Gabriel said:
Hi and Merry Christmas!!, I have a Samsung Syncmaster 2493HM LCD 24" monitor
that will not turn on for a while when the power button is pressed but
everything else is fine as the picture on screen is not bad or does not
flicker so the inverter board should be good but from googleing the power
boards capacitors need replacing. This site below says how to do it and that
I can buy a kit from them :
http://www.ccl-la.com/blog/index.php/reparing-a-samsung-2493hm-24-lcd-monitor/
but are their caps good ones or can I get better ones somewhere?
On their site is this page:
http://www.ccl-la.com/blog/index.php/capacitors/
the above talks about what good caps are. Im using the monitor in question
right now for making this post but as I said when I turn it off for some
houres in time it takes a while to turn on. To my understanding the caps get
heated up from the monitor and become faulty or is samsungs caps not a very
good quality either way is the kit from above going to be good lasting
quality for the heat rating or should I look for some with better heat
handeling? Thanks all, GK
Nichicon and United Chemicon are generally good.
I've heard of the odd Nichicon going bad. Not very many. It's
possible to make electrolytics fail, by mis-applying them
(running too much ripple current through them).
An issue with caps, is counterfeiting. There isn't much to identify
who made them, when they slide a plastic sleeve over the aluminum
to identify the product. It isn't hard for a Chinese shop to
take an un-sleeved capacitor, and make their own sleeves for them
(with anything they want printed on them). Which means, if you're
buying them, you need to deal with someone you can trust to buy
them from the actual company. (There is at least one web
site, where they track reports of counterfeiting, to help
others in the industry to be more careful about who they
buy from.)
With any capacitor company, they make multiple lines of capacitors.
It's kinda hard sometimes, to figure out why they bother with as
many lines of products as they do. Each line has a different
temperature and "thousands of hours" rating. You use the
Arrhenius relationship, to re-rate the caps to a similar
temperature. And then you can select the one with the most
hours at that common temperature. That's how you identify
the best lines of them.
Performance is related to volume. A typical situation you
might find on a motherboard, is caps that are "one size too small".
When you go to buy a substitute, all the substitutes of that
voltage and capacitance rating, are one size too large and
won't fit. That's generally a sign the original parts probably
weren't all that good. A claim I've read is, the companies
that make the caps are pretty well matched on the performance,
so all the sizes of the caps should be similar. If a cap is
"impossibly small", generally it means some parameter has
been compromised.
In terms of doing substitutions, you can't just run off and
substitute anything you feel like. Some switcher designs,
the design is "centered" on "midrange" quality caps, where
the ESR isn't that low. A natural temptation might be
to say "oh, I can replace all of those with one Oscon".
If you do that though, you should go through the datasheet,
and rework the equations, and see if the predicted performance
is still adequate. The application section of the switcher
chip datasheet, will usually have a section on capacitor
selection, which is where I learn about these things.
Now, someone selling a "capacitor kit", might be
Albert Einstein. Or, they might be some pimply
teenager with a nose for business. Your job as
the buyer, is to decide what kind of person this is.
Chance are, they won't be stupid enough to sell you
Oscons because they feel they have superior ratings.
They should be replacing "like with like", in terms
of the "class" of the capacitors. If the original
capacitor was a "switching" class capacitor, the
replacement should be too. (i.e. Nothing you can
buy at Radio Shack, is even close
Even my "good"
electronics store, doesn't have good caps.)
If you want to understand more about this, you'd download
the Nichicon product overview for electrolytics, see
some rated for "switching supplies" and so on. Then see
which families are used in the capacitor kit you're
buying.
Not all the caps on a board, are part of switching
regulators. Some are simple bulk capacitance, for
improving transient response to step loads. They're
"sprinkled" across a board. There's a limit to
how many you can use, because the power supply
feeding the board, is only stable up to some number
of thousands of microfarads of load. Some of those
caps, might not be subjected to the same level
of abuse as the ones in the switching regulator.
You can run enough amps of ripple current through
a cap, to actually heat it up. They use a bunch in
parallel, to share that current. In some cases,
the actual capacitance value isn't all that important,
and the tolerance on total capacitance is sloppy. Not
the same, as if you had an RC timing circuit, and
the value of R times C was critical. In a switcher,
the desired property might be ripple current
rating, as much as anything. If the capacitance was
off by 30%, the transient response would be a little
different, but it probably still works fine. So if
you had a 2200, and only an 1800 at the same voltage
was available, it's probably OK to use it. So
don't freak out, if the capacitor kit happened to
substitute an 1800 for a 2200. They can't go too
far in one direction, because the size of the
aluminum can won't fit. If the original was
"impossibly small" to begin with, they might have
no choice but to sub an 1800 for a 2200. Because
any decent 2200 might be too big to fit.
On the PCBs, I'll never figure out why the caps
have to be touching. They should leave some
clearance around them, to make it easier for
the factory to purchase substitutes. The only
reason for making them touch, is if you plan to
glue them together to make them resistant to
vibration (like you have to do for caps in a
sub speaker+amp). In a monitor, I doubt the
caps are going to be glued to each other
for moral support. They could spread them
out a little more, and leave room for "one
can size larger".
Paul
