Electrolytic caps and Apple analog circuits (semi on topic)

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

This post is about electrolytic caps in general -- concepts which can
be applied to Apple 2 switchmode power supplies -- although it stems
from a June 10 conversation between Michael Mahon, Paul G, and myself
about diagnosing and repairing an Apple Mac 512k analog board.


First, to wrap up the earlier convo.   The problem I was having
concerned the video image being shifted too far to one side (and there
being no adjustments for it) on a spare analog board compared to the
original board -- both boards still working.

<<"Michael J. Mahon" wrote: Although Paul's suggestion to use the
centering magnets would certainly move the raster, the fact that it's
fine with another analog board indicates that it's actually the sweep
timing that's off, not the raster centering. >>

<< Paul G. wrote: OK, I got off my butt and took another look at the
analog bd in my
junk box ... Looks like a leaky C1 would be the likely culprit for a
horizontal offset...>>

<< Michael wrote: I completely agree with your diagnosis. BTW, a
*leaking* (electrically) capacitor is the one failure mode that
"bridging" with a good capacitor won't find. >>

Ok, guys, I bought an Atlas ESR meter from Anatek and all caps on both
analog boards are in spec.  What a great tool to have!   I also
discovered from an Apple service manual for the 512k that aside from
the magnets, there are slip rings on the CRT for adjusting the image
position.   Inspecting the two analog boards, there are some major
differences in flyback design as well as some other parts -- different
rev boards.. so I'm guessing that the image shift is just as a result
of design differences and not a failed component.

ALL THAT SAID... what's the deal with electrolytic cap failures?
Anyone have any perspectives on the topic?
- Why do some apparently seem to last forever -- I have some here from
the 1950's that are still working -- and some fail some damn quickly?

- Are there any patterns to be discerned -- such as the smaller ones
last longer, or lower voltage ones.. etc?

- Are some brands better than others?

This is all very useful info to have when making repairs.    I
occasionally repair Apple power supplies or other analog circuits, but
haven't had enough experience to draw any generalizations.

If this topic has been covered previously, please alert me and I'll
search archives more carefully.

Thanks!
~ J