[sdiy] Korg Delta problem

[sleepy_dog at gmx.de]

Gordonjcp wrote:

> Yes.  You might see them fail if they've been thoroughly hammered in
> cheap badly-designed switchmode power supplies.

I have also seen a computer mainboard where almost every single 
electrolytic had a bulging top. Maybe 10..15 years old.

>> I have been told by more than one EE not to bother using
>> electrolytics that have been laying around for 5 years, not just for
>> things requiring low ESR, not for anything.
> Horseshit.  If you believe that, I've got some lovely speaker cable to
> sell you, complete with lithodynamic vibration mass compensators.

Really? Well, but only if you add an oxide-free audiophile grade mains 
cable!

Anyway.
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.823.6133&rep=rep1&type=pdf

 From that I get the impression that there indeed once existed a rule of 
thumb of a shelf live of 5 years for aluminum electrolytics.
That dude now, under "recommendations", from the findings of this work, 
recommends to extend this to 16 years.
Stills sounds like such caps *do* have a shelf live, a notion apparently 
grounded in far more reliable estimation methods than a wet thumb in the 
wind!

>> I don't have any numbers. But even in some simple old synth PSU with
>> a pair of fat caps and linear regulation, it would be bad if those
>> caps got a huge ESR and reduced capacitance, no? Now I can imagine
>> that this is considered in the circiut designs, but only so much?
> They won't get "huge ESR and reduced capacitance" though.
>
> The electrolyte isn't milk.  It doesn't go off.

 From what I've read, it evaporates over time. There are dozens of 
seemingly reputable sources on the first page of google hits or so 
talking about the limited useful lifetime of such caps because of aging 
(often emphasizing that it's much worse with those caps than other kinds 
of components).

2000 working hours as life time seems like a common value for cheaper 
ones, that's like 5.5 hours per day for one year. Doesn't seem huge.
This does not make it seem odd to me if people want to "recap" some 30+ 
years old synth they got.

Now those "2000h" is apparently for the rated temperature like 85°C, 
doubled for every 10°C less as a rule of thumb.
But I've also read somewhere that ESR does increase over time, which 
also increases heat energy wasted in the device, making the cap degrade 
ever faster.

Now I don't know much about this and this is probably all very 
rule-of-thumbey, and perhaps you know all the kind of design errors that 
have impact on these things.
But still, reading all this, calling aged capacitors a myth seems a bit 
weird!

> People are obsessed with this idea that every fault is "bad caps".  It's
> never a capacitor.
Given the above, it wouldn't surprise me if there were people who recap 
their stuff in regular intervals without anything being wrong, and I 
wouldn't  even find it strange ;)
Like other limited lifetime parts are regularly exchanged. Extreme 
example: relays in industrial settings.
I don't know who does "every fault 'bad caps'".

> It's always something else, although I'll grant you
> in most linear power supplies it's the rectifier diodes which fail and
> *then* take out the big electrolytics.
>
> It's got to the point where I won't touch anything that's been
> "re-capped".  I just assume that if it's been "re-capped" and it's still
> not working, it's now beyond repair.  It was probably a simple fix
> before the poke-and-hope brigade got their grubby mitts on it.

Well, nothing is simpler than doing something that's likely enough 
without having to dig really deep (unless we're talking about a great 
number of parts :D Well, still simple, but laborious) If they go at 
least by some sort of list of common problems tied to aging caps?
I am also surprized by your interesting idea that making sure a subset 
of a circuit's components are guaranteed to be in spec ("recapping") 
somehow could make things worse. I'm not sure how that works. Even if 
other parts are the actual source of the problem - how is having a 
device full of fresh caps worse than one with a lot of old caps?
Do aged capacitors taste better?