Dell 3421 laptop cannot be powered on for repair

Machine model: DELL 3421

No.: 12204-1

Fault symptom: notebook no power on, no power on, standby 0.01

Maintenance process: there is no Short circuit in the whole board, only the ground resistance between CPU and graphics card is relatively low (the ground resistance of diode value of the third generation onboard CPU is generally 3-5 ohms normal), the amount is 3V5V, the reset of E VCC is normal, and AC_IN# is 0V(EC 73 pin is normal).

Dell 3421 laptop cannot be powered on for repair Figure 1.

LID3.3 is normal, but the current of change of RSMRST# and PWRBTN’s VCC keeps flashing, as well as the power working indicator in the lower left corner.

Dell 3421 laptop cannot be powered on for repair Figure 2.

Find RSMRST# and PWRBTN on the edge of EC and cut them off directly.

Dell 3421 laptop cannot be powered on for repair Figure 3.

Dell 3421 laptop cannot be powered on for repair Figure 4.

Measure that VCC on the EC side keeps jumping from 0v to 3v, and replace the EC directly, and the failure will be the same after the replacement. Check the working conditions of EC again, and find that everything is normal except current of change, which keeps VCC and PWRBTN.

We have to go to the oscilloscope to check whether each VCC has stable output, public terminal, 3V5V, and 3V5V linear VCC. The oscilloscope directly adjusts the automatic gear, and the linear 3V5V is normal, but when 3V5V is caught, it is repeated, and VCC will instantly pull down and go up again.

Dell 3421 laptop cannot be powered on for repair Figure 5.

Grab PWR_3D3V_EN2 again. Just like PWR_5V_EN1,VCC will instantly pull down and go up again.

Dell 3421 laptop cannot be powered on for repair Figure 6.

Tracing the source of PWR_3D3V_EN2 and PWR_5V_EN1 is EC, and is connected with the PURE_HW_SHUTDOWN# signal, which in turn controls ECECRST#.

Grab PURE_HW_SHUTDOWN# and ecerst # repeatedly.VCC will instantly pull down and go up again. It is normal to grab ecerst # by directly removing Q2701.

However, PURE_HW_SHUTDOWN# will be pulled down repeatedly, because PURE_HW_SHUTDOWN# has been in several places, and only D3601 can be found after searching for a long time.

Dell 3421 laptop cannot be powered on for repair Figure 7.

If you can’t find anything else, take down the D3601 directly. After taking down the D3601, the board has been displayed normally by switch. It’s normal for half a day after the direct installation and repeated testing, and I’m too lazy to trace the source of PURE_HW_SHUTDOWN# signal.