Troubleshooting NAD C350

[RobHance]

I am having a problem with my NAD C350. Upon power-up the green power light turns on and then fades to red as the protection circuit kicks in. I have inspected the boards and nothing looks obviously blown. All the fuses are good and the power supply is outputting its +/- 40 volts properly. I am not sure what to check next.

I have searched here and other places on the web and have read about many other people encountering similar issues but I have yet to read a solution to the problem. If anyone has a suggestion on what to check next it would be appreciated.

Thanks
Robert

[reddish75]

heres the solution

[RobHance]

Originally posted by reddish75
heres the solution

Thanks for the suggestion. I immediatly checked those resistors but it looks like the manufaturer already beat me to it. The two "resistors" were just jumpers anyway. I guess my unit was made after the original problem was found.

Thanks again, you got me very excited for about five minutes!

Robert

[reddish75]

i'll have another dig around, i'm pretty sure there was another service bulliten for this amp, another thing to consider is that the output transistors have blown, you can check this by measuring voltage on the relay, high voltage (meaning almost always power supply voltage) means transistors have blown somewhere

[RobHance]

That would be great if there was antoher service bulletin. Thanks a lot for checking.

Sorry for the noob question but where is the relay?

Robert

[reddish75]

hi, the service bulliten is not for your problem, however i have found somemore, have a good look around eveywhere under a bright light looking for burnt components if you find any i have service information relating to this, the relay should be a plastic rectangular box maybe an inch or so high.

[RobHance]

I just measured the relay voltage and it is reading 49V between pins 1 and 4 and also between 8 and 5 according to the pin designation of the specification sheet linked to below. The relay is rated for 24V so I am guessing this is bad. All the other pin combinations read 0V.

http://www.ziming.sh.cn/zm/relay/pdf...nqi/ftr-f1.pdf

I did not see any burnt components or burned places on the boards but I will keep checking.

[RobHance]

Update:

Something I did not mention earlier is that this problem is intermittent. That said, I noticed the power light had shifted back to green so I checked the relay voltage again and pins 1 and 4 read 24V while 8 and 5 still read 49V. Referring to your earlier post perhaps the output transistors are blown.

The green light didn't last long. Later in the day it switched back to red and both sides of the relay read 49V.

[RobHance]

The current status of the amp is if it is left turned on for a long time the red protection light goes off and the green light comes on, as if the amp does not work until it warms up. If this indicates that the output transisotrs are blown how do I go about checking them? Do I have to remove them from the board?

Thanks,
Robert

[reddish75]

i, dont think its the output transistors, it could be the power supply caps, ive had this problem with one of the nad classic series before i think the c320, just have a close look at them look for swelling, leakage check there firmly conected to the circuit board. maybe post a picture if you can.