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I've been teaching myself the fine art of monitor repair - replacing capacitors on a trio of lcd screens. Apparently they all had capacitors that had suffered from the plague - though a mild case of it since they bulged a tiny bit, as opposed to leaked all over the place., and replacing them has allowed two of them to be usable, least over the past week.

The third however has exhibited the same symptoms as it did before - one of the two capacitors i replaced seems to have bulged again, as did one of the ones i didn't (made by the same company that made the caps i replaced earlier). Since it was only on the board a few days, and ran for a total of maybe an hour i'm assuming its not capacitor rot.

Did i just get a bum capacitor, or could there be other issues? Is there anything i can test to try to work out the source of the problem?

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    \$\begingroup\$ What manufacturer and model of capacitors are you replacing them with? \$\endgroup\$ – markrages Dec 28 '11 at 1:30
  • \$\begingroup\$ By the looks of it, some company called 'stone' which my friendly local electronics shop had. \$\endgroup\$ – Journeyman Geek Dec 28 '11 at 1:42
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    \$\begingroup\$ You need special electrolytics for a switching power supply. Low ESR, high RMS current handling. General-purpose electrolytic caps will fail quickly. \$\endgroup\$ – markrages Dec 28 '11 at 1:50
  • \$\begingroup\$ @markrages That is what I gathered from a site called "Mike's Arcade World"; apparently manufacturers try to save a little money by using capacitors that will eventually die. Unfortunate. I have an Antec ATX supply (True 380) that has a similar problem. \$\endgroup\$ – Mark C Dec 28 '11 at 5:34
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One big issues with consumer electronics is that they use the cheapest components possible. Electrolytic capacitors in particular are notoriously failure prone.

When replacing failed capacitors it generally a good idea to replace them with ones of equal (or close) value but with a higher voltage rating. For example if the manufacturer used a 6 V capacitor on the 5 V line, you should consider using one rated for 10 or 15 V.

You probably did not get bad capacitors, assuming a reputable source, but if the replacements had the same ratings as the original, and the originals were marginal (and thus failed), the replacements were probably marginal as well.

I should probably add that heat is not your friend. How is the cooling in that part of the board? If the air vents are clogged with dust etc. that could cause them to fail as well.

If you would like another opinion see this EEV Blog forum. The conscious seems to be that you can't go wrong with Panasonic or Vishay capacitors but that many of the lesser brands will be nothing but problems. (Of course there are other good brands as well).

"That means you absolutely have to have low Rs (esr) at some high frequency. I wouldn't touch anything that didn't have a data sheet showing low esr at 100 kHz. If it isn't specified, you don't want it."

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  • \$\begingroup\$ Its relatively clean- i blew off a fine coat of dust with compressed air, but there's no dustbunny colony or anything. The originals lasted between 3-5 years. The original capacitor there was 35 V (there's another 25V cap of the same value which seems fine) as was the replacement, as is the other cap thats failed now. \$\endgroup\$ – Journeyman Geek Dec 28 '11 at 1:25
  • \$\begingroup\$ JG: Capacitors in a properly designed circuit should last far longer than 5 years. \$\endgroup\$ – JonnyBoats Dec 28 '11 at 2:28
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    \$\begingroup\$ As user "Markrages" noted above, the capcitors in switching supplies should have a low resistance and tolerate high current. If the resistance is not low enough they will heat up, and as Jonny warned that will damage them. \$\endgroup\$ – Mark C Dec 28 '11 at 5:38
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It's very simple: Using a cap in ways that are "not allowed according to the design specs" are going to cause it to fail. The obvious way would be to subject it to voltages or temperatures higher than what it's rated for. You could also run currents that are too high, etc. You would have to consult the cap datasheet for a complete list of specs that you must stay within.

Doing this to a cap might make it fail in ways that are not visually obvious. In other words, the cap might fail without bulging, discoloration, etc.

Caps, and Aluminum Electrolytic Caps in particular, have a short lifespan relative to other components like resistors and such. You can increase the life of a cap by not stressing it as much. For example, instead of using a 16 volt cap in a 15 volt circuit you can use a 25 or 35 volt cap. This can dramatically increase the life of the cap. A similar thing applies for temperature and current.

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