laptop motherboard "shorts" when connected to adapter

Disclaimer: I'm sort of a noob, and this is a long post. Thank you all in advance!

summary: completely dead laptop with no signs of life whatsoever (suddenly, for no apparent reason)

Here's the deal:

Lenovo Y470 (a few months past warranty with no water or shock damage). It stopped working suddenly (no lights, no sound, even when connecting adapter with or without battery).

I tried a different adapter (same electrical rating), but no luck.

I disassembled the thing completely, and tried plugging in the adapter and looking for signs of life with all different combinations of components installed (tried all combinations of RAM, CPU, USB power cords, screen, etc plugged in).

no luck. Then, I noticed (as I was plugging in the adapter to try for the millionth time) that there was a "spark" for an instant when I first connect the adapter to the power jack. The adapter's LED would then flash (indicating it isn't working or charging). So, I thought the power jack has a short of some sort (due to bad soldering or something). Scanned virtually every single component on the motherboard, and tested the power jack connections with a multimeter. No shorts or damage to anything on the entire motherboard.

Now I'm thinking I need to replace the motherboard.

But, my actual question: What does this "shorting" when connecting the adapter signify? (btw, the voltage across the power connections and current through it drop to virtually zero when the adapter is connected and "sparks", and they stay that way). The bewildering thing is that there are no damaged components, and the voltage across adapter terminals returns to normal after I disconnect it (so it's not damaged).

Please take a look at the pictures (of the motherboard's power connection and nearby components) and see if I'm missing something completely obvious...

enter image description here

Motherboard model: LA-6881P

Laptop model: Lenovo IdeaPad Y470

Cashew

Posted 2012-11-24T04:22:59.243

Reputation: 71

BTW, I checked for damaged capacitors; couldn't find any... It really doesn't look like a damaged motherboard, yet it just won't work... Has this ever happened to anyone of you before? What should I look for to figure out where the damage is? – Cashew – 2012-11-26T03:23:19.863

I also checked the components marked F12A (see first picture). I'm assuming this is a 12A fuse. It has zero resistance (therefore, I'm assuming it is still functional and isn't the damaged part). Are my assumptions correct? – Cashew – 2012-11-26T03:27:15.417

Can you please upload the images to imgur.com and then link them to here. I can't download the images as they are big in size and I have not a good bandwidth. – avirk – 2012-11-26T04:48:30.333

here – Cashew – 2012-11-26T05:03:07.297

I've embedded them here in the post for make it clear. Hope someone with hardware geek will solve your problem. :) – avirk – 2012-11-26T05:17:49.117

Have you tried a new power supply? – harrymc – 2012-11-26T15:51:45.130

I tried a new power supply (same ratings). Same results. – Cashew – 2012-11-26T15:58:24.653

By connecting adapter, I meant power adapter (where the power supply connects to the motherboard through a 4-pin connector) as seen in the last photo – Cashew – 2012-11-26T15:58:51.267

Have you tried to turn it on with the power adapter without the battery? Can you get another (fully charged battery) and see if it works without the adapter? – EliadTech – 2012-11-26T16:12:40.987

I tried with and without the battery. But I haven't tried with a charged battery. Is there a way I can easily charge the battery (without adapter, because plugging in adapter doesn't charge battery) so I can test it? (How else could I get a charged battery) – Cashew – 2012-11-26T16:34:01.623

Answers

Is there any battery installed in the system (CMOS or regular laptop)? I had an Epson laptop with an external battery that would behave that way (the LED would): If you plugged it in it would light up, but as soon as you tried to turn it on, the light would die. I suspect in your case, the battery is discharging slightly through the cable when it is connected.

K.A.Monica

Posted 2012-11-24T04:22:59.243

Reputation: 5 887

I did notice there is a small battery (it's soldered on to pins, so I cant replace it without some re-soldering). It's reading 2.9V when the motherboard is unplugged and when it is plugged in to power. – Cashew – 2012-12-02T01:05:03.537

Are you talking about the battery that sits inside the motherboard? that's what I'm referring to. I tried running the laptop with and without actual battery that the laptop runs on. – Cashew – 2012-12-02T01:06:36.313

I added a picture of the motherboard battery (I assume it's CMOS) – Cashew – 2012-12-02T01:11:55.937

The spark when connecting the adapter is more-or-less normal for a capacitive load. However, the fact that the voltage goes to zero would indicate a damaged adapter, a short in the internal power cable, or a shorted mainboard.

You reportedly tried with a different adapter, so it's probably not the adapter. (Did you check the on-board voltage with the alternate adapter?) You can connect the internal power cable to the adapter but leave it disconnected from the board, then meter, to see if the internal power cable seems OK.

But if the adapter and cable check out it's sounding a lot like a bad mainboard. The disk drive or DVD drive could possibly cause the short, but if you've tested with those disconnected (and also presumably looked for obvious external shorts between metal components and the mainboard bottom) then the mainboard is strongly suspected.

I think you gave it the old college try with this, but one isn't going your way.

Daniel R Hicks

Posted 2012-11-24T04:22:59.243

Reputation: 5 783

The on-board voltage with the other adapter is also low (close to 0.4V). I also made sure the internal power cable is OK (tested for shorts and tested for correct voltage). I've ruled out the DVD and HDD being the fault (disconnected them). I even removed the USB power cables (my last laptop had USB ports that shorted).

it does look like it's a faulty motherboard. Thanks for the effort (I wanted to make sure before I spent $300 or more...) What a shame (it was such a nice laptop) :( – Cashew – 2012-12-02T01:52:51.303

I couldn't fix it myself, and I sent it to motherboard repair specialists. Let's hope they don't charge insane prices. I gave you the bounty since you were the most helpful... thanks, guys! – Cashew – 2012-12-03T15:37:23.777

@Bash - Good luck! – Daniel R Hicks – 2012-12-06T19:57:16.860