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I have a bad capacitor on this LCD power board. I have rev 2.6, and I noticed there is a rev 2.8 board that looks like it didn't get skimped on in parts. Can I install capacitors in my rev 2.6 board as they are used in the 2.8?

My board:

x20wg-1080p rev 2.6
9 x 470μF, 1 x 100μF, 1 x 22μF, 1 x 10μF

Here's the rev 2.8:

x20wg-1080p rev 2.8
6 x 1000μF, 4 x 470μF, 1 x 100μF, 1 x 22μF 1 x 10μF

The differences are circled in blue. My board seems to use lower rated capacitors, is missing one, and the microchip looks slightly different. The pictures are of the entire boards.

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When attempting to write an answer for you I realized there is a lot of data that you haven't told us here. I have answered with a lot of assumptions, but you will need to tell use more about what the capacitors are connected to before we can really tell you what to do. –  Kellenjb Jun 23 '11 at 17:35
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5 Answers

up vote 6 down vote accepted

The IC that you are pointing out is a 3W (or maybe a 2x 2W) stereo audio power amp.

The one in rev 2.6 is a SM7496L, which I am having a hard time finding any info about, but it is essentially the same thing as rev 2.8 just a different manufacture, so it is probably safe to assume that circuit should be the same around them. This means it should be safe to put a larger capacitance value on it.

For the group of 3 with one missing, it looks like these are next to something that is probably a voltage regulator with a big heat-sink on it. If this is the case, go read the answers for this question Capacitor Sizes for 7805 Regulator

For the other 2 capacitors, these look like they could either be used with the 2 diodes or could also be for the voltage regulator.

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An important consideration is the 'type' of capacitor you intend to replace with. You should seek out the specifications of the part you wish to replace and make sure that your replacement is comparable in terms of ESR, ripple current rating and rated life. If you inadvertently substitute a general-purpose capacitor (which usually has its ripple specified at 120Hz, and often doesn't specify its ESR) where a low-ESR, high ripple capacitor was (which usually has its ripple specified at 100kHz, and does specify its ESR), you're potentially going to be in for a smelly surprise...

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Wouldn't this most likely "just" reduce the lifetime? "potentially going to be in for a smelly surprise" sounds a bit overstated. Could a suboptimal ESR/ripple rating really cause that much heat? –  Rev1.0 Feb 3 at 8:13
    
Absolutely. High ripple caps are often rated for several amps at 100kHz. GP caps are rated for lower current which significantly derates at the high frequencies of a switching converter. Severe enough heating can lead to venting in hours or less. –  Madmanguruman Feb 3 at 12:00
    
Thanks, I wasn't aware this can be so critical. –  Rev1.0 Feb 3 at 15:33
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If the capacitance is between a transformer and a rectifier you need to take into account the conduction angle at the transformer. More C means smaller angle, means higher charging currents, more stress on the rectifier, so a higher current rating required for it; and a shorter duty cycle also means more energy to be dissipated as magnetism during the off cycle, so more magnetism, so more magnetic hum present to be coupled into the circuit via induction rather than conduction, ... and the transformer can only have been designed with a certain set of these parameters in mind. Small increases may be safe, large ones not.

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You can almost always replace a capacitor with one of a higher voltage. This is the limiting factor of a capacitor due to dielectric breakdown voltages that the manufacturer chose.

Varying capacitance gets a little trickier. If the property of capacitance is used for power supply filtering, then it is generally fine to increase the value.

I would verify that all the traces are the same and all the components are the same. If so, it looks like they decided that it needed higher values. This could be the reason your cap failed in the first place.

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Sorry, I'm actually talking about the μF, not the voltage. What do you think about that? –  Louis Jun 23 '11 at 16:40
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@Louis - I edited my answer. Just a tip on terminology, "rated" is generally a idea of how much a component can handle. For resistors, "rated" is in watts. For capacitors, "rated" is generally volts. What you are looking for is higher value capacitors, rather than higher rated one. –  Joe Sacher Jun 23 '11 at 16:45
    
Thank you, I've edited the title. I don't have the rev 2.8 board, so I can't verify the traces. I guess I'll just replace the bad cap. –  Louis Jun 23 '11 at 16:51
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"If the property of capacitance is used for filtering, then it is generally fine to increase the value." Not if the filter's cutoff frequency is important. –  endolith Jun 23 '11 at 18:08
    
Correct, I was talking power supply filtering in this case. I should have been more clear. –  Joe Sacher Jun 23 '11 at 18:34
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I submit the following image. I installed a 1500µF 6.3V motherboard grade cap into a product requiring a 1000µF 6.3V cap. It was too big, so I mounted it horizontally :D, adding hot glue to secure it down.

enter image description here

It works fine.

The product in question is an Advent power line ethernet adapter. I was given a pack of two. The first one died after 3 months and the second one after 5 months, which is ridiculously poor. In both cases, the exact same cap died. The products were new in the box but had been sitting around for about 6 months. Both are now repaired with new caps, and are working great.

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Just because it worked in your application doesn't mean it will work in his. Using this blog post as reference, you are just telling them to "press the green button". Just because pressing the green button might fix the issue doesn't make it a great answer. –  Kellenjb Jun 23 '11 at 19:24
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@Kellenjb Of course; I didn't say that it would work in his, but I showed for a power supply it is plausible. –  Thomas O Jun 23 '11 at 20:11
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