Lenovo M800

[ener_dk]

Hi All,

I have an M800 (10FX) where is completely dead.
The PSU is new and the power switch pulls 3.3 v dc low.

I checked the mainboard for obvious problems as burned component and leaking caps.

What I found was a liquid spill around the chipset.
I cleaned it and tried again without any luck.

Then I check the caps around the chipset and the resistance is about 35 ohm.
So I lifted a lot of the caps and the power FET with same resistance to check for faulty components.
No caps and FET seems to be faulty but the resistance lowered on the mainboard to 0-22 ohms while its hot from the heatgun (set to 350 celsius).

Does anyone have any experience with this?

My next step is to re-flow the chipset.

BR
Ener_dk

[piernov]

Re: Lenovo M800

35 ohms is fine for PCH core power rail.
No point in reflowing stuff randomly except if you want to kill it.

[ener_dk]

Re: Lenovo M800

Thanks for your reply piernov.

No I don't want to kill it - could be nice if I could repair it.

32 ohm on a 5v line is 715mW or 311 mW on a 3.3v line.. so in my world that normally pretty low resistance.
But this is my first mainboard repair so I still have a lot to learn.

I cant google a diagram for it or even another thinkcentre model.
So do you know if the onboard power supply is more or less the same as on a Lenovo laptop?
It would be nice to have an idea on how it works instead of looking in the dark.

A little guidance would really be appreciated

BR
Ener_DK

[piernov]

Re: Lenovo M800

If it's the 5V or 3.3V rail yes it's too low, if it's the 1.05V rail it's fine.

Without schematics it'll be hard to do anything. What's the SIO on this board?

[ener_dk]

Re: Lenovo M800

I found the problem
Its the 3,3 RTC power line that is draining the CMOS battery to fast.
I have checked the dual diode and the passive components around the the battery nothing seems defective.
So a quick fix is that I took a wire from the 3.3 v PSU too the RTC power line to see if that "fixed" the problem and it did.
So thereis something on the RTC power line is half dead?!

[piernov]

Re: Lenovo M800

Could be the dual diode between coin cell battery, 3VSB and RTC power rail.
Or the liquid damage itself under PCH.

[ener_dk]

Re: Lenovo M800

I have checked the diode and it measures OK.
If I ohm the RTC power line to ground its about 6,5K ohm without the diode on the board. I have have also lifted all the resistors and caps around the battery no change in resistance.
PCH? Did you mean PCB?

I tried to locate the RTC ic but no luck - there are some IC that I cant find datasheets on so I guess that one of them is the RTC IC.

But any way the PC works now and it online so every time Windows starts the clock it syncronized.

Thanks a lot for your guidance

BR
Ener_DK

[piernov]

Re: Lenovo M800

PCH = Intel Platform Controller Hub = southbridge.
The PCH itself handles RTC features. Coin cell battery or RTC power rail is also often connected to SIO.
6.5kohms is too low. It should be a few hundred kohms. Current consumption should be a few µA.