Strange VCCSA - Lenovo G470 - LA-6751P

[caspian]

Dear Members,

When the laptop is cold, if I turn it on, it boots fine and gives display.
Then after 10 to 60 seconds (randomly), the VCCSA voltage gets 0v and the laptop shows white screen. then after 3 seconds, the laptop turns off.
Then if I turn the laptop on, VCCSA is zero and the laptop has black screen. then I have to wait about 15-30 minutes to get picture on this laptop.

VCCSA is generated by RT8209B (PU601). Here is the link to the schematics. PU601 is on Page 50:
https://www.badcaps.net/forum/attach...7&d=1591338538

when it gives display, the pins are as follows:
Pin 1 (En): 3.3v
Pin 2 (Ton): 2v
Pin 3 (Vout):
Pin 4 (VDD): 5v
Pin 5 (FB):
Pin 6 (PGOOD): 3.3v
Pin 7 (GND): 0v
Pin 8 (PGND): 0v
Pin 9 (LGATE): ?
Pin 10 (VDDP): 5v
Pin 11 (CS): 0v
Pin 12 (PHASE): ?
Pin 13 (UGATE): ?
Pin 14 (BOOT): 5.8v
VCCSA : 0.9v

when it does not give display, the pins are as follows:
Pin 1 (En): 3.3v
Pin 2 (Ton): 0.02v
Pin 3 (Vout): 0v
Pin 4 (VDD): 5v
Pin 5 (FB): 0v
Pin 6 (PGOOD): 0v
Pin 7 (GND): 0v
Pin 8 (PGND): 0v
Pin 9 (LGATE): ?
Pin 10 (VDDP): 5v
Pin 11 (CS): 0v
Pin 12 (PHASE): ?
Pin 13 (UGATE): ?
Pin 14 (BOOT): 4.6v
VCCSA : 0v

Neither PU601 nor its surrounding Mosfets/resistors/capacitors get warm. the Mosfets are not shorted.
resistance on VCCSA is about 40 Ohms.

Now I do not know whether PU601 needs to be replaced or one of its surrounding Mosfets/resistors/capacitors or the CPU is defective?
Please give your opinion.
I have doubt that the cpu may force PU601 stop its output voltage.

Thanks

[qayyum786]

Re: Strange VCCSA - Lenovo G470 - LA-6751P

CUT THE TRACK BETWEEN "PL601" AND PC605 CAPACITOR
THAN CHECK FOR VOLTAGE on COIL PL601

[piernov]

Re: Strange VCCSA - Lenovo G470 - LA-6751P

CPU is not soldered so most reasonable would be to try another CPU first.
Then try to reflow PU601 solder joints.

[caspian]

Re: Strange VCCSA - Lenovo G470 - LA-6751P

thanks
I replaced PU601. Now when I turn the laptop on, VCCSA is present for a few seconds, then VCCSA gets 0 volts. Then, white screen appears.
When CPU is connected, resistance on VCCSA is about 40 Ohms.
When CPU is not connected, resistance on VCCSA is about 20 KOhms.
I do not have another CPU to test.

Do you think the CPU is faulty or the VCCSA's Mosfets needs replacement?

[caspian]

Re: Strange VCCSA - Lenovo G470 - LA-6751P

Could someone please confirm what resistance exists on VCCSA coil when a second-generation Intel sandy-bridge CPU is installed on the mainboard?

is 40 Ohms too low for VCCSA?

[caspian]

Re: Strange VCCSA - Lenovo G470 - LA-6751P

I replaced the CPU. I replaced PU601 and its Mosfets. But nothing changed. VCCSA is still acting the same.
The laptop gives display only when it remains off for more than 30 minutes.
If I turn it on for the first time, it gives display but VCCSA gets 0 volt after 30-50 seconds.
Then if I turn the laptop on, VCCSA will not be present although PU601's Enable pin is 3.3v. Then, I have to wait more than 30 minutes to have display.
What do you think is defective?

[mcplslg123]

Re: Strange VCCSA - Lenovo G470 - LA-6751P

When display goes off, +1.05V PCH Pwr remains ok??

[caspian]

Re: Strange VCCSA - Lenovo G470 - LA-6751P

yes, the +1.05V rail remains present and stable. However, the +1.05V rail is measured as many as 1.07 volts.
and my multimeter is precise in measering voltage.