NAD C 320BEE weird noise on power up - what might be wrong?

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mikescraigslist

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Joined 2022
#1
Hi guys,

I have a NAD C 320BEE that exhibits strange behaviour and I have no idea where to begin troubleshooting...

The amp powers on into stand-by mode (amber LED) and all input sources are de-selected at this stage (all input LEDs are off). The amp will stay this way indefinitely until I manually push one of the input select buttons. When I do, regardless of which one I push, the amp will default to CD and both the power LED and the CD input LED will go green. At the same time I will hear a loud buzzing/modulating noise come from the speakers that lasts a couple of seconds. After this the noise dies down and everything seems to work fine.

I have the service manual but have no idea where to start looking for faults...

Anyone have any suggestions?
 

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#2
Its a bit unclear whether there is more than one fault here. If the behaviour of the CD function being the default isn't normal behaviour then you have to begin by looking at the supplies to the system control (syscon), this would be the 5 and 12 volt rails. They really need checking on a scope to make sure they are clean.

The buzzing noise shouldn't be directly related to a logic problem in the system control unless the syscon supply is suffering from lots of ripple.

Is C433 OK in the power supply, however without a scope check we are guessing.
 
#3
mikescraigslist said:
Hi guys,

I have a NAD C 320BEE that exhibits strange behaviour and I have no idea where to begin troubleshooting...
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I found that the input cap of the 7805 on the power supply near the torroid was dry such that the input voltage was marginal. I replaced that cap. But I was looking for cause of random fluctuations in the stereo sound. The relay control circuit also mutes the +-18vdc which supplies the preamp circuits. The discrete 18vdc regulators run very hot because of excessive input voltage and poor ventilation so I hacked in TO-220 pre-regulators installed on the chassis sides to spread the heat. Now it works.
FYI: "Hot Heatsinks In Vintage Amps ... Such As NAD C320"
 
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#4
Mooly said:
Its a bit unclear whether there is more than one fault here. If the behaviour of the CD function being the default isn't normal behaviour then you have to begin by looking at the supplies to the system control (syscon), this would be the 5 and 12 volt rails. They really need checking on a scope to make sure they are clean.

The buzzing noise shouldn't be directly related to a logic problem in the system control unless the syscon supply is suffering from lots of ripple.

Is C433 OK in the power supply, however without a scope check we are guessing.
Click to expand...

Actually I got part of the description wrong. It will always default to the last input selected when coming out of stand-by, irrespective of which input button I push to bring the amp out of stand-by.

Defaulting to last input is fine, I believe it is designed to work this way. Weird part is how it stays in stand-by indefinitely until I push an input button, and the noise that follows.

Thanks Mooly for the tip. I'll have a look at that cap you suggested, hopefully this weekend.
 
#6
Hi mikescraglist .. my amp had random fluctuations in audio volume, it was not stuck in standby.
I found the 7805 input cap was degraded and there were hotspots in the chassis. I took measures to reduce the operating temperature and that seemed to be root cause of operational variance. Heat is the enemy of electronics.
 
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