Asus x55VD 2.1 troubleshooting

[questore]

Hi everyone,

I would like to thank you for everyone if your read this and take your time to help me.
I have a laptop mentioned in title. It has a short somewhere almost certainly in QFN chip named "PU8500".
On the top of the chip text is: "JJ EG" And "G27"
I haven't found any shematic or web mention about it.
What is this?
Can anybody help me to find replacement?

[Techtiger]

Re: Asus x55VD 2.1 troubleshooting

how do you say PU8500 short what are the readings you have done, is the chip burn ect

[piernov]

Re: Asus x55VD 2.1 troubleshooting

"JJ " is Richtek RT8202MZQW. On your board it's the GPU VCore buck controller. Low resistance to ground on output is perfectly normal.

[questore]

Re: Asus x55VD 2.1 troubleshooting

@techtiger: chip had brown yuck all over it so I cleaned it, but went further in diagnostic (read below)
Thx for help. Managed to go further here is my steps:
1. PL8500 shown short
2. Removed PL8500.
3. Pin PL8500 pin2 still shorted so removed PCE8501
4. Short still there on PCE8501 pin1 soooooooo
5. I have 10 more capacitors on top board that can be shorted if the problem is not inside GPU chip. Any suggestions how to proceed?

[piernov]

Re: Asus x55VD 2.1 troubleshooting

Well @techtiger question still applies. What are the readings?

[questore]

Re: Asus x55VD 2.1 troubleshooting

Well @techtiger question still applies. What are the readings?

- PCE8501 pin1 - 0.000 Ohm.
- What would you like to know?

[piernov]

Re: Asus x55VD 2.1 troubleshooting

0.000 "ohm"? I'd expect this reading in diode mode, not resistance mode.
GPU VCore will usually read close to 0.000 in diode mode (like 0.001 or so, sometimes a bit more), but in resistance mode it'll read a few ohms, like between 2 to 20 ohms.

In resistance mode, a full short to ground almost never reads 0.000 ohm as a standard multimeter isn't that accurate, and there's also the leads resistance. Full short reads usually at least 0.010 ohm.

tl;dr: confirm you're measuring in resistance mode, no power applied and power from the board fully drained. If that's the case you indeed have a full short to ground.
There are only several decoupling caps and the GPU left on the line, you can either remove them one by one or inject 1V and see what gets hot.

[questore]

Re: Asus x55VD 2.1 troubleshooting

0.000 "ohm"? I'd expect this reading in diode mode, not resistance mode.
GPU VCore will usually read close to 0.000 in diode mode (like 0.001 or so, sometimes a bit more), but in resistance mode it'll read a few ohms, like between 2 to 20 ohms.

In resistance mode, a full short to ground almost never reads 0.000 ohm as a standard multimeter isn't that accurate, and there's also the leads resistance. Full short reads usually at least 0.010 ohm.

tl;dr: confirm you're measuring in resistance mode, no power applied and power from the board fully drained. If that's the case you indeed have a full short to ground.
There are only several decoupling caps and the GPU left on the line, you can either remove them one by one or inject 1V and see what gets hot.

Thank you for the detailed explanation, I have learned a few new things. Measuring in resistance mode gives me 0.1 Ohm.
If I undersand correctly I have indeed a short to ground. I cannot apply 1V my lab power supply is on the wind so I have ordered a new. Meantime I try to decouple capacitors one-by-one and see whats happening.
I let you know where it leads.

[jasko_jacker]

Re: Asus x55VD 2.1 troubleshooting

I think the short is on +VGA_VCORE inside the VGA chip .
With hot hair, leaving one side of capacitor attached onboard, it's relative simple to verify that the culprit is vga chip.

[questore]

Re: Asus x55VD 2.1 troubleshooting

Removing capacitors... Holding my breath...

[questore]

Re: Asus x55VD 2.1 troubleshooting

I think the short is on +VGA_VCORE inside the VGA chip .
With hot hair, leaving one side of capacitor attached onboard, it's relative simple to verify that the culprit is vga chip.

Yepp you were right, all capacitors removed, but short is still there, =>> failty VGA chip. Board goes to donor. I have an X55A that is also faulty but it has no VGA so maybe I'am lucky and spare parts give me a GO.
Thankyou very much for your help. Moving on..

[questore]

Re: Asus x55VD 2.1 troubleshooting

I decided to use this dead beast as a teaching lesson to Learn problem diagnosing.
VGA is shorted but I separated it from everything else and moved on.

After pressing power on button, the laptop starts and power+wifi LED is constantly on. No display.

V3 and V5 voltages are OK. I managed to find that 1.5v is not present on DDR RAM chip.
Posting some measurements on PU8300:
SW REG. RT8207MZQW WQFN-20L

VTTREF =0
P 1v5 VSENS 10 =0
P +1.5v FB 10 =0
P +1.5v S3 10 =4.3mV
P +1.5v S5 10 =3.3V
P +1.5v TON 10 =0.8V
DDR_PWGD =4.2mV
P +1.5v VDD 20 =5.1V
P +1.5v VDDP 20 =5.1V
P +1.5v CS 10 =4.88V
P +1.5v LGATE 30 =0
P +1.5v PHASE 30 =0
P +1.5v UGATE 30 =0
P +1.5v BOOT 30 =0
P +1.5v VLDOIN 30 =0

Whats next? Can anyone help me? Is the chip wrong?

[mcplslg123]

Re: Asus x55VD 2.1 troubleshooting

RT8207MZQW-seems like this chip is bad