Beware of NAD M3 Fire Hazard

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
 

@heaudio123, thank you for your response to my post. I know you are extremely knowledgeable about such matters, and my relevant design experience dates back a few decades.

I would be interested, though, in your comments about the section entitled "Operating Lifetime Model" on page 13 of the following Cornell Dubilier "Aluminum Electrolytic Capacitor Application Guide":

https://www.cde.com/resources/catalogs/AEappGUIDE.pdf

Based on the equations shown there it appears to me that operating such a capacitor at 50% of rated voltage would double "expected operating lifetime" compared to operating it at 90% of rated voltage, which to a close approximation is what is done in the M3.

Regards,
-- Al

Looking at the service manual again that capacitor is a 22,000 uF 80 volt polarized aluminum electrolytic. As can be expected it is connected to the output of a diode-based full wave rectifier. But based on what you (pwb) were told it sounds like something else failed in the power supply which caused that cap to explode. If so, that something else could have been a lot of things, as there are a great many parts in that circuit.

However, although the schematic is a little hard to interpret I’m fairly certain that the DC voltage appearing across that capacitor in normal operation (and across the other three similar capacitors, each channel having two of them, one for a + voltage and one for a - voltage) is between 70 and 74 volts. If so, that seems to me to be a failure waiting to happen, as a cap should be run at a voltage that is much less than its rating, IMO. Preferably at something like half of the rating.

Regards,
-- Al

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

Not long, Jim. The service manual includes a block diagram, a schematic, and a parts list. Which made it pretty easy to determine what fuses are being used, and where they are located in the circuitry.

And a quick look at rear panel photos that are available on the web confirmed that the fuses are not physically located on the panel.

Best regards,
-- Al

Odd the AC line safety fuse did not blow.

@jea48, Jim, I took a look at the service manual for the M3, which can be seen at hifiengine.com if one is registered there. It appears that it has three fuses, all of which are AFTER an EMI filter, consisting of inductors and capacitors, which is connected between the incoming AC and the rest of the amp.

There are two 5 amp fuses (assuming this is a 120 volt model), one for each channel, with the main (rear panel) power switch connected between the filter and these fuses. There is also a 100 ma fuse protecting standby-related circuitry, with a relay connected between the filter and this fuse.

All of the fuses are located inside the unit, and they are not accessible on the rear panel.

All of this would seem to cast suspicion on the EMI filter as the culprit, although secondary damage elsewhere within the unit certainly may have occurred.

Best regards,
-- Al