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NCC200 - Blown Output Transistor(S)?

Suffolk Tony

Suffolk Tony

Aim low, achieve your goals, avoid disappointment.
Hi folks, for no apparent reason, and after running faultlessly for a fair while, it seems one of the NCC boards in my dual-mono DIY has blown one or both of its output transistors, causing the Veellerman board to trip.

My rudimentary knowledge isn't enough to figure out what's gone wrong here, and other than replacing said transistors and checking the settings, is there anything else I can do to diagnose why this might have happened, please?
 
You need to check the drivers as well as the outputs, and check the 0V connection on the input if you lose that it can blow the output transistors.

Pete
 
Which output devices are you using? I ask because I've seen one of these go DC but the emitter resistor took all the current and fried leaving the BUV20 intact. I'm not sure if MJ15003s would fare as well.
 
Thanks folks. Not sure about the output device - I'll have a look later, pretty certain they're MJ15003s. A quick glance didn't reveal anything burnt.
 
One blew in an NCC200 mono of mine. I was unaware of speaker protection until then, as none was fitted..........................

I was/am the third owner, so I have no idea how many hours the board had done, but, if memory is correct, the amp's were about 10 years old at the time. The NCC200 was the version prior to the current one (2?????).
 
I'm not exactly sure how old the amp is - best guess is four years. I'd not been using excessive volume, or messing with connections, so I'm a bit perturbed that the thing's just conked out like that.
 
Practically any fault with the amp board with cause excessive DC on the output, tripping your protection circuit.
The first thing I do with these or any Naim board is remove the output devices. The board will work on the bench as long as all the 0v connections are made. You can test for DC on the output there - it should be within a few tens of mV of zero. Anything other than that and you need to start testing all the other active devices.
 
MJS said:
Practically any fault with the amp board with cause excessive DC on the output, tripping your protection circuit.
The first thing I do with these or any Naim board is remove the output devices. The board will work on the bench as long as all the 0v connections are made. You can test for DC on the output there - it should be within a few tens of mV of zero. Anything other than that and you need to start testing all the other active devices.
Click to expand...

Just in case... don't connect any load to it whilst testing like this!;)
 
^ yes. I forgot that bit.

Common failure modes I've seen on this Avondale/Naim/RCA circuit:
  • Dead output devices from a short circuit
  • Open circuit emitter resistors, most often related to the above fault. (I know one of them is a collector resistor, but convention!)
  • voltage amplifier stage - the XTZ752/3. Also produces a lot of second-order harmonic distortion when faulty but not dead.
  • Fried LTP, often because of nearby lightning.
 
Thanks for all the help folks, it's much appreciated. Events have overtaken matters somewhat - both my boards are now on their way to a kindly gent in Chesterfield for a spot of upgrading (and fixing of course...).
 
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