ASUS X200CA board power issues (schematic and datasheet attached)

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  • gigabyte898
    New Member
    • Sep 2016
    • 6
    • United States

    #1

    ASUS X200CA board power issues (schematic and datasheet attached)

    Hey,

    I've gotten fairly far into this diagnostic but I thought I'd get some input (no pun intended). We had an ASUS X200CA brought in not powering or showing any signs of life. No luck with power flush so I tore it apart to get the board. Found the motherboard schematic (ASUS doesn't like numbering their boards with anything besides the laptop model but they also seem to re-use boards in different models) and essentially just followed power all the way from the 19v DC jack to a step-down switch mode power supply IC chip.

    Datasheet for the chip says it takes the input and switches it to a 3v and 5v output with PWM buck converters as well as handles onboard power-up sequencing, a power good output, internal soft-start, and a soft discharge output that prevents negative voltage during shutdown. Made sense this would most likely be the issue so I tracked down where it was on the board and it was a little gunky around the area.

    Took the chip off, cleaned both it and the board with rubbing alcohol, put it back on, and viola it powered up. My excitement was short lived though as it powered down 30 seconds later. The same thing happens each time I turn it on. Shows the red charging LED, it powers up when I press the power button, and then clicks off entirely shortly after.

    My initial thought is to just replace this IC chip, but part of me is second guessing that's the only issue. Any of y'all ever experienced this before? Any thoughts on if something else could be contributing or should I just go ahead and order the chip and see if it works when it comes in. I've attached the board schematic I'm using as well as the datasheet for the IC in question, Page 79 on the schematic is where the chip is mentioned with the ID PU8701A

    Thanks,
    Brandon
    Attached Files
    Last edited by gigabyte898; 01-30-2018, 11:04 AM. Reason: Added clarification of where the chip is on the schematic
  • khaahk
    Badcaps Veteran
    • Dec 2013
    • 759
    • Estonia

    #2
    Re: ASUS X200CA board power issues (schematic and datasheet attached)

    And during that 30s time, does this ic provide 3.3V and 5V. If it does then most likely the problem is elsewhere. Onboard ram is common point of failure if it is X200CA. The schematic attached is not related to the topic. Which board it is?
    The K40AB most likely problem related to AMD northbridge or video chip.
    Last edited by khaahk; 01-30-2018, 03:19 PM. Reason: corrections

    Comment

    • gigabyte898
      New Member
      • Sep 2016
      • 6
      • United States

      #3
      Re: ASUS X200CA board power issues (schematic and datasheet attached)

      Originally posted by khaahk
      And during that 30s time, does this ic provide 3.3V and 5V. If it does then most likely the problem is elsewhere. Onboard ram is common point of failure if it is X200CA. The schematic attached is not related to the topic. Which board it is?
      The K40AB most likely problem related to AMD northbridge or video chip.
      Thanks for the reply, it does supply the correct voltage before shutting down. Grabbed that schematic from a different site but the guy posting it claimed it was close enough to the X200CA to ID what components are. One thing I did notice after posting this was the area with the IC was hotter than the rest of the board by a lot. I'm sure an IC that downsteps 19v to 3.3v is going to get toasty but it was enough to raise eyebrows. The client this is for ended up just telling us a few minutes ago not to bother with board labor and just replace the entire thing so it doesn't matter too much anymore, but I'll keep in mind what you said about the RAM and IC if this model crosses my path in the future

      Comment

      • BlueMidnight
        Badcaps Veteran
        • Jan 2015
        • 489
        • United States

        #4
        Re: ASUS X200CA board power issues (schematic and datasheet attached)

        Just for future reference... The 3V/5V chips create 4 main voltage rails:

        - A low power 3V linear/LDO output
        - A low power 5V linear/LDO output
        - A high power 3V switching/buck output
        - A high power 5V switching/buck output

        The linear regulators are built into the chip but the switching outputs are generated by controlling 2 pairs of external MOSFETs.

        These 3V/5V chips should never ever be hot. If you have to wonder if it's too hot, it's too hot. An overheating 3V/5V chip is caused either by a bad chip or by a short on either the 3V or 5V linear outputs (or both).

        In the future, you may diagnose this condition by using a multimeter set to diode mode. With the board unpowered, place the red lead on ground, and place the black lead on each of the 4 outputs of the 3V/5V chip (best to check all of them). Anything near 0V on the output means there is a short to ground on that rail.

        Likewise, if the source of the heat isn't the 3V/5V chip itself, but something nearby, then you would check for shorts in that area. Really nothing should get exceptionally warm on a motherboard except the areas with high amperage, like the CPU, GPU and voltage regulation section supplying those.

        Comment

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