Tants. So what's the failure mechanism ... ?

Tantalums have kind of a bad reputation for shorting, and that's been my experience. Not really seen them leaky or open.

The scratchy sound of the pot is typical of DC leakage in such a circuit. Can make a volume pot or similar sound "dirty" or make woofers go "whoomp" when the control is moved.

But then you know this, yes? I know you have loads of experience with this stuff.

I got the "whoomp - whoomp" in a Yamaha receiver the other day - a bad muting transistor put DC on the volume control.

Mark Z.

caps

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I only remember s/c failures. Is there meatalisation either side of a narrow edge? and then metalisation creep over the edge or into a crack ala this mechanism

a useful study on failure mechansim of otherwise trouble free capacitors

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Indeed yes, Mark. That was what gave the clue that it was going to be the cap, and caused me to unsolder it, and reach for the meter ... :-)

I fear that such knowledge, that we were taught as young techs by others much older and wiser than ourselves, is doomed to be conscripted to the 'lost' pile, as we expire one by one ... :-(

Arfa

value,

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disappeared.

I misdirected myself , only last week, removing a perceived scratchy/worn pot, that was fine, other than it had DC on it from a digital delay cct fault.

It seems to me that tants are notoriously intolerant of reverse voltage - even at small signal levels!

These days ceramic chip capacitors can be found upto several uF (maybe even

10) that are small enough to substitute bead types, for larger values I'd be tempted to go for high temp/low ESR aluminium types and maybe add a ceramic chip SMD on the print side.

Mark Zacharias Inscribed thus:

Thats been my experience too. I've seen them glow red hot and burst like a volcano, spraying tiny red hot fragments all over. The fragments burn for a second or so before dying out. Admittedly nearly all that have done this have been bead types.

--
Best Regards:
                     Baron.

Hmmmm. They must have "improved" them. Recall all those gum-drop tantalums in Tek scopes that failed dead short across the power rails? :)

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snipped-for-privacy@repairfaq.org (Samuel M. Goldwasser) wrote in news: snipped-for-privacy@repairfaq.org:

Yes,I do,quite well. 8-)

--
Jim Yanik
jyanik
at
localnet
dot com

On Fri, 19 Mar 2010 02:24:18 -0000, "Arfa Daily" put finger to keyboard and composed:

I've had one particular board where the same tantalum cap (bead) would go leaky.

All other faults were dead shorts across a supply rail. I must have replaced over a hundred of them (bead) in all manner of equipment. I recall seeing one tantalum cap explode and catch fire like a match head.

- Franc Zabkar

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What voltage across it? Maybe they're no good at near zero DC?

I've only seen them fail when reverse connected. Spectacularly in one case with reverse connection on 24V battery --> high pitch bang behind my back, when I turned around to look at the board and there was the remains of the cap, standing on one leg, glowing red hot, cooling off. No sign of the other leg or case...

Grant.

--
http://bugs.id.au/

So far I've never seen one blow - only go leaky in small signal circuits. Wherever possible I try to find a non-polarized cap (film or ceramic chip) small enough to fit the space - that strategy never bounced a repair.

"ian field" wrote in news:2gcpn.3930$ snipped-for-privacy@newsfe29.ams:

In TEK scopes,they were often shorted,and were used in PS decoupling circuits ALL over the scope. Sometimes,I could see a tiny pinhole in the epoxy coating. I've seen them burned,too,even just the leads remaining,sticking out of the PCB pads.

IMO,it's usually OV spikes or polarity reversals that take out tantalums. Or manufacturing defects.

--
Jim Yanik
jyanik
at
localnet
dot com

Oh I love tantalums. "Ashes to ashes, dust to dust"

How interesting. Seems that lots of people *have* seen them go short and even explode, but my experience of them really has been predominantly that of them going leaky, whereas my experience of proper wet electros, has been the exact reverse. Now here's a curve ball then. Most of the people that have a lot of experience of them going short, seem to be leftpondians. Myself and Ian, are both 'rights' ...

Arfa

Arfa Daily Inscribed thus:

Interesting observation ! Since I'm also on the right side of the pond.

--
Best Regards:
                     Baron.

I recall hearing a long time ago that tantalum caps do in fact tend to grow metallic whiskers inside, between the plates, which ultimately bridge the cap. Whether it's tantalum or some other metal used in construction, it is a poor conductor, and in a power supply, will immediately burn out and the cap will return to normal. If the cap is left for a long long time without power applied, the bridge can become quite substantial, and the heat generated next time power is applied can blow the cap apart. I remember the moral of the story is that tantalum caps are good only for decoupling and power filtering, where there is guaranteed to be sufficient current to deal with the shorts. They are attractive for low signal applications because of their size, but I've never used them for the above reasons.

Now, is any of this true? I have no idea. But it would account for Ta caps > Anyone read anywhere what the failure mechanism is for solid dielectric c=

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some useful info here

Are we not all on the right side of the pond??

I dont think there IS a wrong side :)

'Cept for the land down under :)

Grant.