Please Help , Asus Zenbook Motherboard repair

[roxboy1984]

Please could someone help or advise me. I have an Asus Zenbook UX305UA that I am trying to repair. I'm still a newbie but I'm learning. Under a thermal cam I noticed that the TI 1285A IC is getting very hot. I checked the resistances to ground of all the pins connected to this chip, and found that pin 3 has a very low resistance of 1.8Ohms so I checked the Ceramic cap which is next in line, on continuity mode and the cap appears to be shorted. This is a 4.7uF 6.3V capacitor. My question is, do I need this cap? can I jump it? I have some donor boards laying around. What Cap would be an ok replacement if need be.

Apologies if these are stupid questions. I'm still learning.

[mon2]

Yes, the capacitor is required to filter the rail and to assist during heavy current draw on the rail. Without this part, the voltage rail will dip under load.

Where is this regulator on the board? Where is this cap on the board? The boardview will offer a part index. If not available, post a few pics of the area otherwise we will be searching for 'Waldo'.

[roxboy1984]

Yes, the capacitor is required to filter the rail and to assist during heavy current draw on the rail. Without this part, the voltage rail will dip under load.

Where is this regulator on the board? Where is this cap on the board? The boardview will offer a part index. If not available, post a few pics of the area otherwise we will be searching for 'Waldo'.

Thanks so much.. So the PWM chip is PU8701 and the cap is PC8702.

[mon2]

See attached.

The critical parameters (in order) are:

SMD size 0603
voltage rating of 6v3 (or higher)
capacitance of 4.7uF or higher

[SMDFlea]

Thanks so much.. So the PWM chip is PU8701 and the cap is PC8702.

Please post the boardview in this topic where everyone will be able to find it https://www.badcaps.net/forum/troubl...05ua-boardview ,then post a link to it from this topic. Thanks.

[pc_okay]

Just to be clear, never jump caps and generally never jump any component or at least think twice before doing This. (For example if you jump a burnt fuse and there is still a short, you Will do hume damage!)

Caps are often attached to ground on one side and current is not supposed to pass through. caps are open component so the current don't pass to ground through the caps (exept when they are failing, in that case the circuit is connected to ground and that's why you have a short to ground)

On each Line all component are connected together, so if one caps is failing and is connected to ground all the Line become connected to ground and Of course all components are connected to ground.
Sometimes you have Line With hundred component, you Will not replace all !?

you have to determine which caps is dead:

First by observation look for oxydation m, cracked burnt, component on the Line and on the board

if you can't find anything you have to inject a Little current on the Line starting at very low voltage like 0,5V. And look for heat With thermal cam, cold gaz, colophane… because when short attract current and when a lot Of current pass through a component it heat.

[roxboy1984]

Please could someone confirm this for me. I have the following donor board that I'm looking for the necessary donor capacitor on. If I'm reading correctly the Cap "PC4723"on the Dell board would be a sufficient replacement for the Asus board?

LINK to schematic https://www.badcaps.net/forum/troubl...=112466&nojs=1

[roxboy1984]

Ok so I'm now stuck. I don't think that the Cap is actually shorted as the resistance to ground with the cap removed on pin 3 of the PWM is still very low.

The PWM IC is getting really hot. how would I test and see if the PWM IC itself is dead? Appreciate the help.

[roxboy1984]

is the TPS5128A and TPS5128B PWM IC interchangeable?

[mon2]

Since the rail is still low, you will have to pluck off the parts to hunt down the shorted part OR inject a low voltage if the resistance is low enough to cause the shorted part to heat up. What is the exact resistance to ground at this time (without the cap you have removed)?

Given that the PWM is heating up, suggest to carefully flux and remove this PWM IC. Then allow the board to cool and test the resistance to ground again of this low resistance rail. Did the resistance increase to a normal range? If yes, the PWM is defective and must be replaced. If not, keep hunting.

Take pics of the entire area before starting any rework.

[SMDFlea]

is the TPS5128A and TPS5128B PWM IC interchangeable?

Read the datasheet https://www.ti.com/lit/gpn/TPS51285A

[roxboy1984]

Since the rail is still low, you will have to pluck off the parts to hunt down the shorted part OR inject a low voltage if the resistance is low enough to cause the shorted part to heat up. What is the exact resistance to ground at this time (without the cap you have removed)?

Given that the PWM is heating up, suggest to carefully flux and remove this PWM IC. Then allow the board to cool and test the resistance to ground again of this low resistance rail. Did the resistance increase to a normal range? If yes, the PWM is defective and must be replaced. If not, keep hunting.

Take pics of the entire area before starting any rework.

So the resistance to ground with the cap removed is still very low. Around 1.3ohms. So I don't think the cap is the issue. I removed the PWM and checked the resistance on pin 3 and it was still low. Should I inject voltage on pin 3 with PWM removed and see what gets hot?

[mon2]

Yes, that would be the next stage. What I am suspecting is that the PCH who consumes the 3v3 rail may be the fault. Inject 1v or lower and test the thermals without a heatsink on the CPU.

[mcplslg123]

A short on +3V LDO will very likely lead to a shorted KBC and/or PCH. If you are lucky, it could be KBC only.

Warningont connect adapter while tps1285A is removed.