Re: Electrolytic caps and Apple analog circuits (semi on topic)

["jsnospam@cimmeri.com" <js@cimmeri.com>]

> Forming can be maintained by occasionally applying
> working voltage to a capacitor.

So for the on-topic generalization for Apple computers, how often at a
minimum do you think folks should be powering them up, and for how
long?

Kinda would have to know the rate of oxide breakdown to hazard that
guess, I guess!

---

> If an electrolytic is properly sealed, evaporation is not a
> problem--of couse, not all *are* properly sealed...

I'm reading in a few places that the electrolytes are now more paste
like or even solid, and are designed to absorb moisture from the air
to stay functional longer... so if that claim is true, the canisters
must be breathing.  Does that seem plausible?  Doesn't to me,
otherwise, why would the top of the can be scored?  You've already got
some other vent which would rupture.

---

> The easiest test for leakage is to lift one end of the cap and
> see if the DC voltage on the other end shows up on the free
> end.  Digital VOMs will usually have megohm-range
> input impedances on DC ranges, so it only takes several
> microamps of leakage current to read as a significant
> voltage.  

Hm.  Are you sure, Michael?  I know that works for non-electrolytics,
but I thought that all electrolytics leak at least some -- eg, I
quote: "A forming current of 100 microamps to 1 milliamp is a typical
maximum allowable current."   So you would always see the DC volts on
the other side.

--

> The other effective "test" is replacing the capacitor and
> confirming that the symptom of leakage is gone.  ;-)

With the Mac 512k running, I put a scope on that horizontal sweep
blocking cap, and at least noted that DC was on one side and not the
other.   But then I put my finger on the cap and noticed it was very
hot.    Displeased, what I ended up doing was replacing it with 4 1uf
monolithic caps in parallel.   Much tinier, no electrolytes, and
totally cool to touch now.    Makes me wonder why they put that elec.
cap in there to begin with.  End result?  No change in video. lol.
So, yoke slip rings, it became!

~ J