Beware of NAD M3 Fire Hazard

@ pwb

Odd the AC line safety fuse did not blow. The fuse should have blown protecting the unit from becoming a fire hazard. Is the AC Line fuse the original OEM fuse that came with the amp?

If it is the original OEM fuse I suggest you contact NAD service Tech support.

Jim

Al, that amp has a lot crap in it! How long did it take for you to find the 3 fuses?


pwb said:

I took it to the NAD authorized repair facility and they said a faulty power supply caused the round cylinder ( not sure of part name) to explode

round cylinder in the power supply???

I should have mentioned in my previous post that series arcing will not cause a fuse to blow. In the case of parallel arcing the current may not be high enough to cause a fuse to blow. The arcing can, will, create sparks though.

Jim
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https://ibb.co/T1VBM5G

I’ve never seen the top vent of an electrolytic cap blown off like the one in the photo.

That’s just plain scary!



heaudio123 Said:

Could be anything, but a shorted diode in the bridge passing AC and cooking the capacitor would be my guess.

Wouldn’t that cause a high current load on the secondary winding of the toroid power transformer causing the primary winding to overload and cause the 5 amp (I assume a slow blow) fuse to blow?

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heaudio123 Said:

Could be anything, but a shorted diode in the bridge passing AC and cooking the capacitor would be my guess.

jea48 response:
Wouldn’t that cause a high current load on the secondary winding of the toroid power transformer causing the primary winding to overload and cause the 5 amp (I assume a slow blow) fuse to blow?


heaudio123 Response:

heaudio123100 posts

03-15-2020
3:19am

No it would not blow the fuse. The capacitor is not a dead short when reverse biased, but a low enough resistance that it will heat quickly and blow up.


heaudio123,
Thank you for your response.
I think I understand what you are saying in your above statement. The cap had a lot of stored energy and it was suddenly fed with a reversed polarity by the shorting diode. Kind of like connecting two 12Vdc car batteries in parallel out of polarity with one another.

I noticed the vent cap on the other cap did not look like it was affected by the event. It doesn’t look like there is any bulging in the top vent cover. Wouldn’t the shorting diode have affected that cap as well?

heaudio123 Said:

Could be anything, but a shorted diode in the bridge passing AC and cooking the capacitor would be my guess.

heaudio123,

Would you please explain the event in time of the shorted diode. How long of an event time did the short of the diode last before, I assume, the diode blew apart breaking the short circuit on the secondary winding of the toroid power transformer?

Jim
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almarg9,445 posts

03-15-2020
11:20am

I noticed the vent cap on the other cap did not look like it was affected by the event. It doesn’t look like there is any bulging in the top vent cover. Wouldn’t the shorting diode have affected that cap as well?

@jea48 Jim, good question, but the answer is that the schematic shows that there is a separate diode bridge (and other circuitry) associated with each of the two capacitors that are in each channel. One set of diodes/capacitors/and other circuitry provides approximately +72 VDC, while the other set provides approximately -72 VDC.

Best regards,
-- Al

Al, thanks for the quick response. You provided the reason why.....



Any thoughts on my comment on this part of my previous post.

heaudio123,

Would you please explain the event in time of the shorted diode. How long of an event time did the short of the diode last before, I assume, the diode blew apart breaking the short circuit on the secondary winding of the toroid power transformer?

From the OP:

pwb OP9 posts  

03-14-2020 
 3:03pm  

Thanks for the responses. As far as I know the fuses are intact. I stopped the sparks shooting out the top by unplugging it.

AC Mains was still supplying energy until the OP unplugged the amp from the wall.

Jim

pwb OP9 posts  

03-15-2020  
 10:01am  

Thank you everyone for the useful information. I guess I will just have to take the loss and move on. I will never purchase an NAD product again. Anyone have a recommendation for a good integrated under 2k. It’s for a second system driving Merlin tsm-mmm speakers. Thanks.

pwb,

I wish you would take the time and contact NAD tech support here in the US. Send them the photos you provided on this thread. The photos speak volumes to the damage that was caused by the cap in the amp. Make sure you tell them the sparks were still flying when you walked into the room. Let them know if you had not been home there is no telling what the damage may have been to your home.
Be polite! .....

I would hope NAD America would want the amp to determine exactly, if possible, what caused the catastrophic event that caused the electrolytic cap to blow its’ top the way it did. I would hope NAD America would pay for the shipping.

Jim
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