lenovo g700 will not turn on

[merkin]

About a year ago a friend brings a g700 with broken power jack.
It was an easy fix since the jack is on a harness..no soldering required.

6 months later I got a call that the laptop will not turn on. I noticed that no status lights were illuminated when the laptop was plugged in.
I disassembled and confirmed that power was getting to the motherboard.
After reassembly I decided to leave the battery and the RTC coin battery removed and held the power button for 30 seconds to drain the caps.
I then plugged in the laptop (without the batteries), and noticed the power status light came on. So I pressed the power button and the laptop fired right up.

Recently I got the call again that the laptop will not turn on. I am now wondering what is causing this issue? (polyfuse?)
here is the teardown manual

I am looking for the schematic, but cant find it. If anyone has a link please share.

I noticed something strange in the manual. On page 21, there is a test for checking the power adapter. I get 0vdc when trying that test.
However, there are contacts inside the plug walls (next to pin 1). If I put the common lead from my DMM(fluke 87v) to the contact on the inside wall and the power lead to pin 1, the DMM shows -20.13vdc. So that conflicts with what the test in the manual shows in two ways below.
1. Center ping is Negative
2. Outside wall of adapter is not grounded. The DMM shows 554.5 ohms resistance between the center pin and outside wall. The DMM shows infinite resistance between the center pin and inside wall.

I also did the battery test on page 22.
There is 0vdc between pin 1 and pin 7.
There is ~10k ohms resistance between pin 5 and pin 7.

Has anyone ever externally charged a laptop battery? What did you charge it with? I would like to charge the battery to see if the laptop will turn on with just a battery.

If there is any insight on what could cause this issue, please share. Thanks.

[BlueMidnight]

Re: lenovo g700 will not turn on

The diagram in that manual is just very misleading. It's not pointing to the center pin. It's pointing to the inner wall. The outside is ground and the inside wall is positive. The center pin is an ID pin that has a resistor to ground. The value of the resistor lets the motherboard know how much power the AC adapter can deliver. 550 ohms means you have a 90W AC adapter (120 ohm, = 45W, 280 ohm = 65W, etc).

You measured -20.13V because you were essentially just measuring from positive to ground with the probes backwards. The resistor doesn't affect the voltage measurement since there's not really any current flowing through it (Ohm's Law).

So the AC adapter has the correct voltage. But be aware that sometimes AC adapters measure good but then have trouble when under load.

You are probably measuring 0V on the battery because the charge got too low. When that happens, the protection circuit inside the battery disconnects the cells to prevent them from getting any lower. It might still charge in this state or it might not. It depends on the condition of the cells and how the protection circuit is designed.

It's possible to manually charge Li-ion batteries, but you must take care. This shows how to charge a single cell. I don't have time to look now, but I'm sure there are many videos and pages describing how to charge a multi-cell notebook battery. It's the same rule for the current but the voltage is more like 12.3-12.6V (if we're talking about a 10.8V-11.1V battery).

Did you already try what worked last time and it's not helping this time?

[merkin]

Re: lenovo g700 will not turn on

Thanks for your explanations..makes perfect sense.

I tried what worked last time, but unfortunately its not helping this time. I maybe did something inadvertently last time and cant remember what it was.

I also just had a thought and I think the original AC adapter was also damaged when the power jack was damaged (the laptop fell while plugged in).
The replacement adapter looks like an identical replacement (not one of those cheap ebay replacements from overseas), but who knows they get better at copying each year. I am sure I did buy it on ebay though. I will look for the receipt to confirm.

You got me wondering if this replacement AC adapter is failing under load..but it has worked as intended for a while now.

A schematic sure would help me right now. Of all the testing equipment I have unfortunately a smart bench PSU is not one. Unfortunately I might have to chuck this thing.

[BlueMidnight]

Re: lenovo g700 will not turn on

There are definitely things you can do without a schematic. There are many similarities between boards, and the individual chips usually have datasheet available.

For example, if the attached pic is what your board looks like, I can easily find the input MOSFETS. You should measure the 3 indicated points to make sure 20V is going through both of them. If not, then there is a problem with the input/charging circuit.

If power isn't getting through both MOSFETs, then measure the pins on the input/charging chip. But if there is voltage all the way through, then check for voltage on the main 3V and 5V rails. If those don't exist, then measure the pins on the 3V/5V chip.

You can confirm which chip is which by doing a web search for the the part numbers.

[merkin]

Re: lenovo g700 will not turn on

look at this post here: https://www.elvikom.pl/lenovo-g700-m...em-t44197.html

that is the exact same experience i have.

i am getting 20vdc from the fets.
i am not getting any voltage on the 3/5vdc rails, however i am getting 20vdc across those four small caps just above the inductors.

so i focused my attention on the chip next to the RTC coin.
but this is where i f*cked up..at first i was getting 3vdc and 5vdc across the caps next to the chip, but i accidentally short circuited two of those caps with the DMM lead because i was not being careful. now i do not see the voltage across the surrounding caps. i must have caused even more damage. i definitely saw a spark..

i am also getting 2.7vdc across pin 1 and 7 of the battery connection.

[BlueMidnight]

Re: lenovo g700 will not turn on

I think that's happened to all of us at some point. Unfortunately, it might make the repair far more difficult or even too difficult/expensive to be worth doing. It just depends on what got shorted to what. For example, one of the pins on that chip is 20V and if that were shorted to the 3V or 5V rail, then there might be a lot of dead chips on the board right now.

The best way to deal with this is to just start over. Make sure the AC adapter is unplugged and replugged from the wall so that it can reset (just in case it went into protection mode due to the short). Then check that the 20V is getting through both input MOSFETs.

If so, then get the datasheet for the 3V/5V chip and find the 4 outputs. There are 3V and 5V LDO outputs and 3V and 5V switching outputs. Check them all for shorts to ground (multimeter in diode mode, red probe on ground and black probe on the pin you're testing... a short will read as less than 0.1V).

If you find shorts, then they can be investigated. If not, then the 3V/5V chip may need to be replaced before dealing with anything else.

[merkin]

Re: lenovo g700 will not turn on

here is the datasheet: https://cdn.badcaps-static.com/pdfs/...2c6e6f33b6.pdf

i basically shorted pin12 (VIN) with pin13 (VREG5).

before i shorted the pins, i can say with 100% certainty that there was no voltage at the inductors next to the two polarized caps in the third photo.

also before i shorted the pins, i can say with 100% certainty that there was 3vdc across the pins of the RTC coin holder.

ill check the 3/5v outputs for shorts to ground.

[BlueMidnight]

Re: lenovo g700 will not turn on

The inductors are on the main switching/buck outputs. So those were not outputting voltage originally. That might have been due to a short on one/both of those rails, you had a bad 3V/5V chip, or the 3V/5V chip wasn't receiving the enable signal to turn those outputs on. But now they definitely won't work since the LDO outputs aren't working.

The circled blobs of solder in the attached pictures are solder jumpers. And they are awesome. You can disconnect the 3V/5V outputs from the rest of the board by just using some desoldering braid on those points. It can really help narrow down where problems are.

There appear to be at least 3 solder jumpers. There are two by the inductors for the switching outputs, and there is one more on the opposite side of the board for the 3V LDO output. The 5V LDO output may not have one, but if so, it's going to be nearby.

Just go ahead and check for shorts at the inductors and also at pins 3 and 13 of the 3V/5V chip. If there are any shorts, disconnect the solder jumpers and then test both sides of the jumper to see which side the short is on.

[Kostas7GTR]

Re: lenovo g700 will not turn on

Hello,
I have the same problem with the G710 laptop. When i got it from my friend it wasnt charging at all and was working only with battery, didnt power up from charger at all.
I replaced the mosfets with new ones SI4835D and i could power on from the psu and the battery was charging fine. After 10 days or so my friend told me it stop working again. I got it back and it had burned again one SI4835D. I replaced both again and the third one SI4835D under the black plastic glued cover though it wasnt shorted and tested it out. It start again working from the plug and charging but after few minutes it didnt power on or charge at all. I checked the SI4835D they were ok no short so i put the cpu/gpu heatsing powered up and worked again for few minutes. That goes on and on even i checked that the SI4835D arent burned it does that thing. Power jack with cable is new and replaced. I dont know what to check there is no visual dmg to other parts. I got some mosfets to replace again but the old ones arent shorted. Any ideas ?

[Kostas7GTR]

Re: lenovo g700 will not turn on

Also i have to say that it still works with a charged battery without a problem.
I also got a watt meter and i saw that when plugged in it was pulling 22watts while charging (off) and about 50+ when on desktop charging. When it stopped getting power from the plug and charge the watt meter show like 2w consumption after few seconds like 5-7 then 2 and sometimes i got again 22w (normal charge) but that last only like one minute max then back to 2w standby(no charge). Any ideas what to check ?

[Kostas7GTR]

Re: lenovo g700 will not turn on

Any thoughts if it would be possible to power it directly from the battery connector or the 3.3v /5v line ?

[slv-uk]

Re: lenovo g700 will not turn on

Hello, please tell me that for MOSFET at the output of TPS 51225 3.3v and 5 v (marked as ALW 4GAA)

[mferna14]

Re: lenovo g700 will not turn on

Can anyone inform me what kind or the correct MOSFET Identification #s are used on this board. The MOSFET on this motherboard are ALW 2YAM.. What is the type of MOSFET (P or N Channel) used on this board??? All 4 MOSFET in the top .jpg photo have the same numbers. I googled the number ALW 2YAM but cannot find anything. Can anyone help, thanks

[SMDFlea]

Re: lenovo g700 will not turn on

Hello, please tell me that for MOSFET at the output of TPS 51225 3.3v and 5 v (marked as ALW 4GAA)

Can anyone inform me what kind or the correct MOSFET Identification #s are used on this board. The MOSFET on this motherboard are ALW 2YAM.. What is the type of MOSFET (P or N Channel) used on this board??? All 4 MOSFET in the top .jpg photo have the same numbers. I googled the number ALW 2YAM but cannot find anything. Can anyone help, thanks

The mosfet is a Vishay SiA432DJ https://www.badcaps.net/forum/showpo...7&postcount=10

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