Laptop Model: Dell 2421
Board Number: 12204-1
Fault: Laptop Does Not Turn On. Standby Voltage Is Present.
Repair Process:
The Fault is that it does not turn
on. After detailed inspection, the results are that the protection isolation is
normal, the standby is normal, the switch to EC jumps normally, and the PWRBTN
of the EC to the bridge is 3.3 and has not changed.
The EC and PCH of this
motherboard use the SPI shared BIOS, so now the EC does not send out the boot
signal, and the BIOS is involved. The BIOS has been flashed. I habitually flash
the BIOS I modified, but the fault is the same. Then continue to measure
the detailed conditions of EC.
AVCC and VCC are both 3.3V,
85-pin EC has 3.3V, the oscilloscope has measured the instantaneous pull-down
action, 73-pin ACIN# is 0V normal, 17-pin LID 3.3V is normal, 20-pin RSMRST is
0V abnormal, 117
The PWRBTN of the pin is 3.3.
If the switch does not jump when the switch is pressed, it is abnormal. The
problem is locked on the EC side. First, replace it with an NPCE885P.
Then test
again and the fault remains the same. The problem is that RSMRST is not high.
When the power
is on, the diode pin is measured to be more than 400 to ground, which means
that this pin is not pulled low by EC or PCH when the power is on.
After ruling
out the possibility that the two chips were damaged, I then tested the BIOS
with an oscilloscope and found that pin 1 was always chip select.
This is not
right. Either the wire is broken or the bridge is weakly soldered. I ran the
line and measured the resistance of the 33 ohm resistor from EC to BIOS and PCH
to BIOS.
The resistance
to ground is also there, which means there is no open circuit or soldering.
When I measured When I got to R2108 and R2110, I saw a tin bead stuck in the
middle of the two resistors.
When I measured the two resistors, they were
connected. then the problem appeared. Just turn it off and turn it on. Repair
completed.
The RSMRST was measured again and it was 3.3V. The waveforms of the BIOS
pins 1, 2, 5, and 6 were also normal with intermittent waveforms.
Happy Repairing !!
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