On Feb 9, 12:42 pm, Wesley Furr wes...@megley.com wrote:
I guess the first question would be...if they are just sitting around not
being used, are the caps leaking more?
They are probably more likely to leak when not in use. Some kind of
process occurs in the capacitors which reforms the dielectric layer
out of the electrolyte when the electric field is in place. But I am
not certain. In any case, they definitely leak, sitting unused.
Whether they're less likey while actually being used, I'm not certain.
Next question...are all of them SMD surface-mount caps?
It depends on the machine. The SE/30 has two axial electrolytic caps
in addition to eleven surface mounts. The IIci has four axial caps,
as well as ten or thirteen surface mounts. I don't know the count in
the LC.
That is what it is
looking like from browsing around online a bit... I've replaced a good
number of capacitors in things, but have never worked with surface-mount
capacitors. Any tips on how to do this successfully?
There are a number of useable methods. I prefer to use two soldering
pencils, one in each hand, and apply them simultaneously to the pads
of the SM capacitor until it easily lifts. Don't grind the pencils
in, and don't force the cap up. A pair of 40 watt pencils seems to do
the job quickly. I've done it with a pair of 15 watt pencils ($10
at Radio Shack) but it takes a few minutes to get the pad hot enough
to melt the solder with 15 watt pencils.
I prefer this method because if you are careful and patient there's no
chance of stressing the pads, as there is when you apply physical
forces to the cap.
As long as you are patient, the two pencil method is really easy. It
also helps to have some liquid flux on hand. Applying liquid flux
before desoldering makes the solder melt easier.
Then of course the next question is how to
re-install! Anyway...any tips folks could provide would be appreciated!
Clean all the old solder off of the pads using good desoldering
braid. I like the Chemtronics brand Chem-Wik line. Other brands
don't seem to pick up the solder as well. After the old solder is
cleaned off of all the pads, thoroughly clean the board.
Once the board is clean and dry, tin (apply a small amount of solder)
to just one pad for each position where a capacitor will be
installed. Place a capacitor on its pads. Position it carefully.
Gently press down on it with something like the blade of a flat
screwdriver to hold it in place. Apply soldering pencil to the
tinned pad. Wait for that end of the capacitor to sink through the
solder down to the pad. Check the positioning. Remove the soldering
pencil. Remove the screwdriver. Now, solder the other contact.
It's really very easy. I find it simpler than through-hole
capacitors.
And for cleaning up the leftover leaked stuff...how should that be cleaned?
Q-tip with rubbing alcohol good enough? Or is it safe to just leave it
alone?
There are many opinions on this as well. Well, not on whether to
clean or not. Definitely clean the board thoroughly. The leakage is
corrosive.
In the following, get as pure of alcohol as you can. Rubbing
(isopropyl) alcohol is fine, as long as it does not contain any
colors, moisturizers or scents. The ingredients should be alcohol and
water. However, the common stuff is only 70% alcohol. Drug and
grocery stores around here sell 91%, so get that if you can. The
hardware store (Home Depot) sells gallons of nearly pure methanol, of
you want to go hard core. And there are other sources of mostly pure
alcohol.
You can:
1) Spray the board with solder flux remover. Scrub stubborn or thick
accumulations with a soft toothbrush. Follow with an alcohol rinse.
Follow with a distilled water rinse.
2) Skip the solder flux remover and just use the alcohol, followed by
the distilled water.
3) Skip the distilled water in either or both of the above.
4) Put the board in a dishwasher with no detergent and the drying
cycle turned off or set to no heat.
And lastly...what should I replace them with?
I recommend tantalum surface mount capacitors. They are affordable,
and won't leak. They may wear out some day, but that's likely to be
many many years in the future. Make sure the capacitance value is the
same and the voltage rating is the same or higher.
I recommend against replacing the SM caps with axial or radial caps.
SM pads are easily lifted from the circuit board. If you attach
leads to the SM pads, you provide a lever which can pry up the pad.
Don't do that. They're a little cheaper, but it's not worth it.
The new solid caps that look like electrolytics are also good. I
can't remember their exact name at the moment. They tend to be more
expensive than using tantulums.
Finally, for a vast amount of discussion on this topic going back at
least six years, search the forums over at 68kmla.org, especially the
Compact Mac forum.
Jeff Walther
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