ASUS G771JW 5 Volt switched rail undervolted/missing

[spirman]

So i've been working on a board of mine from an asus laptop. I've checked for shorts to ground which there are none. 5v 3.3v 1.5v and all power supplies are operating at their correct voltages. However when i got the laptop there was a blown SOT363 what looks to be a double mosfet. I've since replaced it but i still have issues. On the 5 volt rail, the fan does not spin, and one of the USBs doesnt power up. I checked the 5 volt rail there with my multimeter and it says 1.66Volts.

Does anyone know what could be causing such a drop of voltage? Is there some kind of faulty controller that would cause it to be like this? For reference, the rail shows 1.66volts while the power supply coil on the board shows 5 volts. Any help would be much appreciated. Thanks!

[caspian]

Re: ASUS G771JW 5 Volt switched rail undervolted/missing

this seems simple to solve (probably). trace the rail from the 5v coil to the point of 1.66v. and find the point where it drops. use the boardview if it is available in the Internet.

[piernov]

Re: ASUS G771JW 5 Volt switched rail undervolted/missing

This may be a short to ground on the 5V rail, so measure the resistance between the 5V and ground.

[spirman]

Re: ASUS G771JW 5 Volt switched rail undervolted/missing

This may be a short to ground on the 5V rail, so measure the resistance between the 5V and ground.

Diode mode shows no shorts to ground on any of the rails. This is a very confusing repair.

[BlueMidnight]

Re: ASUS G771JW 5 Volt switched rail undervolted/missing

The fan is supplied by +5VS. And +5VS is enabled by a load switch (RF4E080BN) connected to +5VSUS. Measure all pins of the load switch.

USB port 2 is supplied by +5VSUS through a fuse. USB port 1 has additional device charging logic, and is supplied by +5VSUS through a controller / load switch IC (TPS2546). If your problem is with USB port 1, then measure all pins on the controller IC. If your problem is with port 2, then measure both sides of the fuse and/or fuse resistance.

[spirman]

Re: ASUS G771JW 5 Volt switched rail undervolted/missing

The fan is supplied by +5VS. And +5VS is enabled by a load switch (RF4E080BN) connected to +5VSUS. Measure all pins of the load switch.

USB port 2 is supplied by +5VSUS through a fuse. USB port 1 has additional device charging logic, and is supplied by +5VSUS through a controller / load switch IC (TPS2546). If your problem is with USB port 1, then measure all pins on the controller IC. If your problem is with port 2, then measure both sides of the fuse and/or fuse resistance.

I tested the load fuse and its the exact component you showed on the board. Before the load switch, there is 5 volts. After that loadswitch there is 1.66 volts. Ive shorted these pins and attempted to turn it on, and for a moment the fan spinned up but shut back off again (Likely as a protection mechanism).

Perhaps this is relating to another component on the board that was visibly burnt and damaged, which i replaced with a P type sot363 double mosfet i pulled from another board. The pins of this blown presumed mosfet are tied to 5 volt, as well as the 260khz VCLK pin of the DC to DC converter. I have no clue what was supposed to be there, and if it could be the main issue. The original SOT363 that was there, is now a charred mess on my desk xD.

I have no schematics, or any way at all to know what this SOT363 used to be, i presumed it was a P type mosfet given its location, but now im not even sure.

Also how did you find that image? Its the exact board of mine?

[spirman]

Re: ASUS G771JW 5 Volt switched rail undervolted/missing

Please ignore the Flux mess xD

[BlueMidnight]

Re: ASUS G771JW 5 Volt switched rail undervolted/missing

I just googled for those images and then cropped them and added the annotations.

You didn't measure pin 4 (the gate) of the load switch. But it looks like there is a bigger problem...

The original component was a BAT54S and contains 2 schottky diodes. It's part of the charge pump circuit that generates 12V to be used for switching on various N-channel MOFSETs on the board (including the load switch that I asked you to measure). It absolutely must be replaced with another BAT54S (that "S" at the end is very important).

[spirman]

Re: ASUS G771JW 5 Volt switched rail undervolted/missing

I just googled for those images and then cropped them and added the annotations.

You didn't measure pin 4 (the gate) of the load switch. But it looks like there is a bigger problem...

The original component was a BAT54S and contains 2 schottky diodes. It's part of the charge pump circuit that generates 12V to be used for switching on various N-channel MOFSETs on the board (including the load switch that I asked you to measure). It absolutely must be replaced with another BAT54S (that "S" at the end is very important).

Thank you so much, but also the component you mentioned, the BAT54S, It looks like it only has 3 pins on it, whilst the original SMD had 6?

[spirman]

Re: ASUS G771JW 5 Volt switched rail undervolted/missing

Also is this component resposible for why i dont have full 5 volts? And if so can you explain to me if i may ask? I'm new to board repair and if i can learn something new it can help loads.

[BlueMidnight]

Re: ASUS G771JW 5 Volt switched rail undervolted/missing

Oops yes I'm sorry. I meant BAT54SDW, where the "SDW" is super important. So important that I wrote it wrong heh

I still need to know the gate voltage. But yes, if it's not high enough, the load switch won't be on all the way and you won't get 5V.

[spirman]

Re: ASUS G771JW 5 Volt switched rail undervolted/missing

Alright ill do more research and ill get back to you

[spirman]

Re: ASUS G771JW 5 Volt switched rail undervolted/missing

Also as for the load switch, Using this datasheet When the laptop is switched on, Pin 4 voltage is 1.66v and pin 3, which looks more like the gate, has 0 voltage, and the 4 pins connected to 5volt power supply, are 5 volt.

[BlueMidnight]

Re: ASUS G771JW 5 Volt switched rail undervolted/missing

Yeah pin 3. I'm all typos today. Of course, it can't possibly get the right voltage on the gate without the charge pump circuit working. So that does explain it.

[spirman]

Re: ASUS G771JW 5 Volt switched rail undervolted/missing

Thank you so much, you've helped me a lot. I would have never known the correct part. Also how did you come up with that part name, and how did you know what it did? Experience?

[spirman]

Re: ASUS G771JW 5 Volt switched rail undervolted/missing

oddly enough i just found another forum somehow mentioning the same problem? https://www.abcelectronique.com/foru...d.php?p=854427

I guess this problem is more common yes? xD

[BlueMidnight]

Re: ASUS G771JW 5 Volt switched rail undervolted/missing

Ah yeah, it looks like you're not the first to see that part burn up.

I have a boardview from a similar model that I was able to grab part numbers from. Although it is possible to figure out the type of part from examining the circuit itself, and then go browse parts for something compatible.

It being tied to VCLK is a big hint that it's part of the charge pump. Along with buck regulators and a few other things, those are an important type of circuit to understand and learn how to spot on a board or a schematic.

[spirman]

Re: ASUS G771JW 5 Volt switched rail undervolted/missing

By the way do you happen to have that motherboard diagram of the similar board? Unless of course you paid money for it or something then i understand.

[spirman]

Re: ASUS G771JW 5 Volt switched rail undervolted/missing

I dont have one of the components on me readily, but i managed to manufacture my own .

[spirman]

Re: ASUS G771JW 5 Volt switched rail undervolted/missing

Ill let yall know as it goes