Beware of NAD M3 Fire Hazard

As an authorized NAD servicer for over 40 years this is the first time I’ve heard of this.
I can’t see how this would occur UNLESS the user keeps their electronics on all of the time.
The user did not mention they were listening the time it failed, so I guess they do keep it on at all times. In standby, the capacitor that failed is not in the circuit.
If powered to a fully on condition, the capacitor is live and before any regulation. If there are issues with incoming AC power they will translate thru to these 4 capacitors.
If the user does not keep the unit on at all times this can still occur, but it’s less of an issue.
Capacitors can and do fail due to many reasons but there is nothing endemic in this model.
If the incoming AC power is above the nominal voltage (typ. 114-126) for long periods of time, this can put a stress on filter caps in pre-regulated circuits.
Here is a good suggestion for all electronics... do not place anything combustible above the equipment and the "burning" component will not have anything to ignite.
I do agree with the poster directly above this as the highly corrosive electrolyte is likely spewed onto many parts and the PC board, too.  It will cause metal component leads as well as. PCB traces to corrode and fail. Also, the unit uses double sided PC boards and many small "plate-throughs" (micro rivets that connect one side of the board to the other) that are easily damaged from the electrolyte.

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.

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
.

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