Question about NAD C370 2-Channel Amplifier
When switching on amp in the morning after overnight shutdown, only one relay click heard, and no sound. 30-60 seconds after this, another relay heard to click and sound then fine until turn on next day after switch off. Although i could just leave switched on permanently, worried that something is on it's way out that may go permanently in future.
My thoughts are that problem may be temperature related?? however this is not caused by ambient room temp, as this is same as always.
Any help / advice gratefully received.
Thanks in anticipation,
The first click is the power relay connecting up the main transformer, the second click you normally hear is the speaker protection relay. When a fault condition is present in the amp, the protection circuit prevents the speakers connecting to the amp by NOT operating the relay. As the unit "settles" and works after the delay you noticed, what ever issue the amp is balancing itself it. It sounds like a leaky capacitor in transistor in the output stages. This can often be located with quick bursts of freeze coolant spray. It may also be a dry joint in the soldering about the driver and output transistors, always pays to have a visual look at it whilst you are in there. I also check power regulator stages for dry joints. The fault will be on one channel if it is signal path related, and both channels if power supply related. If you are familiar with electronic repairs, get in and have a look yourself. If not, get a NAD service guy to take a look at it for you. Good luck. Feel free to ask me any questions you may have about it.
Posted on Aug 14, 2008
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Posted on Jan 02, 2017
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I have had problems with this stereo mixer amplifier which had the symptoms of no sound output from either channel .
I opened up the unit and discovered that the amplifier was correctly amplifying my input signal up to a relay which switches the output through to the speakers .
This relay did not activate at all.
When the power switch is turned ON you should hear a click approximately 2 seconds later , if no click then there is a problem with the circuitry associated with this relay .
I traced out all the connections associated with this relay and made a circuit diagram .
The relay is a 24v relay and I will explain the way that I can see that this section of the amplifier works.
It has been designed to ;
This it needed because otherwise there is a current surge and sparks from the relay turning on immediately.
Q804 turns On Q805 which turns On Q806 to activate the relay .
Voltages on transistors
Q802 and Q803 2sc1815
Base 0V Collector 12.9V Emitter 0V
Base 12.9V Collector 0V Emitter 15.1V
Q805 2sc1815 (driver for Q806)
Base 0V Collector 7.2V Emitter 0V
Base 0V at Turn ON then after 2 secs increases to 0.7V
Collector 2.8V ( transistor is activated as a switch Turned Fully ON ( saturated) to turn on the Load which is the relay .
I have replaced the following transistors to fix my amplifier q805 2sc1815 and q806 2sc1815 .If you are going to use a different type of transistor it may not work and also the lead connections of a 2sc1815 for the collector and emitter are different to those such as on a 2n2222( transposed).
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