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

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

On Jul 8, 1:51 pm, "Matt J. McCullar" <mccul...@flash.net> wrote:
> I worked on jillions of Apple Computer switch-mode power supplies.  I've
> worked on far more various types of video monitors of all kinds, too.
>
> The biggest enemy of any electronic component is heat.  That's what degrades
> electrolytic capacitors, and why some seem to last forever, whereas others
> have taillight warranties.  Some caps are exposed to high-temperature areas,
> while others are not.  There's plenty of heat inside a video monitor, and
> dust tends to collect inside.  That traps even more heat.

This is absolutely correct.  Unless an electrolytic capacitor is
stressed
by large ripple currents or a marginal voltage rating, elevated
temperature
is the main killer.  Electrolytics, as the name suggests, are
basically
*chemical* devices, and are subject to chemical degradation at a rate
which is strongly dependent on temperature.

> The older a video monitor is, the more likely that it's got at least one bad
> electrolytic capacitor -- perhaps several.  It's been my experience that
> it's not a change in the capacitance value itself, so much as a big increase
> in a capacitor's Equivalent Series Resistance -- or ESR.  It's like a cap
> has been bridged with a resistor of just a few hundred ohms.

Actually, bridging a capacitor with a (usually) moderate resistance is
the equivalent of a *leaky* capacitor, that no longer blocks DC
current
well.

High ESR is the equivalent to a low-value resistance *in series* with
the capacitor.

> That causes
> all kinds of weird things to happen.  True, some caps do just flat-out open
> up.  And a cap can change its capacitance value dramatically without
> affecting its ESR.  But in my experience, the increase in ESR is the most
> common failure.  It's best to just replace them all if you can, at the same
> time.  Reflow the solder joints while you're at it.
>
> When I worked on Apple II power supplies, the most common failure was the
> same electrolytic capacitor.  It wasn't on the DC output sections, it was in
> the feedback loop for the main switching transformer.  That caused the
> failure of all output voltages.  At the time I didn't have access to the
> gosh-wow cap testers I have now, so I don't know offhand if it was higher
> ESR (which I now consider likely) or capacitance change, or what.  I do know
> that replacing that capacitor cured the problem.  Soon my job consisted
> mainly of knocking out rivets or removing screws from those power supply
> lids, replacing that cap, and Voila!
>
> As for the video image shift in a monitor, that's more likely due to some
> sort of timing issue.  There might be a potentiometer adjustment inside the
> monitor for that.  A phase shift in the horizontal sync signal (or even a
> lengthening or shortening of the sync pulse) could cause that, too.
> Critical timing circuits tend to use high-precision capacitors such as
> tantalum, not electrolytic.  Look for a horizontal sync adjustment pot; it
> may be a horizontal phase adjustment pot, too.  Look for a pot marked HSYNC
> or HPHASE.  Be sure to use a magic marker to note exactly how the pots were
> set before adjusting them.

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
(current, not
electrolyte!).

Although my first guess was horizontal timing, since it is unusual to
have a DC
blocking electrolytic in the yoke circuit, after viewing the
schematic, I'd be more
suspicious of that coupling electrolytic.

BTW, the centering permanent magnets are usually two rings with tabs
that
are on the back of the yoke, and can be rotated with their tabs.  The
magnitude
of the field is adjusted by the relative alignment of the two rings,
and the rotation
of the rings as a pair adjusts the direction of the field.

Any smaller permanent magnets located near the front of the yoke are
usually
linearity or "sweep magnification" magnets, and should not usually be
adjusted,
nor should they need adjustment.