Bulging Capacitor with Low ESR?

[paul_g_787]

Hi. I have encountered a strange fault condition here that I have never seen before.

I have here a NAD C325BEE Amplifier to repair. Both of the main power supply smoothing capacitors have failed, they have bulged up and leaked their electrolyte everywhere.

They are big chunky 15000µF 50V capacitors.

However they are both reading a low ESR of 0.008Ω!? 

In my experience when a capacitor bulges and the electrolyte comes out the ESR goes up drastically.

How could this be right? I have verified on two different ESR meters..

Has anyone else encountered this before?

[The Electrician]

Maybe the capacitor has been damaged enough that the anode and cathode foils inside are touching at some point creating an 8 milliohm short.  If you measure the resistance with an ohmmeter what do you get?

[paul_g_787]

My thoughts initially too but the weird thing is they are both basically the same. 0.008ohm ESR on one and 0.009 ohm ESR on the other.

They both read 1.28M DC resistance (and the same with the probes reversed).

After I de-soldered them I noticed that if you squeeze them a little you hear the remaining gases escape. Also if you shake them you can feel the innards knocking about, unlike the new replacements. So they are definitely dead.

I would try to read the capacitance but all the meters I can find seem to only go to 100 microfarad.

[The Electrician]

Could you have a failed rectifier so that AC was applied to the caps, causing them to fail quickly due to overheating, but still retaining some capacitor-like behavior when they cooled back down?

If you figure it out, be sure to let us all know what happened.

[wraper]

Electrolyte needs to almost completely dry out before ESR skyrockets. If they ruptured recently, there may be enough electrolyte left to keep ESR low and fault was caused by electrolyte shorting other components.

[SeanB]

Yes, just replaced PSU capacitors, and while the ones that had vented were of various values capacitance wise, from 22pf up to 300uF, for a 2200uF 10V unit, the ones with some capacitance, just bulged, had ESR in the 0.1r range, and looking at the Chongx ones next to them, all still nominally the rated value, but ESR around 0.5R, they were not long for life.Decided to replace every one over 1000uF in the power supply, using what is available, Nichicon 1800uF 16V, as I needed 10mm diameter ones, and that value is locally stocked, and vaguely has a current rating for ripple near the originals. Not bothered to use lower voltage rating for the 5V and 3V3 rails, the ripple current rating is lower for the parts, so used the 1.9A ones all over. Hopefully they will last better, and while there taking the power supply apart I lubricated the fan as well, sleeve bearing and at least 11 years old, seeing as this is a 2010 made unit. Capacitors 7k hour endurance, hopefully will last better than the Jwelect ones, with the 10k fiction life, they put in originally, Most were bulging, though of those that were not, they all were well cooked, and starting to weep out the base.

[paul_g_787]

Could you have a failed rectifier so that AC was applied to the caps, causing them to fail quickly due to overheating, but still retaining some capacitor-like behavior when they cooled back down?

If you figure it out, be sure to let us all know what happened.

No the rectifier diodes are all fine. I checked these already as this is a common fault with this kind of issue at you are obviously also aware

[paul_g_787]

Electrolyte needs to almost completely dry out before ESR skyrockets. If they ruptured recently, there may be enough electrolyte left to keep ESR low and fault was caused by electrolyte shorting other components.

Possibly! I expect it will work fine once the new ones arrive!

The power transistor heatsink is radiating directly onto these capacitors which is a ridiculous design in my opinion...

Have a look at this guys video. The thumbnail clearly shows the two main capacitors next to the heatsink:


And funny enough in his description all of the exact same capacitors are faulty as in my one. 

I have probed every solid state part I can find and also the power rails now the capacitors are removed and there are no short circuits I can see! 

Hopefully this is it.

Will report back.

[paul_g_787]

So my capacitors finally came! I have installed them and now the voltage rails are measuring good! However the amplifier is still not working.

I am going to start a new thread in "Repair" regarding the fault.

Edit (New thread here): https://www.eevblog.com/forum/repair/nad-c325bee-will-not-power/