Motherboard capacitor witb buldging top

[pchan]

Hi All,

Several months back my desktop PC started having long beep during boot up. I suspected it was the memory module issue. I took one module the problem and beep went away. I swapped the module, the beep came back. I assumed that module was faulty. I put back good module and make do with 512MB. Several days ago, it failed to boot. While inspecting the motherbaord, I noticed the capacitor beside the RAM module has a buldging top. I happen to have a similar motherboard that is non-functional. I manage to salvage a good capacity from this board and replace the one with buldging top. It booted up and I put back the "faulty" RAM module. It worked. So, it was the faulty capacitor that caused the POST to detect the RAM module as bad. If you have spare RAM modules and motherboard, don't throw away. You may need it in the future.

One of my friends said it's OK to use a higher rating capacitor if a similar rating cannot be found. Wouldn't this change the characteristic of the circuitry ? Anyone expert care to comment.

[Constant Gardener]

Usually there's a fair bit of slop in designs that use electrolytic capacitors. You can expect a larger capacitance value, say twice the original, to work, and even one that's a bit smaller will probably be okay. But don't use a capacitor rated for a lower voltage, or lower temperature.

[redwudz]

Most times those type of electrolytic capacitors in that type of application are used to 'smooth' down the voltage pulses and usually the capacitance specification isn't that critical if you go a bit higher for capacitance, though the voltage rating is. Higher voltage caps are usually larger and may be harder to fit. Lower voltage caps may fail fairly soon. Most times, you want to use as close as you can to the original specifications of capacitance and voltage.

Some other types of capacitors, usually the solid and much smaller types, are more often used for timing and some types of specific filtering and their capacitance specifications may be critical to proper circuit operation.

I have also replaced a few of the electrolytics surrounding the CPU on a MB, using ones of a bit higher capacitance, but at the same voltage rating. Of course, if one failed, another beside it may be about to fail also. But you may be able to get a bit more use out of the motherboard. Worst case if you fail in replacing a dead electrolytic, you aren't any worse off as the MB wasn't working anyway.

[pchan]

Thanks for your promt reply.
The capacitor concerned has the following rating.
1000uF, 6.3V, 105 degree celcius
So, if I stay above these figures, I should be OK.
The dead mobo has several buldging top caps. So, the one I replace may no last.
I am asking this question in anticipating more buldging top capacitor.

[redwudz]

So, if I stay above these figures, I should be OK.

At or above those figures, yes. Larger capacity or higher voltage caps may be larger in size, but would still work.

I would probably replace the caps with new ones as ones from another MB may also be old and their leads are a bit short to work with.

[SLK001]

If the capacitor is used for power supply ripple filtering, then using a larger value is no problem, although sticking to the same value is usually the better idea, since you may not be able to install (phyically) larger parts in the space provided on the board. Also, cheap electros are known for bulging, especially in high humidity areas. If you really feel bold, replace all of them with quality parts such as Panasonic - the more expensive parts here are really worth the extra cost. You're in Singapore, so I don't know about local suppliers of these, but Digikey has a large selection of quality electros. Just remember to observe the polarity of the part when installing (electros installed backwards will EXPLODE when powered).

[Steve Stepoway]

If you haven't already, take a look at www.badcaps.net. He seems to know a lot about bad electrolytics on motherboards.

[ranchhand]

I've replace them, including even TVs, and always use higher ratings; better that way, a bit "beefier" to (hopefully) last longer. I never have had a problem and saved a bundle. Just my opinion, but I see an increase in bad caps lately, and I think it's going to get worse as cheap junk is used by manufacturers' suppliers from 3rd world countries. One thing I notice, I never had any caps go bad with an Asus board (no guarantee of course).

[TBoneit]

If the capacitors with the bulging tops on the dead motherboard are the same brand and line of capacitor as the one you salvaged then it will go bad too.

Example if the bad ones are marked KZG and the salvage one is also marked KZG it will fail. The KZG line is from a good maker but a dud line.

Capacitors need to be replaced based on Capacitance, Voltage, ESR, Temperature rating, & physical size may or may not matter but lead spacing does matter.

ESR is a way of measuring how good a job it will do of filtering.
As an example I've seen 1000uf caps that are much smaller than others for the same 100uf and 6.3V. That means they won't filter the same.

[pchan]

You folks at VH are just super. Such a wealth of knowledge. I am a lot wiser dealing with cap.