Blown capacitors in PC

[JonathanAnon]

Hi lads,

trying to repair a PC for a friend. The PC has an issue that when you power on its POSTs but then immediately shuts off before booting to the hard disk. I spotted that a couple of the capacitors beside the processor looked like they had blown.

When I took out the bored I was able to wipe off the dielectric from the top of the caps, and now the PC is booting.... I thought once they were blown they were blown and that was it.. I am gonna replace both, but is this behaviour (me wiping off the dielectric and this resulting in the PC then booting) consistent with what you would expect ?

 

[GreenGiant]

your wiping off the caps could have inadvertently removed a short somewhere that was causing the computer to fail after post, the other thing could be that the caps are merely filters and you wiping the dielectric brought the capacity back to an acceptable level

I would recommend going with your plans to replace them, but that is my take on it, its a fluke that will not last long.

 

[shrtrnd]

I agree with GreenGiant.
Your electrolytic caps are leaking the electrolyte. Possibly due to circuit usage, possibly
due to the heat from those big aluminum heat sinks.
You'll probably be fine just replacing the two caps you indicate.
If it was me, I'd blow the dust out of the chassis, and replace all seven of the caps
between those heat sinks, just to be sure. They were all made at the same time.

 

[KrisBlueNZ]

It's probably just a coincidence that it passed a POST after you cleaned the electrolyte off the caps.
A few years back there were many cases of defective electrolytics on motherboards due to cheap components that used the wrong electrolyte formula. If this is another case of the same thing, I would replace all of the large electrolytics (and perhaps any small ones too) that have the same brand name as the leaking ones.

 

[KJ6EAD]

A few years back there were many cases of defective electrolytics on motherboards due to cheap components that used the wrong electrolyte formula.

The problem was so pervasive that it came to be called the capacitor plague.

http://en.m.wikipedia.org/wiki/Capacitor_plague

 

[JonathanAnon]

Hi lads, thanks for all the responses .. very informative... That's interesting about the capacitor "plague" as well, never heard about that.. It's a Packard Bell PC, so no doubt the stuff put into it is gonna be the cheapest stuff...

See my only ambiguity is the fact that I (stupidly) did two things at the same time .... i) I removed the board from the case and set up the components on my desk and ii) I rubbed off the electrolyte from the top of the capacitors....

So half of me is thinking that it may have been a short between the board and the case, and the other half is thinking that it's the capacitors plain and simple... The capacitors seem to be connected to the power rectifications, so it would be used in this context to filter off any AC components (to give a purer DC signal) ?

 

[shrtrnd]

Caps don't leak like that, unless they've gone bad.
I would've suggested what KrisBlueNZ said. Your Packard Bell was put
together at the same time, I'd worry about all those caps, but I figured you
wanted to try to save money. Who knows how much longer the PB will last.
I'm with you KrisBlueNZ, you can work on my gear anytime.
Yeah that Chinese electrolyte problems was well documented.

 

[CocaCola]

The problem was so pervasive that it came to be called the capacitor plague.

http://en.m.wikipedia.org/wiki/Capacitor_plague

I actually came across a site offering cap replacement bundles for several motherboards many years ago, at first I thought it was a joke or just some scam to turn a quick buck by insisting people needed to replace them, until I looked into it further...

 

[JonathanAnon]

It's not a happy story.. Replaced the two caps with ones from another motherboard, and now it is behaving EXACTLY the same as it was when it was given to me... i.e. fans start for literally two seconds, POST screen comes on ..... the all power goes...

Thought I did a clean enough job on the solder but apparently not :-(

 

[KrisBlueNZ]

That's why we're suggesting that you replace ALL the caps of that brand. It's quite possible that the caps that are actually causing the problem are not showing any clear signs of failure. The fact that ANY of them are showing the tell-tale signs means that ALL of them are suspect.

 

[JonathanAnon]

Yeah I know Kris, I dont mean to be disrespectful by ignoring the agreed course of action, but unfortunately I have no way of getting seven new capacitors in time (nearest store is 100Kms away and have to have it done for tomorrow). Also, I thought the two that were definitely blown were in positions that were doable for me, the others I'm not so sure.. I'm gonna see if I can get a schematic for the board and see if I can learn something from it..

And I have to ring a glazier to fix this motherboard shaped hole in my window..

 

[(*steve*)]

Leave it powered up for a while and then try booting it again.

Capacitors generally have an ESR which lowers with temperature. The fault causing the higher ESR makes them heat up fairly quickly. Often they will heat up to the point at which the circuit will work (an early symptom is that the motherboard fails to boot the first couple of times and then works OK).

It's not a cure, but maybe the fix you've put in is good enough to have it working when warm.

Also possible is that the worst affected capacitor(s) were not the ones leaking.

 

[JonathanAnon]

I had to get the PC back to my friend, so I replaced the board / processor / RAM from an old PC... thankfully I managed to get his hard drive to work with it, and got the XP license registered. However, I'm gonna fix the other board in my own time.... It shall not beat me..

One question with regard to the capacitors... I know that capacitors have different "response" times, like those used in cameras.. I am currently looking for replacement capacitors on the basis of capacitance / voltage / temp ..... Is it okay that I just look up my electronics provider's catalog for any electrolytic that matches those three values?

 

[KrisBlueNZ]

In this application, ESR (effective series resistance) is a very important parameter (as well as capacitance and voltage, and to a lesser extent, temperature). These capacitors are providing smoothing on voltage rails that may be generated by a switching regulator (which produces ripple) and the rails are used by devices that have irregular, non-static current consumption. Both of these factors mean that the capacitors have to be "stiff" in their ability to keep the rail voltage steady, which means they must have a low ESR.
Some switching regulators can be unstable if the ESR of the output capacitance is too low, but generally you should be safe going for replacement capacitors with lower ESR than the originals, especially in this case where many capacitors are used in parallel. When capacitors of the same value are paralleled, the ESRs of the capacitors can be considered to be in parallel (ignoring PCB copper impedance, which is minuscule when power and ground planes are used, as is normal in multi-layer boards), so the resulting bulk ESR is lower than the ESR of one individual capacitor. (The formula for combining resistances in parallel is 1/Rtotal = 1/R1 + 1/R2 + 1/R3 ...)
If you don't know the ESR specifications of the original capacitors, I recommend getting the lowest ESR parts you can find of the same type, without breaking the bank (a few dollars maximum, probably under a dollar, per capacitor).
Higher voltage parts will also usually have lower ESR for the same capacitance but make sure the replacements will fit on the board!

 

[jackorocko]

http://www.badcaps.net/pages.php?vid=31

I would highly consider using the capacitors recommended on this site as replacements. Where you buy them is up to you. When I repaired my video card I bought the rubycon MBZ and MCZ series.

 

[JonathanAnon]

If you don't know the ESR specifications of the original capacitors, I recommend getting the lowest ESR parts you can find of the same type, without breaking the bank (a few dollars maximum, probably under a dollar, per capacitor).Higher voltage parts will also usually have lower ESR for the same capacitance but make sure the replacements will fit on the board!

Hi Kris, thanks for that advice.... If I take an example, there is a monitor that I'm also repairing, that has a number of blown caps in the power circuitry (see below) but there is no information about ESR for any of them. I found a spec sheet for the CapXon KF range which says that they are "low impedance" but does not give an ESR value..

EDIT: Hang on I think I might have it in this document
http://www.capxongroup.com/files/KF Series_1767828587.pdf

I have clarity on ALL the other parameters ... but have no even an indication as to the ESR of the CapXon caps..

 

[KrisBlueNZ]

The CapXon data sheet you pointed to DOES state the ESR, in the "Impedance" column in the table. Every capacitance / voltage combination has a different ESR, so ESR is listed independently for each combination in the table. In fact, "impedance" is a more appropriate name than ESR for this characteristic. It would be better to call this parameter "ESI" (effective series impedance), but I've never heard that term used.
These caps have fairly low ESRs, but not as low as the ones used on computer motherboads. Other signs that these capacitors aren't critically low-ESR are the single-sided PCB (to get truly low-impedance power rail smoothing, you also need low-impedance copper paths on the circuit board, and this is impossible with a single-sided PCB), and the fact that there are no components that need very "stiff" voltage rails. Components that do are ones where (a) the supply voltage must not be allowed to jump around much, because they are high-speed semiconductor devices and will malfunction if that happens, and (b) the device's current drain is irregular and varies a lot over a time frame of nanoseconds to microseconds to milliseconds. This means fast CPUs, GPUs, and custom high-tech devices, which incidentally have lots of pins, which is another reason why the PCB will be multi-layer.

 

[JonathanAnon]

Yeah, I edited that post Kris, the PDF I found from CapXon which I had originally included did not have the impedance info..
http://www.spezial.com/doc/capxon/kf.pdf

I think I've worked out the chart and where my capacitors fit in to it, so I'm gonna order them now... thanks for your help..

 

[(*steve*)]

Is that from an Acer monitor?

If not, it's a remarkably similar design. I replace those all the time.

There is enough room to replace those capacitors with something a little larger.

I also replace all 5 even if only one of them looks swollen (in fact I'd probably replace them anyway). Measuring the ESR of the capacitors after they've been in use for some time compared with new ones is very revealing.

After you've replaced the capacitors make a note on the metal chassis so the next person who looks inside knows what you've changed and when. I use permanent marker.

 

[KrisBlueNZ]

After you've replaced the capacitors make a note on the metal chassis so the next person who looks inside knows what you've changed and when. I use permanent marker.

EXCELLENT advice Steve.
But advice that's rarely followed - I've NEVER seen any notes on previous repairs inside any electronic equipment I've looked at. I'm not a professional repairman though, so I haven't see that many. But even when I was an apprentice at an electronic repair outfit, I never saw any notes. The best we could do was to look for previous job numbers for the same customer, and even then the job cards didn't have much detail.
You can get clues from looking, like fresh solder on components, but even then you can't know WHY the part was replaced, or even whether it was replaced or just resoldered, and you can't tell how long ago the work was done.
If everyone made a note of who, when, and what they did, it would be very helpful to to the next technician. In fact it can be helpful when YOU are the next technician, because you may not remember what you did and when and why, unless you keep a notebook.
I guess most people think it's not worth the time. But it doesn't take long. Perhaps subconsciously, techs don't WANT to help other techs. I don't know.