Lenovo F41 Notebook Water after the water does not add electricity repair

A water f41a, do not add electricity, ask the customer how bad, he also said half a day also did not say a Sanlai, did not say not understand, look at standby------current between the 0.002~0.003 rhythm, with experience like the main 19V filter capacitor wear or upper mos pipe wear, Light motherboard Board 19V resistance regardless of isolation protection, or isolation protection after the normal is puzzling, depressed ... The visual motherboard has no obvious fault points except for the gray dots. Crazy Dizzy ... Well, that's going to be a step-by-step approach.

First of all, the main board of major power supply to the ground resistance is normal, and then measured 3v5v No, this machine standby should have 3v5v, power supply chips for the common max8734a

After measurement found that 8734 power supply foot 19V has, vl5v, PIN6 shdn# for high level, normal, PIN8 REF 1.97V, normal, open signal ON3 normal, but PIN4 ON5 0.24V, abnormal, should be high level, the small pieces of resistance around this foot are normal, disconnect resistance PR322, 8734 PIN4 ON5 foot voltage from 0.24V up to 5V (note: This 5V for vl5v voltage) then determine the fault in the Mainpwon this road, at this time still no 3v5v (Forced to Mainpwon high level is still so) test Mainpwon resistance is normal, then check it

Look for the drawing, turn over Ah, turn to 39 pages, sure enough, this page is very critical DC in detection circuit

It appears that the Mainpwon signal is derived from the comparator LM393 (note that PD6 is a Schottky diode, voltage from LM393 pin7-->pd7--->8734 PIN5) to measure its LM393 PIN5 to 0v,pin6 3.3V ( LM393 need PIN5 voltage >pin6 voltage PIN7 output High level, otherwise PIN7 output low level) originally PINI5 no voltage, no wonder mainpwon for low level, along the PIN5 to look up, found that this voltage is B +, measured PR18 resistance normal

Measured PR18 both ends have no voltage, measured PQ1 PIN3 have 19V input voltage, PIN1 No, B + at all did not come out, no wonder when 8734 PIN5 for high power and forced to mainpwon high power at ordinary times still have no 3v5v output. and PIN1 to PR18 has been broken. Because the customer is anxious to take, directly from the PIN3 to PR18, in the welding process, smell the---beer taste, that taste, repaired the water machine all understand ... It seems that the water leads to disconnection, but the visual is just some gray, completely do not see any water traces, the palm of the Shell keyboard no traces, oh, flying good line after standby immediately restore normal--0.017, press the power-on button, familiar with the Lenovo appeared, this machine repair.

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