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

["Michael J. Mahon" <mjmahon@aol.com>]

On Jul 9, 6:59 pm, "jsnos...@cimmeri.com" <j...@cimmeri.com> wrote:
> On Jul 9, 3:43 pm, "Michael J. Mahon" <mjma...@aol.com> wrote:
> <<  Unless an electrolytic capacitor is stressed by large ripple
> currents or a marginal voltage rating, elevated temperature is the
> main killer.>>
>
> Interesting.   Is that for the simple reason that higher temp equals
> higher evaporation rate?

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

Higher temperature means much higher chemical reaction
rates, for both desired and undesired reactions.

> What is it I've read about the electrolyte tending to dry out in the
> absence of a sufficient rejuvenating voltage?   What's the best way to
> keep electrolytics happy?  Use power supply once a month, for
> instance?

The actual dielectric in an electrolytic is a very thin film
of insulating compound (usually aluminum oxide) formed
on the cathode during manufacture in a process called
"forming".  ;-)

When an electrolytic sits uncharged for a long period,
chemical reactions can de-form the oxide layer, causing
it to be much more susceptible  to leakage and breakdown
when full voltage is suddenly applied.

Forming can be maintained by occasionally applying
working voltage to a capacitor.  Re-forming can also be
done by slowly raising the voltage across the capacitor,
allowing it to "soak" until falling leakage current indicates
that the oxide layer has re-formed sufficiently for the
applied voltage.  When the forming voltage has been
successfully raised to the working voltage (+10%), then
the capacitor is again ready for use.

As this suggests, partial de-forming is usually a cause
of excessive leakage (in the miliamp range rather than
the microamp range), rather than a cause of high
equivalent series resistance (ESR).

High ESR is more often associated with drying out or
other degenerative changes in the electrolyte.

> << Position shift is also readily caused by DC leakage in the sweep
> circuits that drive the yoke.  As the schematic showed, the
> electrolytic cap in the horizontal yoke coil circuit would produce
> just such a shift if it were leaking>>
>
> How does one test for leaking current?  Is a good ESR not good
> enough?

Leakage and high ESR are orthogonal problems.

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.  Note that it may take several seconds to charge
the capacitor before a stable leakage current is evident.

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

> OT MAC STUFF:
> << BTW, the centering permanent magnets are usually two rings with
> tabs >>
>
> Yes, that's what's there... as well as small cubic magnets all around
> the yoke.   The Apple service manual seems to suggest it's normal to
> adjust the centering using the rings.

That's correct.