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Old 06-15-2021, 04:38 PM   #1
mferna14
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Default Lenovo ThinkPad W520 Dead, shorted

Hi all., working on a Lenovo ThinkPad W520. When I got it I put in the Adapter & the Amp went way too high meaning there is a short to ground on the main motherboard. Removed the MB & found 5V rail is shorted to ground. Removed the coil L32 5V coil & injected 1.5V (Pin 1) to this rail. With the multimeter this rail is totally shorted out (ZERO ohms) & when injected 1.5V ~ 2V, No components are turning hot. Is there any other way to find this shorted component. I am assuming it a ceramic capacitor BUT which is it in these thousandth. Thanks in advance for any tips.
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Old 06-16-2021, 09:12 AM   #2
Spider1211
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Default Re: Lenovo ThinkPad W520 Dead, shorted

Post schematic/picture for your board. When you removed 5V coil, it your main rail still shorted?

Assuming this is your schematic: https://www.badcaps.net/forum/showthread.php?p=949155 , remove R1090 and R1091 to see if short is on VCC5M_SOURCE or VCC5M_OUT.

If it's on VCC5M_OUT, it can be those "all in one' driver chips like U138.

When I get devices with such drivers, I avoid injecting voltage but rather remove the output coils and narrow down the short circuit. Also, since they are soldered to high thermal mass areas on the board, it'll longer to detect heat.
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Old 06-16-2021, 10:53 AM   #3
mferna14
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Default Re: Lenovo ThinkPad W520 Dead, shorted

Thank you very much... I was assuming that chip like U138-9-40 are shorted. I have to removed them. I narrowed down the trouble shooting.. I removed the CPU the short is in this section, but when I put back the CPU the short is all over the board. Guess one of these chips are shorted.
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Old 06-16-2021, 01:01 PM   #4
Spider1211
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Default Re: Lenovo ThinkPad W520 Dead, shorted

It could also be a dead/shorted CPU depending on which FET in the driver chip failed(high or low side).

You can check but first remove the inductor (L96 in your case);
To check low side mosfet, place 1 probe on ground and the other on inductor pad (driver side) -> you should not get continuity.

To check high side mosfet, move probe from ground to Vcc (VCC5M_OUT in your case) and keep the other probe same place -> you should not get continuity.

Does the short go away when the CPU is removed? Newer generation CPU have fairly low resistance. If you have a known good CPU that fits your motherboard, try it and check its resistance.

Also, with L96 removed your short on L32 should disappear if indeed it was shorting from one of the driver chips.
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Old 06-16-2021, 05:02 PM   #5
mferna14
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Default Re: Lenovo ThinkPad W520 Dead, shorted

Very impressive... I shall do as you say. But the CPU is not shorted. Even I remove the CPU, the short is still there... Hopefully by removed L32, I can short list my trouble shooting, then you are right, L32 should not have the short. You will not belive me at once stage upon testing components there was no short at all. I measured every caps which were shorting, they were all ok, but as soon as I placed the CPU in the slot, & inserted the power jack, at once the short appears & the coil started to make noise, my app meter showed 4.5 amps. This is why what you said is right to remove the L32 & then check. Maybe I could find the short ot component. Thanks dear
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Old 06-17-2021, 04:13 PM   #6
Spider1211
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Default Re: Lenovo ThinkPad W520 Dead, shorted

Just to make sure I understand correctly. The short is on the 5V rail or is it also on the main (19V) rail? What is resistance to ground when you measure sense resistor after 2nd input mosfet?
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Old 06-17-2021, 05:03 PM   #7
mferna14
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Default Re: Lenovo ThinkPad W520 Dead, shorted

When I add the CPU then the 19V rail get short too. It becomes 0 ohms, clear short to ground. But I know that the CPU is 100% good as I have checked it on another laptop. Anyway I have ordered U138 (5 pcs), I am 90% sure that the problem lies within these 4 chips, But I have to do this one at a time & check which is shorted. Let me get them first & will keep you updated. Thanks
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Old 06-18-2021, 03:55 AM   #8
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Default Re: Lenovo ThinkPad W520 Dead, shorted

Remove CPU (ideally also L96) and use post #4 to find out which driver is faulty. Once found, remove it, insert CPU and check if short is still present. Do not apply power.

That way you can check if there isnt other issue with your board while waiting for replacement parts.
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Old 06-18-2021, 09:27 AM   #9
mferna14
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Default Re: Lenovo ThinkPad W520 Dead, shorted

Quote:
Originally Posted by Spider1211 View Post
Remove CPU (ideally also L96) and use post #4 to find out which driver is faulty. Once found, remove it, insert CPU and check if short is still present. Do not apply power.

That way you can check if there isnt other issue with your board while waiting for replacement parts.
yes, sure, good idea, will do so & at lease will know which driver IC is defective. Thanks for all the help.
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