Laptop Model : Lenovo G480
Board Number : LA-7981P REV1.0
Fault : RAM Section Repair
Repair
Process :
The plug-in test current is 0.8
and there is no jump. The upper and lower case lights do not light up. It is
judged that there is no memory. Even after replacing the memory, it still
doesn't light up.
I took out the motherboard and
checked that there is no short circuit on the whole board. I measured that the
memory power supply is 1.5V and it is normal.
The memory VTT power supply is 0.75V, which is normal.
Flashing the BIOS
failed. The oscilloscope measured the memory data waveform and found that it
was at a low level.
Open the drawing and
check that the CPU reset BUF_CPU_RST# 1.05V is normal.
Check DRAMPWRGD, 1.5V is normal, and the SMBUS waveform captured
by the oscilloscope is normal. Strangely, after replacing the memory
again, there is still no data waveform.
Thinking that there was another CPU memory module that I had not checked,
I quickly found the power supply of the CPU memory module on the drawing and
found that it was abnormal, only 0.4V.
Continue to trace the source,
which comes from Mosfet conversion. When measuring the D-level Mosfet,
1.5V is normal, while the S-level one is only 0.4V.
Notice that there is an isolation point next to it, and observe that
there is indeed an isolation point on the motherboard. This is easy to handle.
Remove the Mosfet, and use the Solder at the isolation point to illuminate the Laptop.
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