2 smd cap burned, unknown value. Help - Page 1
Forum section: electronics / repair

author: azigta
author_rank: Regular Contributor
timestamp: November 06, 2016, 06:43:59 pm
content: Hi, I'm trying to repair a chinese mechanical keyboard that I got on a lan party because smoke started comming from the inside...I opened it and I found that an smd capacitor burned, burning with it also the trace that was comming up to a pin connector that also got burned. It also has another capacitor burned but the pads look ok.I attach some pictures so that you can see what I'm talking aboutPicture of the boardPicture in detail of the 2 burned capacitorsI can solder new capacitors with no problem, but I don't know what value do they have.I've worked soldering smd but never had to replace burned smd's so I have no idea what to solder there...Could I use a higher value capacitor? Does anyone know what capacitor could I put there?Thanks

author: Gyro
author_rank: Super Contributor
timestamp: November 06, 2016, 07:11:34 pm
content: I think those are ferrite beads for EMC filtering, not capacitors. the one on the left is labeled FB836.It looks to me that you have a significantly larger problem than just replacing the two parts. The track between the right hand one to the connector pin and the board around it are clearly badly burned. You have a severe short circuit of some kind.I think you need to try to find schematics and try to fault find it. Just replacing the two parts isn't going to help you at the moment, you need to trace the fault and repair the PCB damage first.

author: azigta
author_rank: Regular Contributor
timestamp: November 06, 2016, 07:46:30 pm
content: Quote from: Gyro on November 06, 2016, 07:11:34 pmHi, thanks for the reply. The board around the trace that's going to the connector pin it's a ground pad (one on each side of the track). I don't know what happened here but I think that maybe the component on the right just burned because it's just bad quality, and that maybe burned the other ferrite bead . I don't know much about electronics so I'm probably wrong but trying to replace the components seems like the only thing I can do based on my experience and knowledge, which unfortunately isn't much.I contacted a chinese company asking for schematics or boards that I can buy and they told me this : "Sorry we didn't have spare parts and will pass your message to original."I don't know if that means they are going to contact "the original" company that makes the pcb's. I don't quite understand it.It looks like without the schematic I'm pretty much done, but I don't have anything to lose here so If you think that replacing it with some other ferrite bead or other component could work, just comment here please.

author: wraper
author_rank: Unknown
timestamp: November 06, 2016, 08:08:49 pm
content: Quote from: azigta on November 06, 2016, 07:46:30 pmZero chance for it. Something is powered through those ferrite beads, and that something certainly is shorted.

author: Gyro
author_rank: Super Contributor
timestamp: November 06, 2016, 08:15:00 pm
content: Hi, no problem.Ferrite beads are just very low value inductors for interference suppression, they can normally just be replaced with a wire link unless you're worried about EMC specs. You may even find that they still have continuity as they are low resistance to start with.I don't know what those pins connect to, but I suspect maybe power supply?The first thing to check is that the one on the right really is a FB. There are identical looking parts connected to the other pins, check that they are conductive, not capacitors. The one upper left isn't in much doubt due to its FBxxx part ident. Check that it still has continuity.The next thing it to make any repair to the PCB. Scrap away the charring if possible as it might be conductive now. Repair the PCB track with a wire link as necessary.The main problem is to find the cause of the short. Schematics will obviously help of the manufacturer can get them, it is encouraging that they are at least trying. Other than that it's a matter of pencil and paper and tracing out the connections from that pin across the board - you say that it's a ground pad, but thet FB must be leading somewhere. I think you can discount the FBs being faulty, not if the PCB burned too!It may be as simple as a shorted decoupling capacitor somewhere on the board, or a fault in whatever is plugged in, an accidental short somewhere. Try repairing the damage, plug everything back together and probe to see if that pin is still shorted to(gound, the other pins?). I think it's down to some detective work. It may be a really simple fault, or it may be more complex. Any information about what happened when it failed - did you see it happen? Was there a thunderstorm maybe? Clues will help.Edit: Damn, I didn't notice the chip, and can't see well enough if it is burned, or just cleaning residue. I would maybe have expected something closer to the chip (or a leg) to burn first. Hard to say.

author: azigta
author_rank: Regular Contributor
timestamp: November 06, 2016, 08:16:39 pm
content: Quote from: blueskull on November 06, 2016, 08:03:47 pmHi, the chip at the center of the board doesn't look like it's fried at least not on the real thing, maybe in the image it looks like so, but the real thing just has a little bit of transparent substance that looks like it could be some kind of glue. It is also present on the lower 8 pin chip and as you can see on the image, the black connectors are sticked to the board with a transparent substance. So it could be that the machine that puts the connectors and components or the human soldering the things or whatever left part of the substance on other components of the board. But the chips don't look fried at least not on the board.I don't know which computer was this keyboard been used in, but as the usb connector that was connected to those pins is burned, including the plastic part that was inserted on the board, I also suspect that there was a severe overload.It looks like it didn't fry any other components apart from those 2 ferrite beads and the trace+grounds.Thanks for the reply

author: Gyro
author_rank: Super Contributor
timestamp: November 06, 2016, 08:24:54 pm
content: Ah, now that's information that we didn't have before - it's important to include ALL relevant information. If it's USB and the cable burned to that extent then is sounds very much like a nasty ground fault between the Keyboard and the computer! You need to check carefully what the keyboard case, connectors etc might have come into contact with. Likewise check the grounding of the computer.

author: azigta
author_rank: Regular Contributor
timestamp: November 06, 2016, 08:30:18 pm
content: Quote from: Gyro on November 06, 2016, 08:15:00 pmThanks for the detailed comment!Well, The thing that was connected onto those pins was the usb connector cable. I didn't see it happen, but it was on a 500 people lan party and it is highly probable that the guy had a short on the psu/motherboard or there was some kind of electrical problem. The chip thing , I'm pretty sure it's cleaning residue or some kind of glue.The ferrite bead on the left does have continuity, and the other ferrite beads close to the burned one too. The burned one doesn't have continuity but there's an arrow indicating the name of the component and it's labeled FB15. So it's clearly dead :\Tomorrow I will try wiring the lower part of the FB to the pin and connecting it to the rest of the keyboard and a safe testing cheap pc. Thanks