Lenovo G580, COMPAL LA-7982P

Problem with this laptop is +1.5V_CPU_VDDQ missing.

Sometimes laptop works after getting a bit temperature by its own.

I've found no voltage in pin 1,2,3 of mosfet U3 (+1.5V_CPU_VDDQ) due to pin 4 (gate) being only 1,1 volts.

+VSB is present and mosfet Q4, R56 and C97 replaced but same result.

Heating Q4 only a little bit, Voltage at its drain normalizes and +1.5V_CPU_VDDQ is there.

I've found also that without charger nor battery, (no power at all) there is a short (2.5 ohms) at pin 1 of Q4, and no voltage in its gate. (there's nothing powering laptop)

So what can make this low resistance at drain pin of Q4?

Really I don't know, but "shorting" the gate of Q4 whith 1 Mega-ohm resistance, problem disappear, so probably the high input impedance of Q4 and a open circuit somewhere in the board (there is no boardview and many modifications) makes this problem rather difficult to solve.

With this little modification, laptop are working with no problem.

Regards.

Just a few clarifications:



Problem with this laptop is no video, due to voltage +1.5V_CPU_VDDQ missing.



There is no boardview and also manufacturer makes so much modifications that makes this problem rather difficult to solve. (some traces run below cpu socket)



Adding a high ohm resistor between Q4 gate and ground, in order to limit input impedance of Q4.