Asus FX504GM shorted

[Kuhanzzi]

Hi,

I need help troubleshooting this laptop.

When I got this, seller said it might have water damage.

It had blown mosfet near charger connector, so there must be short elsewhere what caused this.

I measured few capacitors, and many of them shows there is short somewhere.
I cant find any schematic for this motherboard (DABKLIMBAB0 REV: B)

I also tried lifting another leg of inductors to isolate shorted area.

What should I do next?

[ktmmotocross]

Re: Asus FX504GM shorted

measure short on main power rail. if ok, measure short on every power rail

[Kuhanzzi]

Re: Asus FX504GM shorted

measure short on main power rail. if ok, measure short on every power rail

I don't have schematics or knowledge to resolve rails.
Here is photo of measure points where short circuit is present.

[ktmmotocross]

Re: Asus FX504GM shorted

short is only when resistance is under 2ohm. so check again

measure on big grey coils

[Kuhanzzi]

Re: Asus FX504GM shorted

Hi,
I injected low voltage and found that this ic is heating up really fast. I removed it and short disappeared. Any idea what it is? I cannot find anything on google with markings

"
5X=5C
225
"

[Stevedb]

Re: Asus FX504GM shorted

It is not always the chip test the outputs from the ic for short.

[SMDFlea]

Re: Asus FX504GM shorted

Richtek 5X =

Take a look at page 9 "Typical Application Circuit" ,you will need to replace the 2 caps and resistor on the top right of your picture.The cap connected to pin 3 might be ok if resoldered.

[jaxxwolfe]

Re: Asus FX504GM shorted

Hi, did you solve the problem? I have the same issue and my warranty just expired.

[gustavind]

Re: Asus FX504GM shorted

Hi

I have the same motherboard- as mentioned above, and it also have a short somewhere. It shorts the moment the psu is connected, activating the auto shutdown in the psu. I have tested the connector and the 4+4 connectorpins on the board, and they are all ok. My question is: Where do I go from here? Knowing that the short occures befor powerup, does that narrow the search down?

[mon2]

Re: Asus FX504GM shorted

See attached.

Often the 2 x DCin mosfets are the suspects. Usually the 2nd one is found to be defective and has to be replaced but start your investigation.

Remove all power. Meter in resistance mode.

Check the resistance across the mosfet pins and post your readings:

source (1-2-3) & drain (5-6-7-8)
source (1-2-3) & gate (4)
gate (4) & drain (5-6-7-8)

Check each of these mosfets.

[gustavind]

Re: Asus FX504GM shorted

See attached.

Often the 2 x DCin mosfets are the suspects. Usually the 2nd one is found to be defective and has to be replaced but start your investigation.

Remove all power. Meter in resistance mode.

Check the resistance across the mosfet pins and post your readings:

source (1-2-3) & drain (5-6-7-8)
source (1-2-3) & gate (4)
gate (4) & drain (5-6-7-8)

Check each of these mosfets.

Thank you very much for youre reply!!

The readings are as follows:

PQ21
source (1-2-3) & drain (5-6-7-8) - 0 Ohm
source (1-2-3) & gate (4) - 0 Ohm
gate (4) & drain (5-6-7-8) - 59 Ohm

PQ22
source (1-2-3) & drain (5-6-7-8) 75,3 kOhm
source (1-2-3) & gate (4) - 59 Ohm
gate (4) & drain (5-6-7-8) - 75 kOhm

I hope it makes sens..?

[mon2]

Re: Asus FX504GM shorted

Both mosfets are with low resistance readings.

Can you confirm the meter scale used for each check ? If possible, post a pic of your meter.

Is it possible that the meter was showing 'OL' = over limit so the next higher scale on your resistance setting should be selected.

[gustavind]

Re: Asus FX504GM shorted

The meter I'm using sets the scale automatic. The readings on PQ21 are all very low, but the readings on PQ22 are:
75300 Ohm
59 Ohm
75300 Ohm

I really appreciate your help :-)

[mon2]

Re: Asus FX504GM shorted

The readings are very low. It is very possible that one mosfet is causing the other to look bad. Bad influence from the same family of mosfets. Movie to follow soon...

If you wish to proceed and have the tools, remove the 2nd mosfet @ PQ22.

Then check the resistance values again. Just note which part is placed where and also the orientation of pin # 1 - a picture will be very helpful before you start.

A good idea to also check the resistance of the part after it is removed and on your work table. This will offer a good idea if the low resistance is being influenced by other parts on the logic board or not.

Start with remove PQ22 and if required, remove PQ21. These mosfets are very common and can be replaced with other vendors and parts with the same fit and function. Interested in the resistance readings of the pads on the circuit board.

Before you proceed, use flux if available, low air pressure so the other parts do not go flying + tweezers + circular motion till the solder melts. Do not force the movement of the parts else the PCB traces / pads will tear off!!

Be patient and the mosfet will eventually just swim on the solder pads of the PCB which can then allow you to remove with tweezers.

[piernov]

Re: Asus FX504GM shorted

PQ21 is clearly the one that's shorted. And there'll most likely be a short to ground on main power rail.

https://www.badcaps.net/forum/showthread.php?t=90356

[gustavind]

Re: Asus FX504GM shorted

Thank you both very much for incredible helpfull pointers.
I'll return to the workbench sometime during the weekend, and update as appropriate.
Have a great weekend both of you!!

[gustavind]

Re: Asus FX504GM shorted

Hello again - a short update as of now...

First - I better correct the info on the resistance on PQ21...

PQ21
source (1-2-3) & drain (5-6-7-8) - 0 Ohm
source (1-2-3) & gate (4) - 59 Ohm !!
gate (4) & drain (5-6-7-8) - 59 Ohm

... I'm sorry, my first reading was faulty - I don't know why..?

When that is said - I'm more confident now, that you are both right - One or both of PQ21 and PQ22 has shorted. After reading the excellent thread - https://www.badcaps.net/forum/showthread.php?t=90356 it seems I should also check the main power rail. I can't inject power as of now, as I'm waiting on my bench power supply (wuptidooo :-) ) being delivered hopefully late next week. I have tried to make some readings on the resistance between VA+ and ground and found:



I'm not sure it tells a lot, but ...?

So - I'll wait for my bench PSU, and use the time (after hours) training using the hot-air solder machine on some scrap PB (print boards) I have...

I'll keep you posted...

[mon2]

Re: Asus FX504GM shorted

With enough flux on the part, remove PQ22 mosfet. Be patient to allow for the metal underside pads to melt the solder. Nudge the part now and then to check if the part is able to swim on the surface before plucking it off the board.

Never push this step. The solder will melt if you apply small circles over the topside of this body.

After the part is removed -> check the resistance to ground again for the main power rail.

It should certainly not be a 0 ohm resistance to ground which is concerning.

Is your meter auto-range or you select the resistance scale ? Post more details.

[piernov]

Re: Asus FX504GM shorted

With enough flux on the part, remove PQ22 mosfet.

Why would you remove the good MOSFET not the bad one?

After the part is removed -> check the resistance to ground again for the main power rail.

DC-in MOSFETs have no connection to ground, they won't be responsible for a short to ground on the main power rail.

[mon2]

Re: Asus FX504GM shorted

When I got this, seller said it might have water damage.

Only to rule out suspected parts.