Re: Hardware Question about RAM and Capacitors
On Thu, 12 Dec 2013 14:11:32 -0500
"Goren Buckwalk" <goren.buckwalk@mail.com> wrote:
>
> Are three rusty capacitors and 3 bad slots just a coincidence? Thanks.
>
>
The exactly matching numbers is a coincidence, but overall it isn't.
Dying capacitors mean that the power rails they are attached to are no
longer nice and clean and smooth, but contain voltage spikes and
steps. When the voltage including these transients falls outside the
range in which RAM (or anything else) works, then it starts causing
errors. RAM draws quite a bit of current, and the higher the current,
the worse the spikes, so removing RAM may well take the voltage
back into the working range. The worse the capacitors get, the more
you need to reduce the current taken to get things working again.
Capacitors degrade prograssively rather than dying instantly, and you
can't see the loss of capacity, so the number that are visibly damaged
is a coincidence. They will all be on the way out.
So yes, replacing the capacitors is the only option. And as has been
said, it isn't a trivial job. It's easy in principle, but the board is
thick and the holes aren't that much larger than the wires, and the
wires are connected to large copper areas, so it's difficult to melt the
solder. Too small an iron simply won't do the job at all, you want at
least 50 Watts with a short bit or an equivalent hot-air rework tool.
Having removed the capacitor, it may be even more difficult to remove
the remaining solder from the holes, or at least enough to fit a new
component. If you succeed in removing the dodgy capacitors, look around
their positions to see if any electrolyte has leaked out, and if so,
clean it up. It's water-soluble and corrosive, and generally eats
copper and other materials, so don't leave it there, even if it looks
dry. It *will* eat the PCB tracks nearby.
Finally, these capacitors are chosen for very high ripple current
ratings at high frequencies, so any replacements need to be designed to
do this particular job. Obviously, use the same capacity and voltage
rating (not larger in capacity, as that may over-stress the switching
regulator which they work with, and not higher in voltage as it won't
do any harm but you'll be trading that for ripple current) and
preferably about the same physical size. The smaller the capacitor, the
lower its ripple current rating will be. The ripple rating won't be
marked on the capacitor, though you may be able to look up the model
number in a datasheet. Just get the highest ripple current you can find
for the voltage and capacitance and physical size, looking for figures
quoted at tens or hundreds of kilohertz rather than 50/60Hz values.
Capacitors designed for switching regulators won't usually have a 50Hz
value quoted anyway.
--
Joe
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