Guidance on troubleshooting

[urad29]

Hi,
I was wondering if anyone could advise if I have gone the right route for diagnosing my laptop (Acer Helios).
I have attached the schematic.
The laptop was not turning on and there was no signs of power.
I injected voltage at the power input and measured the motherboard under a thermal camera. PU4501 was getting hot. I gather that PU4501 is the converter creating the 3.3v and 5v power rail.
I removed PU4501 and noticed a short on the 100K resistor on pin 7.
I injected voltage at the resistor and measured heat coming from U3801 and U3802 which I believe are the USB controllers.
From there I measured a short on resistors C3812 and C3801. When injecting voltage there, the chipset got hot.
I therefore gathered that the chipset is kaput.
Am I right? What would your approach be?

Thanks in advance.

 

[BradtheRad]

Your thermal camera is certainly ideal for tracking down overheated components!

Yes, a power supply problem is the first suspect when a unit won't power up.

A computer executes a POST (power on self test). If all is well then you see a startup screen. However it sounds as though it detects a short circuit right off the bat and shuts down immediately.

A USB port is exposed to the world. Easy to become damaged by drawing too much current, or, by abuse from malfunctioning peripherals.

If you're skilled and determined, you might go on to the next diagnostic step: disconnect U3801 U3802, then re-install PU4501.

PU4501 may or may not be ruined, depending on how robust it is. The POST may be sufficiently sensitive so as to shut down in time.

 

[urad29]

Hi,
Thank you for your response.
I have removed U3801 and U3802 and there is still a short on the 100K resistor, even before reinstalling PU4501.
When I inject voltage at the shorted end of the resistor (with PU4501, U3801 and U3802 removed), the chipset gets hot.
Does that mean that the chipset is gone?

Many thanks.

 

[BradtheRad]

At times like this don't we wish we'd taken readings of the unit's normal operation? Then we could notice abnormalities more easily.

Laptop computers are notorious for developing high temperatures. IC's are miniaturized so they can be densely packed inside a slim enclosure.

And being miniaturized the boards are so difficult to repair that technicians commonly tell customers it would cost more to repair the laptop than the unit is worth.

If your problem were in merely one or two chips, then your diagnostic steps appear straightforward and correct. You uncovered faulty behavior where it seems to be the prime cause for failed startup.

However it seems frequent that when one device goes bad, it causes other devices to weaken and break. Apparently this occured in your laptop.

I've never heard of the POST creating a log at startup. If you can find such a log (in an IC or hard disk) then it may report the tests it did and which failure caused it to shut down. To do this requires specific technical know-how.