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Question ASUS TUF Z270 MARK1 problem. Faulty chipset?

Jul 12, 2020
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Hi everyone,

As a beginner electronic enthusiast I recently started to diagnose a faulty ASUS TUF Z270 MARK1 motherboard that I purchased from a friend.
I would like to use this as an opportunity to learn the basics of how desktop motherboards work and hopefully be able to identify the faulty component.

My friend was trying to backup an old HDD using an ide to usb adaptor. The HDD was connected as follows:
  • HDD IDE --- IDE TO USB ADAPTOR ---> USB3 HEADER MOTHERBOARD (FRONT PANEL)
  • HDD MOLEX ----> PC POWER SUPPLY
The problem occured when he removed the power cable from the HDD while the PC was still running. The PC suddenly shut down and never started again.
  • the pc will not boot and there is no fan spin
  • the intel chipset z270 south bridge is getting hot while the pc is in shutdown state and will stay hot until psu is turned off
  • the UZ2085g standby voltage regulator is getting really hot while the pc is in shutdown state and will stay hot until psu is turned off
  • there is a power light led on the motherboard which will light on for a few seconds but will die off once the power voltage regulator gets really hot
From what I learned so far the UZ2085g is used to provide the 3V standby voltage to the super i/o, southbridge and power button which will take care of the power on sequence. The fact that the southbridge is getting hot while turned off and the PSON signal on the ATX 24 pin power connector is not activated during the power button press suggest me that there is a problem with one of the components responsible for the power on sequence:
  • standby voltage regulator (UZ2085g)
  • super i/o (nuvoton NCT6793D)
  • south bridge intel z270 (sr2wb h65005 01 edg x702D271)
  • some other passive components (capacitors, ...)
In order to track down the problem I started breadboarding the PC and made the following measurements:
  1. 3VSB 2.8V is present at the output of the UZ2085g --- 2.8V seems a bit low...
  2. PSIN 2.5V measured at the super i/o and will drop to 0 when power button is pressed --- voltage is low but power button logic is ok.
  3. PSOUT 2.2V measured at the super i/o and will not drop to 0 when PSIN is 0 --- Super i/o data sheet specifies that PSOUT needs to drop with PSIN
  4. 0.5 kOhms are present at the output UZ2085g and ground --- I was hoping to see 0 because that would suggest that there is a short to ground in one of the components although I'm not sure if 0.5 kOhms is high or low value for this motherboard
  5. Knowing that the issue occured at the USB header I measured the pins and found that both IntA_P2_D+ and IntA_P2_D- are shorted to ground --- From what I've read so far the southbridge is responsible for usb communication so I'm having a feeling that this is the faulty component
Measurement #5 and the fact that the southbridge is getting hot makes me think that the southbridge is gone. Not seeing a short to ground from the UZ2085g's output and PSOUT not changing with PSIN raises some doubts.

I feel like I learned a lot until now but without any motherboard schematics and spare parts I think there is not much I can do and probably just take it to a repair shop :). Before that I was hoping maybe someone else can think of another measurement test I could use to identify the faulty component?
 

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