Bad surface mount cap?

[EasyGoing1]

Hello,

So I have this roughly 10 year old PC power supply that won't power on. When I disconnected all of the cables, then shorted the green cable to ground, the power LED on the power supply went on for about half a second then nothing.

So I took it apart and one of the surface mount caps had this brownish residue around the edges of the solder joint. So I have a couple of questions:

1) Do surface mount caps have gel like material in them as electrolytic caps have? If not, what is that residue that seems to have come out of the cap? It scraped off like a crusty brownish residue like burnt brown sugar ... kinda...

2) The cap has no print on it of any kind. It is a light purple color. Does the color somehow determine the value? There is another cap right next to it thats identical. I assume if I remove it and measure its value, I could replace the one that seems to be bad with that same value???

3) Since I don't have any surface mount caps laying around, I'm going to try and find one from another circuit board that is no longer in use ... if I find a value that is higher than the one being replaced, is it reasonable to think that it should be fine in the long run? And am I right in thinking that a lower value would NOT be recommended?

Thank you,

Mike Sims

[Andrew F. Ali]

Re: Bad surface mount cap?

That crusty brown stuff is probably residual flux from manufacturing. I think aslightly lower or higher value may not affect the circuit much, not too low or high from the original value.

[diif]

Re: Bad surface mount cap?

Its flux. As always photos help.

[stj]

Re: Bad surface mount cap?

maybe.
simple test, touch it with a soldering iron.
if it stinks like cat-piss then it's electrolyte.

[EasyGoing1]

Re: Bad surface mount cap?

Its flux. As always photos help.

OK, if it's flux, then why am I not seeing it anywere else on the board?

I'll see if I can get a pic decent enough, but I already scraped it off the board so ...

So assuming it is flux, how CAN you tell if a surface mount cap goes bad?

[Andrew F. Ali]

Re: Bad surface mount cap?

Surface mount caps don't usually go bad.....If they do, though they would short circuit. I don't think you have a bad SMC. Your problem is due to some other component/s.

[EasyGoing1]

Re: Bad surface mount cap?

Here's a picture:

[EasyGoing1]

Re: Bad surface mount cap?

Surface mount caps don't usually go bad.....If they do, though they would short circuit. I don't think you have a bad SMC. Your problem is due to some other component/s.

So the symptom is that when I power on the supply by shorting the green wire to ground, the LED comes on for about half a second and then it goes out and it doesn't turn on at all ... any ideas?

[PeteS in CA]

Re: Bad surface mount cap?

That surface mount capacitor is a multilayer ceramic capacitor (MLCC). inside are basically alternating layers of thin ceramic sheet and metal film. There's no liquid inside.

So, when you apply AC power, what is your 5V Standby voltage? If you have an oscilloscope, what happens to the +5V (or +12V or ... pick one)? If you don't have an oscilloscope, monitor using a DMM and see if the voltage starts to come up or not. I hope you have a 'scope, as it will tell you if an output is tripping over-voltage protection, causing the PSU to shut down.

[jiroy]

Re: Bad surface mount cap?

Maybe it's that small amount of red glue they usually use to fix smd components prior to soldering . SMD capacitors do not contain any kind of liquids .

[stj]

Re: Bad surface mount cap?

it's probably flux, now we have a foto.
there is a lot of flux on that pcb, and some has gone white - so it's been in contact with water.

there are also a lot of joints that need resoldering.
(dark rings forming)

lets see the other side of this psu