ASUS UX31A showing no signs of life. Help needed - Page 1
Forum section: electronics / repair

author: croc
author_rank: Newbie
timestamp: April 16, 2022, 09:37:27 pm
content: A friend came to me with his broken laptop. 2 years ago he put it in sleep mode, the next day laptop was dead. Nothing lights up, there's no reaction, no fans spin. I found online that it could be because of the keyboard (the power button is a part of the keyboard, with no external cable), but I have no way to prove or disprove this without ordering a new keyboard (we already cleaned and reseated the connector). Voltage on the battery connector was fluctuating at 3.5V+-0.5V which seems awfully low as it has to charge a 7.4V battery. Attached are the schematics of mobo (look at page 60 and 88), photo of the motherboard with some markings and oscilloscope measurements on the power MOSFETS. PQ8801 drain is showing this rippling pattern but that's what comes out of the power brick, so it's not the MOBOs problem. all the other legs on both mosfets are flat, the attached photo of PQ8801 source was taken earlier, because after poking around for a while it started showing completely flat, just like all the other legs (I don't think was me shorting anything but I can't prove that). Are the power mosfets dead? Could the charge controller be dead too despite the fact that it was sending (albeit low) voltage to the batteries? Do you have any other theories? I'm very scared of desoldering stuff from the board to check anything because the elements are absolutely tiny, I also don't have a hot air station so I can't even desolder the charging controller.

author: mon2
author_rank: Frequent Contributor
timestamp: April 17, 2022, 09:58:20 pm
content: Hi. If possible, repost all of these details on badcaps forum -> laptop repair. Myself and others monitor that thread and someone will post ideas to proceed with your repair.For starters, a hot air tool is a must for such repairs. The tools are not expensive and required to remove / replace SMD parts. Review for example the BEST 863 hot air tool (Quick clone) on Aliexpress. There are many others on the market but we use the BEST tool since the vendor claims they can offer us spare replacement part, if ever required to service our machine. Like the fact that we can dial upto 550C on the temp but also suggest an underside pre-heater for heavy copper plane motherboards - like Asus. We have seen cases where even with full heat + high air pressure, we could not remove the SMD BIOS SOIC-8 device.Moving on, remove all power. Meter in lowest resistance mode (2k scale is ok). We wish to check if your first 2 mosfets that hit the DCIN are ok or defective.Measure the resistance between:source (1-2-3) & drain (5-6-7-8)source (1-2-3) & gate (4)gate (4) & drain (5-6-7-8)Post the readings. Repeat the test for the 2nd mosfet mated with the charger IC.Post the readings. We are checking for a low resistance reading that will indicate the mosfet is defective and must be replaced. If you see 'OL' = over limit on your meter, then change the scale to the next higher setting. A high resistance reading is good and may confirm that the mosfet is ok.If the mosfet readings are indeed high, then proceed to very very very carefully, measure the voltage (DC scale of 30v on your meter) on the TI charger IC and post the readings.Be careful on these measurements so that you do not short the charger IC pins and cause damage. We are interested in the voltage on each pin of the charger IC such as ACOK, ACDET, etc.Also, this charger IC will boost the DCIN voltage (19vdc) to +6 to enable the GATE of the first 2 mosfets to allow for the voltage to pass through onto the main logic board rail.If you DO NOT see the DCIN voltage on the main power rail then remove all power again -> meter in resistance mode -> check the resistance to ground of the 2nd mosfet, pins 5-6-7-8 = DRAIN. We are checking to see if there is a LOW resistance on this rail which often means there is a shorted cap on this rail. With a shorted cap (tantalum is often the bad boy), the charger IC is measuring the current and turning off the mosfets to prevent further damage.First start with the mosfet resistance values -> then the voltage readings on the mosfet pins.

author: croc
author_rank: Newbie
timestamp: April 19, 2022, 08:24:48 am
content: Hey,thanks for replying! I'll do all this and get back with the results, but I might not have time before the weekend. I don't have 130€ to spend on the station, but I'll ask my friends and try to look for something second hand in my area.

author: croc
author_rank: Newbie
timestamp: May 03, 2022, 02:15:46 pm
content: Hey,It took longer than I expected but we measured the MOSFETs and the charging IC. Here are the results:PQ8801:D-S 14.6MΩD-G 1.1MΩS-G 16MΩPQ8802:D-S 14.4MΩD-G 1MΩS-G 6.2MΩThe charging IC measurement results are attached in the zip file as photos of the scope (most pins were not stable enough to measure them with a DC multimeter, and that gives a better picture of what is happening). Please note the voltage it's showing, as some photos were taken with 5V/div and some with 1V/div. The time is 0.5s/div in all of them. Also the scope was measuring ground as +0.2V (didn't know how to reference it) so the results need to be offset by that amount. Please use readings from pins 15 and 7 as reference for ground, as it's at the 0% mark between two lines.

author: mon2
author_rank: Frequent Contributor
timestamp: May 04, 2022, 01:36:05 am
content: The mosfet resistance are high and that is a good sign. For now, do not suspect the first 2 mosfets to be defective.The scope pics are difficult to review for the voltage values but ACOK sticks out as incorrect. This is an open-drain output that should have a local pull-up resistor.See attached.Measure and post the voltage reading of the local 100k PU @ PR8812 - pin # 2. What is the voltage on this pin ?next, remove all power - meter in resistance mode (> 100k scale) - check the resistance of PR8812. Is it ~100k ohms ?