Beware of NAD M3 Fire Hazard

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
 

I haven’t seen capacitor application notes in decades, but I believe heat still applies, right? That is, the capacitor can be aged while on, not necessarily because of the voltage applied but because of the heat in the amp. Further, higher temperature caps also have longer life spans at a given temperature than lower temp caps, or am I mistaken?

I agree, unfortunately, that the failure here seems not worth fixing. That’s a lot of cleaning to do, with no guarantees.

If it were MY personal unit, I would attempt to clean it, and see if the remaining power supply works, and if the logic/preamp circuits appear in tact, then and only then would I attempt to rebuild the burned out side. Still, lots of work with no guaranteed happy ending.

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

I will never purchase an NAD product again.

EVERY product suffers one-off failures.

NAD has made millions of units with very high reliability and excellent sonics for very reasonable prices.

NAD are well engineered. Electronics are made of component sourced from dozens of manufacturers. A dozen or more maybe involved in the power supply alone. All are potential points of failure for which NAD unjustly suffers the blame.

It's possible that a line surge coupled with a weakish part at precisely the wrong moment caused the only catastrophic failure on the planet.

The probability than another NAD unit would suffer a similar fate is small.

The unit is an audio computer and computers are replaced quite frequently.  The last firmware update was a decade ago, so the unit could be nearing EOL.

Some questions:
- did you buy it new?
- was the unit left on all the time?
- what is the local line voltage?