I purchased a NAD C 370 amp in pretty much immaculate condition a number of months ago. The amp has little use over the 20 or so years. It has worked perfectly for the past few months until last weekend where the sound became distorted. I switched off the amp and on again and it went into Protection Mode(Red light).
I unplugged the amp and put it aside. till I could test it properly.
Conditions-
Main Inputs L&R grounded, so no stray signals feeding the power section
Soft Clipping=Off
Bridging Mode=Off
8 Ohm B&W CM2 Book Shelves on Speakers A (I've also tested with McIntosh ML 1C Speakers as well
Behaviour
Switch on from cold, goes through the normal startup procedure
I get what seems like a 100hz hum/buzzing on both speakers
Switch off, then on again, goes through the startup procedure, then into Protection Mode
----
Leave it for a bit of time and it repeats the behaviour above, hum/buzzing then on second power on; into Protection Mode
I would lean towards the Zener Diodes degrading the Capacitors in the protection circuit if;
Has anyone experienced this type of behaviour?
Any advice would be appreciated.
I unplugged the amp and put it aside. till I could test it properly.
Conditions-
Main Inputs L&R grounded, so no stray signals feeding the power section
Soft Clipping=Off
Bridging Mode=Off
8 Ohm B&W CM2 Book Shelves on Speakers A (I've also tested with McIntosh ML 1C Speakers as well
Behaviour
Switch on from cold, goes through the normal startup procedure
I get what seems like a 100hz hum/buzzing on both speakers
Switch off, then on again, goes through the startup procedure, then into Protection Mode
----
Leave it for a bit of time and it repeats the behaviour above, hum/buzzing then on second power on; into Protection Mode
I would lean towards the Zener Diodes degrading the Capacitors in the protection circuit if;
- The Amp has had a lot of use over the years
- The Amp is producing a 100Hz hum, and only going into Protection Mode after power cycling it
Has anyone experienced this type of behaviour?
Any advice would be appreciated.
Search for "NAD protection" and visit some of the links about troubleshooting various NADs.
Also, the C370 might have a UPC1237, like in C320, for protection.
Also, the C370 might have a UPC1237, like in C320, for protection.
First checks should be for power supply voltages. Any suspicious bulges of supply caps, etc? Hum present with volume at 0? Check DC voltage at amp outputs.
Do you have a scope?
Do you have a scope?
20-22 years of heavy use. Bet every cap in it is high/inconsistent ESR-loss or is near the end. Caps can be any degree "semibad".
Especially when the OEM's use 50V devices for 47 rails , or 63V for 60V rails. So common. Most oem aux supplies will do 16V for the 12V rails.
Junk aux. supplies ,always seems to be where is board looks burned !
OS
Especially when the OEM's use 50V devices for 47 rails , or 63V for 60V rails. So common. Most oem aux supplies will do 16V for the 12V rails.
Junk aux. supplies ,always seems to be where is board looks burned !
OS
Thank you all for your input. I've traced the fault supply (+68V and -68V) to the first stage of the power amplifier. I only get +27V and -27V from the supply. That would explain the hum/buzzing on both channels equally and then a valid protection mode.
I'll look at replacing the large Caps on the power supply and see if it restores the +68V & -68V
I'll look at replacing the large Caps on the power supply and see if it restores the +68V & -68V
The +/-68V regulators use the preamp power supply (+/-34V) as a reference, so it may be a good idea to check those voltages, too. Generally speaking, the C 370 runs quite hot inside, esp. around those regulators on the right side of the main board, which leads to caps losing capacitance, gaining ESR and often leaking electrolyte. While you are at it, consider re-capping at least the main board with some quality caps rated for 105°C.