Asus Tuf gaming fx506l no power, mosfet does not open

[Thomas Smith]

Hello everyone, I have an ASUS FX506LI laptop that does not turn on, and I need help diagnosing the issue. Here is what I have checked so far:
Initial Symptoms:

• The laptop has no power at all (no LEDs, no fan spin).
• No 3.3V or 5V present on the board.
• The power button (NBSWON#) reacts but only changes from 0V to 0.5-0.11V.

Power Rail Checks:

1. Charger input (FETs PKCH2BB)
   • Drain: 20V, Source: 0V, Gate: 0V (FETs not switching).
2. BQ24780S Charger IC:
   • VCC and ACDET are present.
   • ACN / ACP, ACDRV, BATDRV, LODRV, HIDRV, ACOK, SRN, SRP – all 0V.
   • Shunt resistor has 10Ω to ground.
3. Power rails:
   • No 3.3V (VPCU), no 5V.
   • PU8 (power management IC) gets no input power.

Actions Taken:

1. Replaced BQ24780S – no changes.
2. Checked MOSFETs & resistors around the charging circuit – all seem fine.
3. No shorts detected on major power rails.
   
   
   Could this be an EC ( IT8987VG ) chip or not? I have a new one. But I don't have a programmer. I read on a forum that it can program itself, is that true?
   Please help me, I have little experience with laptop repair 😅

[Thomas Smith]

Main power rail is shorted.

Shunt resistor has 10Ω to ground.

small update. I cleaned the board again and saw that the capacitor near the mosfets of the video chip was damaged. I unsoldered it, now there are kiloohms on the shunt. But the nvdd resistance (inductors) is still the same low 0,3.

Could you tell me what to do next?

[FriedFred]

You need to measure the standby current. Without battery for the first attempt. It must be around 10mA. This would be a resistance about 2 kOhm, not 20 Ohm.

[mon2]

Main power rail is shorted.

Shunt resistor has 10Ω to ground.

[Thomas Smith]

You need to measure the standby current. Without battery for the first attempt. It must be around 10mA. This would be a resistance about 2 kOhm, not 20 Ohm.

Yes, it shows exactly 10mA.
Very interesting... A bit more information, in case it's important:
This laptop has never been damaged by water, but at the same time, when I disassembled it, I found that it had never been serviced since purchase... There was a lot of dust and dried-out thermal paste.

All inductors have correct resistance. The battery was completely discharged to zero. The laptop just suddenly turned off and that's it. If you have any ideas, I'd be happy to hear them.

For now, I'll try to figure out the low resistance on the shunt…

[Thomas Smith]

All inductors have correct resistance.

Oh God, my bad. NVDD 0.5 Ω,
Could be GPU dead due to overheating?

[Thomas Smith]

Main power rail is shorted.

Shunt resistor has 10Ω to ground.

small update. I cleaned the board again and saw that the capacitor near the mosfets of the video chip was damaged. I unsoldered it, now there are kiloohms on the shunt. But the nvdd resistance (inductors) is still the same low 0,3.

Could you tell me what to do next?

[Thomas Smith]

0.8volts to +nvdd, only one videochip is heating up...
(contacts MOSFETs kOm)
Can anyone confirm that this is a dead GPU?

[mon2]

Wait on others to comment but the GPU may not be dead. It is very possible that the GPU has a low resistance to ground by design. Assorted past posts on this forum have noted low resistance measurements for the GPU - especially Nvidia.

[Thomas Smith]

Wait on others to comment but the GPU may not be dead. It is very possible that the GPU has a low resistance to ground by design. Assorted past posts on this forum have noted low resistance measurements for the GPU - especially Nvidia.

Okay, you're right, I'll wait and try to find something. I'm waiting for someone's help. Thanks in advance ❤

resistance on the video memory 100 ohms

[Thomas Smith]

Wait on others to comment but the GPU may not be dead. It is very possible that the GPU has a low resistance to ground by design. Assorted past posts on this forum have noted low resistance measurements for the GPU - especially Nvidia.

I found this spreadsheet on the forum. Well, this is normal resistance, right?

[mon2]

Looks normal to me.

[pc_okay]

Why not injecting low voltage like 0.8v on the shunt resistor and look if something is heating?

if you dont want to take any risk, You can search the same résistance (10ohms) on all coils.
You may have a shorted high mofset connecting directly the main power rail to a secondary circuit.
like chipset / cpu / ram… by the way try without ram to see if résistance rise on the shunt resistor…

[mcplslg123]

pc_okay, you seemed to missed post#6. there is no more short on CLR.

The resistance to GND reading is perfectly fine for GPU as well as Vram.

Connect everything and check if mb turns on.

[Thomas Smith]

People, thank you so much for your help! Everything is working now!

I ended up resoldering the original BQ24780 back in because I noticed that the new one wasn't soldered properly, and the original was actually fine.

I really regret not spotting that faulty capacitor earlier. I did so much unnecessary work... but I guess I can blame it on my lack of experience 😅.
Tips for anyone dealing with this laptop:


🔹 Tip #1: If your laptop hasn't been exposed to liquid damage or other major issues (aside from dust buildup), carefully inspect the contacts of components that may have overheated. In my case, the damaged capacitor was near discolored inductor contacts (see photos).

🔹 Tip #2: Always measure the standby current first. Then check the resistance on all major power rails.

🔹 Tip #3: Check the MOSFETs. If one isn't switching on, verify the contacts on BQ24780S (or similar controllers). A non-working MOSFET doesn't always mean it's faulty—it might not be receiving the correct signals.

🔹 Tip #4: Don't rush your decisions! Start with the easiest solutions before moving on to complex repairs. This will save you time and effort.

And of course, always check the boardview and datasheets! 😁