NAD C355BEE

[Joe bags]

Hi folks

I have a NAD C355BEE amp in for repair. It was in with me before but the problem rectified itself.
Unit was in protect mode but when I went to plug it in to repair it worked fine for me. So I gave it back.

The unit was returned to me with the same issue after about 6 months of working fine.
Issue was unit would power up then go into protect after about 5 secs.

I checked all the output transistors and voltages on the board and all seemed fine.
One thing I did notice was in the 5 secs between coming out of stand by and going in to protect.
I measured a big DC offset increase on the speaker outs. I think it got up to about 500mv before going in to protect.

Anyway put it back on the bench today to have another go and the problem has rectified itself again.
It turns on and plays fine now.

So followed the alignment procedures from the service manual. Setting the idle current etc.

Now i'm not sure what to do to make this amp more stable. As I assume if I give it back like this it will inevitably end up back on my bench in another 6 months.

Any ideas where to root out any leaky transistors/caps/diodes or out of spec resistors?

Am I looking at comparing voltages on the L+R sides of the amp to see if there's any differences?


Regards

Joe

[olddawg]

Are you an authorized servicer? With problems like this, especially under warranty, I would always contact the manufacturer. There may be a bulletin or something. If it only fails every 6 months good luck. Most of the time it was "ship to factory for evaluation" or they just replaced it. Honestly. I would suspect a strange load/use situation.

[The Dude]

I've never worked on an NAD anything that didn't suffer from extreme heat with no place to go, though I'm not familiar with this particular model, so maybe it's different. Thoroughly check all solder connections, particularly in high heat areas. If there's evidence of high heat anywhere (darkened PC board, resistors with bands burnt off, etc.), check all electrolytic caps in those areas and near heat sinks.

[Joe bags]

No i'm not an authorized servicer for any brand, but it's something i've been meaning to look into.
Must have a look on this forum for advice about that.

I've had difficulty in the past finding spare parts for Nad equipment, cd mechs, lenses etc.

Don't think it's a strange load issue as it happened on the customers set up and mine.

A thermal issue sounds more plausible, I will investigate that suggestion.

Thanks guys!