 units I worked on. I actually moved the zener diode to the back of the protection module board to get it away from the caps on the parts side of the board, when I recapped it.

In general that era of NAD amps should be re-capped on sight. Total garbage caps in there that fail due to the fact that the units run very warm.
 
You must log in or register to reply here.
Back
Top Bottom