Acer E1-531 sort on RAM PSU

[Askic]

Hello guys,

Today I have experienced a stubborn short to ground problem that I'm not able to solve.

Attached is part of the schematics that I consider relevant for this topic (RAM PSU).
Using multimeter I have found MOSFET PQ25 shorted to ground (both drain and source). Also, PQ24was also shorted to ground on the source side.
I have removed PQ25 and it was OK, so it is not shorted D-S.
After closer examination of RAM power supply controller PU6, it was evident a physical damage on the surface of the chip, so this IC is removed using hot air tool. The short was still present so, PL9 was removed from the board. After removing PL9, PQ25 MOSFET was no longer in short to ground, so natural conclusion was that +1.5VP line was in short to ground. So the next step was to remove electrolytic cap PC74. However, the short to ground was still present.

When chip PU6 removed (and PL9 removed), pin nr 5 (VDDQ from PU6) on the board was still short to ground.
What are the possible other places where +1.5VP can be shorted?

What to look next? According to the schematics this +1.5VP goues to +1.5V and there are many many places this voltage is used.

[ammarben]

Re: Acer E1-531 sort on RAM PSU

try to inject 1v and see what component get hot

[piernov]

Re: Acer E1-531 sort on RAM PSU

Post exact resistance to ground measurements.

[lapfix]

Re: Acer E1-531 sort on RAM PSU

As piernov ask , use diode mode on multimeter to take the impedance on all the large inductors with red probe on GND and post it

[Askic]

Re: Acer E1-531 sort on RAM PSU

PL 9 side (3,4) has 1.1 ohm to ground. SIde 1,2 shows something >100 kohm.
pad nr 5 on motherboard (where iC chip PU6 was placed) has 1.1 ohm to ground,
pad nr 19 (VLDOIN) on motherboard (where iC chip PU6 was placed) has 1.1 ohm to ground.
What I'm trying to achieve here is to learn correct methodology in finding the cause and not to solve this particular problem. This mobo is already designated to junk, so I'm using it for learning purpose.

The next step I would do is to find the jumper links shown on the attached picture and to remove them. In this way, +1.5VP would be separated from 1.5V. This way I would now whter the short was around PU6 (this is reasonably to assume since this chip was blown up with a small hole on the surface).

There is a possibility +1.5VP to be shorted via PR94 and PR95, but this is very unlikely that both resistors are shorted.

My best bet is to try to separate 1.5VP and 1.5 V, but I'm not sure how to recognoze attached jumper symbols.

[Sametbey]

Re: Acer E1-531 sort on RAM PSU

You can more easily find where the short circuit is by removing the PL9 coil

Must be near PU6, example, like J6