Do these caps fail often in Tek gear?

[FlyingHacker]

Hi,

Does this type of cap (see attached image) fail often? They are in Tektronix plugins. They are listed as electrolytic. I am not sure if they are Tantalum or not.

I have a DD-501 plugin for a Tek Mainframe with lots of them. The plugin is from the 80s I think.

Any info is appreciated.

Thanks!

[Cubdriver]

That certainly looks like a dipped tantalum, and while my personal experience with them is limited, my understanding is that they like to fail as shorts, and occasionally imitate matches when they do so if whatever they're connected to can provide sufficient current.

-Pat

[FlyingHacker]

LOL. "Imitate matches"

Thanks.

[timb]

Hi,

Does this type of cap (see attached image) fail often? They are in Tektronix plugins. They are listed as electrolytic. I am not sure if they are Tantalum or not.

I have a DD-501 plugin for a Tek Mainframe with lots of them. The plugin is from the 80s I think.

Any info is appreciated.

Thanks!

I've restored dozens of Tek TM500 series plugins (1970's vintage) that contain large number of those "Ticks Tripping on Acid" dipped tantalums and I've never seen one fail. Keep in mind these are from plugin modules known to run hot, and have been in service, without replacement for 30 to 40 years. Odds are, if they're still running fine, they'll keep running fine.

So, if it ain't broken, don't fix it! That said, if one of your boards isn't working, check those caps first.

(As an aside, when I first started repairing these old plugins, I replaced all the tants on the first couple of units, with equivalent electrolytics, and aside from being a huge pain in the ass, every tant I took off measured perfectly fine and had lower ESR than the brand new aluminum caps I was replacing them with!)

[David Hess]

Those are dipped solid tantalum capacitors.  The color code can be found online.

I have seen them fail but not more often then other dipped solid tantalum capacitors in instruments of that age.  Dipped solid tantalums of both styles *are* one of the more common failures; early on they were often not voltage derated properly and the importance of pure materials and good processing may not have been fully appreciated.  I saw an old marketing ad which optimistically listed the lack of a need for voltage derating as a benefit.

When I find a bad one, I check for others operating at the same voltage and with the same voltage rating and replace them also.  If aluminum electrolytic capacitors are used as replacements, then 2 to 4 times the capacitance will yield about the same ESR.  Keep in mind that aluminum electrolytic capacitors are not as good at high frequencies.

I have a cheap local source at an old style electronics store for NoS solid tantalum capacitors but I burn them in first and check for leakage before installing them as shown in the photograph.  I would not bother doing this with new solid tantalum capacitors.

[wn1fju]

Maybe I've just been unlucky, but I've had a few of them go in 500-series Tek plug-ins.  All of them have partially shorted to about 10 ohms.  And since Tek likes to run voltage rails through low ohm resistors and then to these bypass caps, when they do go, the first indication is smoke pouring from the resistor.

[guido]

Had to replace one just yesterday. 100uF 20V in a 7A13 (code brown, black, purple). It also went short.

[David Hess]

I think the first one I replaced was in a 7B92A.  I had just bought it because I mistakenly thought it supported peak-to-peak automatic triggering and wanted to use it in place of my 7B53A but that was false; the 7B92/7B92A actually does not.  Anyway, it immediately dropped my 7834 power supply into tick mode so for diagnostic purposes I installed it into my 7603 which uses a linear supply and the linear supply had enough grunt to open the shorted tantalum capacitor by force and the fan spewed the resulting smoke out the back of the mainframe.  Then the previously shorted dipped solid tantalum capacitor was easy to find.

Since that episode, I have had a couple more dipped solid tantalum capacitors short out; one did so while I was watching it.

Incidentally, the 7B92A did not work as specified anyway; it produced a backwards sweep at certain settings.  Coincidentally, it was marked "Physics" in red ink on the side and was produced in the UK.  I now refer to it as the physics or Tardis timebase although I repaired the backwards time aspect which was caused by a problem with the cam switch.  Maybe I should have left it alone and waited for The Doctor.

[FlyingHacker]

Remember never to reverse the polarity of the neutron flow.

[lowimpedance]

As David has said its more general to 'tants' and not just TEK gear. I have had some in TEK gear but also in other manufacturers stuff and the common factor is mostly age.
If you have power issues they are a good place to start particularly if some BBQ ing has just taken place  .

[FlyingHacker]

Ok...  But it sounds like blanket replacement with no symptoms is not called for.

[David Hess]

Ok...  But it sounds like blanket replacement with no symptoms is not called for.

That is my position.  The only times that I replace working dipped solid tantalum capacitors are if they have insufficient voltage derating or sometimes if one of them with the same voltage derating has already failed.  If a 6 volt tantalum on a 5 volt supply failed, I would replace all of the other 6 volt tantalums on the 5 volt supply and the 16 volt tantalums on the 15 volt supply if they existed but not the 35 volt tantalums on the 18 volt supply unless one of them had failed as well.

[FlyingHacker]

Makes sense. Thanks for all the info, everyone.

[orbiter]

[cncjerry]

I have about 30 plugins.  Repaired quite a few.  Never had one of those fail or the caps that look like little rolled-up proclamations with two arms.

So adding to above, you take one of the ones I'm talking about and you have "hear me! hear me!".

[FlyingHacker]

A Tantaluminion? 

[chris_leyson]

Only had two dipped tantalum caps fail in 7000 series plugins, both of them in 7L12s oddly enough.

[free_electron]

LOL. "Imitate matches"

Thanks.

All Tantalums have pyromanic suicidal tendencies...

[T3sl4co1l]

LOL. "Imitate matches"

Thanks.

All Tantalums have pyromanic suicidal tendencies...

Except the liquid ones.  They'll burn your face with concentrated sulfuric acid (which will probably then cause fires anyway), if you ever get them to explode...    (They're also ridiculously expensive, and rarely seen outside of mil gear.)

As for the regular kinds, they are most vulnerable to current surges.  They are frequently used in RC timing circuits (where the low leakage, stable capacitance and high density are a win), where there is no danger of ignition.  These do not need to be replaced, unless they have managed to fail shorted somehow.  Those used for power supply filtering, are subject to inrush and ripple currents, and are vulnerable.  Most notably: avoid hot-plugging modules with tantalums!

Tim

Tim

[David Hess]

As for the regular kinds, they are most vulnerable to current surges.  They are frequently used in RC timing circuits (where the low leakage, stable capacitance and high density are a win), where there is no danger of ignition.  These do not need to be replaced, unless they have managed to fail shorted somehow.  Those used for power supply filtering, are subject to inrush and ripple currents, and are vulnerable.  Most notably: avoid hot-plugging modules with tantalums!

There is something else going on as well though.  Some of these old dipped epoxy solid tantalum capacitors are failing at times other than when surge current is present.  I *watched* one fail after replacing another similarly voltage derated part (6 volt tantalum on a 5 volt supply, 16 volt tantalum on a 15 volt supply, and 20 volt tantalum on an 18 volts supply Tektronix?  Really?) and after having applied power for about an hour.  Previous to this I had several old dipped epoxy solid tantalum capacitors fail 10s of minutes to hours after power was first applied and fewer which apparently failed immediately at turn but how do you tell when powering on "new" use equipment?  They could have originally failed do to surge current or while in operation.  And why would the failure be decades later?

When asked, I now recommend powering on "new" used equipment and leaving it running only under supervision for the first few days of operation.

One of the AVX application notes mentions damage caused to the tantalum slug do to a mismatch in temperature coefficient of expansion between the solid tantalum capacitor pellet and the epoxy casing.  NASA has one discussing field crystallization and the growth of defects in the dielectric layer over long time periods but they say it only applies at voltages greater than 20 volts where the dielectric layer is thicker.  Either way the failures described above would seem to be do to a new defect and not a surge through an existing defect.

[T3sl4co1l]

Maybe self-healing.

It's really the underlying cause, brought about in different ways.  Surges exacerbate cracks (thermally and due to leakage current into the crack), causing local heating and potentially causing, uh, catastrophic ignition.

Overvoltage, obviously, is most sensitive around a vulnerable zone.

Normally, a crack self-heals, because the local heating decomposes the MnO2 solid electrolyte into insulating Mn2O3, and the released oxygen creates more tantalum oxide.

Obviously, if this turns into a runaway process, it's screwed.  Normally, it doesn't.

A crack might fail shorted, if insufficient energy was provided to self-heal it (ironically enough), which might explain random failure of timing caps.  In that case, the cause would probably be thermal stress.

Tim

[David Hess]

My current hypothesis is that the self healing process fails when a defect grown through field crystallization becomes large enough.  NASA said this does not happen below 20 volts but I suspect the difference here is that these capacitors were produced early in the history of solid tantalum capacitors and have much worse defects do to higher levels of impurities in the raw materials.

Do the hermetically sealed solid tantalum capacitors (not the wet ones) fail in the same way?  I do not remember ever had one one these fail but they are rarely used in anything except extreme reliability applications.  If not then it argues that the problem is indeed do to the mismatch in thermal expansion of the epoxy packages.

Anyway, my solution is to properly voltage derate the replacements to 2/3rds or even 1/2 which AVX showed significantly reduced surge related failures to zero in some cases.  When I buy NoS tantalum capacitors, I run an extended leakage test at their specified working voltage to weed out bad parts but so far I have never found any.

[T3sl4co1l]

Dunno about hermetically sealed ones, but they're also rather large. I have some 10uF 50V parts that are as heavy as silver slugs (well... they're mostly Ta, so that's not far off!), and comparable to the size of a 220uF 50V electrolytic (maybe more).  Could be that they're made with unusually coarse Ta (the Ta grains are loosely sintered, then anodized, impregnated with MnO2, then coated with a better conductor), and built to a much higher test voltage than rated (say, 100 or even 200V).

The larger physical size would also give less power density per event, and less than stoichiometric MnO2 to Ta if they do ignite.

Epoxy could be a common factor.  Also moisture ingress, perhaps, since it won't be as good a diffusion barrier as glass and metal.

They ought to be pretty graphic if they do go off, though.  Hermetically sealed, n'all...

Tim

[TerraHertz]

Ah, this year's tantalum discussion. Anyone recall other threads? Just for the records.

20140226
https://www.eevblog.com/forum/projects/fe-fi-fo-fum-i-smell-tan-tal-um/

20150918
https://www.eevblog.com/forum/projects/is-the-bad-reputaion-of-tantalums-justified/

My favorite of all comments on tantalums so far, is from the 2015 thread:

Slightly off-topic -- I've worked with rockets and fireworks hobbyists on all sorts of ignition circuits, and a very cheap and reliable way to ignite a charge is using a 90's era dipped tantalum capacitor. The ones I've used are 10uF, 10v ones.
When you hook them up in reverse and pump around 100 milliamps at 50V into them they blow up instantly and very impressively for their size. I don't think I've ever seen them fail to ignite black powder, and sometimes they're hot enough to ignite candy rockets with no additional chemicals (though you'd probably want to sprinkle a tiny amount of magnesium powder to increase the temperature of the ignition).
Their advantage over heating wire (nichrome) is that you can hook them up using extremely thin wire-wrapping wire, much thinner than you'd usually need to use, so they're great for pencil-scale rockets and tiny remote-activation circuits.

After you use them in that way for a while, you get used to *always* do a reverse bias test every time you start using a new model of tantalum capacitor. And I would never use an older dipped tantalum capacitor in any circuit, no matter how cheap I manage to get them. They're a huge fire hazard.

Rocket igniters. 

[SoundTech-LG]

The pattern I have seen on Tek TM5000 series plug-ins (and other equip as well) is that at first, they start running hot to the touch, then they smell, then they turn a deep brown, then they burst into flames, or just frigggin' explode. Tantalizing...