Acer aspire 4750 power on no display

Model : Acer aspire 4750 Series
Board id : JE40 HR MB 10267-3

Lapop have strange behavior. Sometimes laptop is running and display ok (Without HDD or ODD). With HDD or ODD, lapop is running but no display. But, sometimes laptop is running and no display even without HDD or ODD. When running no display, there is only 0.4V on PL4501. RAM supply volt 1.5V is ok on PL4601, but PU4601 Pin-10 (PWR_0D75V_EN) is 0V and Pin-13 (RUNPWROK) is also 0V. Need help.

Hello!

Measure the voltages to ground on pins 1, 3, 7, 10 of chip PU4501, when there is no picture on the screen.

PU4501 ,
Pin 1 - 0V
Pin 3 - 0V
Pin 7 - 5.1V
Pin 10 - 4.9V

Perform the following measurements, when the machine is turned on and there is no picture on the screen:
- Measure the voltages to ground on pins 5, 10, 11, 12, 13, 14, 15, 16, 22, 23, 24 of chip PU4601.
- Measure the voltages to ground on pins 1, 2, 3, 4, 6 of chip PU4701.

PU4601 pins,
Pin 5 - 0.8V
Pin 10 - 0V
Pin 11 - 3.4V
Pin 12 - 0.8V
Pin 13 - 0V
Pin 14 - 5V
Pin 15 - 5V
Pin 16 - 4.9V
Pin 22 - 6.5V
Pin 23 - 1.6V
Pin 24 - 0V

PU4701 pins,
Pin 1 - 0V
Pin 2 - 3.4V
Pin 3 - 3.4V
Pin 4 - 5V
Pin 6 - 0.5V

Measure the resistance to ground on pin 6 of chip PU4701.

Resistance to ground on pin 6 of chip PU4701 is 343 Ohm.

Replace the chip PU4701. You can use the chip RT9025-25PSP or G9661-25ADJF11U.
If it still doesn't work, remeasure the voltages to ground on pins 1, 2, 3, 4, 6 of chip PU4701.

Replace the chip PU4701 and laptop is running with display. Only one problem exit. Can't detect HDD and ODD. I found problem mosfet U3607, U3604. Replace these two mosfet and laptop is running well. If you don't mind, i want to ask one question. How you decide PU4701 can be problem device?

How you decide PU4701 can be problem device?

Based on the measured voltage values.
To better understand the operation of the RT9025 just read its datasheet.

Measured voltage values are ..

with HDD or ODD
Pin 1 - 0V
Pin 2 - 3.4V
Pin 3 - 3.4V
Pin 4 - 5V
Pin 6 - 0.5V

but without HDD or ODD
Pin 1 - 3.3V
Pin 2 - 3.4V
Pin 3 - 3.4V
Pin 4 - 5V
Pin 6 - 1.8V
and laptop is running fine. So i make a wrong decision PU4701 is ok and problem devices is something others.

cygron писал(а):I found problem mosfet U3607, U3604.

Were those old transistors damaged?

- Measure the resistance to ground on pin 8 of transistors U3607, U3604.
- Measure the voltages to ground on pins 1, 4, 8 of transistors U3607, U3604 when the machine is turned on with and without HDD, ODD connected.

For now, replace PU4701,U3607, U3604 and laptop is running fine with HDD or ODD. I only need to know how matic decide the problem device is PU4701 for edecational purpose only.

cygron писал(а): I only need to know how matic decide the problem device is PU4701 for edecational purpose only.

The PU4701 is a quite simple LDO regulator chip RT9025:

The most basic things to proper operation of this chip are two power supply voltages VDD and VIN (pins 3, 4) and the enable signal (pin 2).
In your case the output voltage (on pin 6) was too low (0.5V instead of 1.8V) despite the present and correct power supply voltages VDD, VIN and enable signal. The resistance to ground on the output was also in range of normal, so everything indicates this chip is damaged.
Well, it's also possible that there could be something wrong with the "feedback" circuit on the ADJ pin, but I've ruled that out, since in your case this chip worked correctly from time to time.

Thanks for your helping and teaching. Best regards.