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Can I fix a missing capacitor by just soldering the spot short? (lemmy.blahaj.zone)

submitted 6 months ago by fnafdesktopfan111@lemmy.blahaj.zone to c/askelectronics@discuss.tchncs.de

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So, I've got a laptop screen that's giving up on me. 2/3 of the screen runs alright but the 1/3 on the left edge is acting weird. Half of the broken section displays an image but the image smudged and weird, while the other half is just dead. I opened it up to see what's up and, lo and behold, a wee tiny capacitor is missing (I know it's a capacitor 'cause I looked of the board marking, C248).

Now I'm wondering, since ordering a single capacitor just for fixing this screen is not worth the effort, can I just... put some solder in there to at least get power to where it needs to go? I know it's definitely not ideal but, this is an ancient laptop. Putting in the effort to fix it perfectly is not exactly a great value proposition. What I want to know most is, will the screen be damaged if I do this, or what could go wrong if I do this?

I'm pretty new to DIY electronics fixing so sorry if this is a stupid question. Thanks in advance y'all. Cheers!

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[–] MajorHavoc@programming.dev 18 points 6 months ago* (last edited 6 months ago)

The purpose of a capacitor is to build up charge slowly, and release a larger charge suddenly, under very specific conditions. The specific conditions are determined by the exact model of capacitor.

A direct connection at a point where a capacitor once was won't do the job. It'll both provide too much power (compared to zero) and too little (compared to the burst of extra power on release).

Shorting a spot where a capacitor once was will almost certainly damage other components.

If you're sure there was a capacitor there, and can find out which one, then you want to install (solder in) a fresh capacitor of the exact same make and model.

[–] Flexaris@discuss.tchncs.de 18 points 6 months ago (1 children)

No, you can't short the pads. If there's no capacitor there then there likely never was unless you can see obvious damage. Not all components are placed when manufacturing. A picture would help but it could be a decoupling capacitor which has been deemed unnecessary or not required for the model. So it's unlikely to be the issue. It could still be other capacitors that are getting old but it would be the electrolytic ones.

[–] fnafdesktopfan111@lemmy.blahaj.zone 11 points 6 months ago* (last edited 6 months ago) (1 children)

I'm confident it's that capacitor since I can see the pads aren't rounded but jagged (there are rounded ones on the board, and I know those are supposed to be empty). That would suggest that the capacitor was torn off by the plastic of bevel of the display assembly being pushed in too far. I can also see that there's a hole in the protective tape covering board, right where the capacitor should be. It's a bit awkward to describe but yeah. Plus, the previous owner also described scratching that bevel and symptoms showing up after they did it too often. So all in all, I'm pretty sure it's the cap. Still, thank you for the answer!

[–] Flexaris@discuss.tchncs.de 2 points 5 months ago

Okay, that certainly sounds like damage then. Most capacitors are pretty easy to solder and extremely cheap, you can buy them in small quantities. I would try a repair if you know what value capacitor

[–] soulsource@discuss.tchncs.de 2 points 5 months ago (1 children)

will the screen be damaged if I do this

Yes. Probably immediately and violently if the capacitor was used as a low-pass filter or a similar configuration. If the capacitor was used in a high-pass filter (or similar), then the damage might take some time to appear (if at all), and it might not be as violent.

what could go wrong

If it was a low-pass filter or similar, then expect other components to overheat quickly. Depending on what type of component dies first, you might be lucky and just get a light-emitting resistor. If however an electrolytic capacitor dies first, then you'll probably have to deal with a lot of nasty smelling smoke, that might take days to get rid of. Also, overheating electrolytic capacitors can explode if you are unlucky enough, what might do physical damage to the circuit board, or even the casing or panel.

[–] fnafdesktopfan111@lemmy.blahaj.zone 1 points 5 months ago

Thanks for the clear and concise answer, my man. Cheers!

[–] JohnnyCanuck@lemmy.ca 2 points 6 months ago (1 children)

I wonder if the cable connecting the screen to the main part of the laptop is the issue. They can wear out over time and being bent over and over.

[–] fnafdesktopfan111@lemmy.blahaj.zone 2 points 6 months ago

I also thought that was the issue at first. I've tried reconnecting and putting more slack/less slack but the symptoms stay the same. Plus, if it was worn out from open/closing the screen, I think it'd affect the backlight as well as the display when I wiggle the screen back and forth. But they're both is stable from what I see.