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NAD C370 protection circuit repair (amp fails, goes into protection mode)

A C370 with a defective protection circuit generally fails in a specific way.  The amp will work fine when cold, but after being on for several minutes, the protection circuit will activate.  Power cycling will not help, unless the amp is allowed to cool fully before the next attempt.

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.

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 through-hole capacitors on a printed circuit board.

My C370 ripped apart on my workbenchSo it turns out the design issue with these amps is that NAD decided to put a hot-running diode right beside (touching!) some electrolytic capacitors in the protection circuit. The heat from the diode ages the capacitors prematurely, which causes their values to drift.  The new values tend to cause the circuit to activate unnecessarily.

The protection circuitLuckily, the fix is simple.  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 recommend 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 SchematicWith 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.  Warning: make sure you keep the proper orientation of the diode when putting it on the other side of the board.

Diode on the back of the boardNow apply power, hope there aren’t any sparks, and enjoy the music!