DELL 14-5420 notebook is not powered on for repair

machine model: Dell-5420

Fault phenomenon: connected to Nand no booting

Maintenance process: insert Nand on no booting, do not plug Nand can start, is Nand broken?A Nand is still the same.Test Nand interface VCC, found + 5V_Run this VCC is abnormal, only 2.6va.

DELL 14-5420 notebook is not powered on for repair Figure1.

After checking the source, it was found that Nand VCC was converted from standby 5V, that is, the secondary VCC.If the standby 5V wants to pass, pq16 must be turned on, and pq16 is controlled by pq19.

DELL 14-5420 notebook is not powered on for repair Figure 2.

In the standby state, 2 pin RUN_ON of PQ19 is at a low level, and then PQ16 is turned off by pulling down the upper 5V conduction. When we press the power-on key, the RUN_ON sends out a 3.3V high-level conduction to turn off PQ19A, and the upper +15V_ALW high-level conduction of PQ16 realizes the secondary VCC conversion.

When I measured PQ16, I found that the G-class was only 5V, which was definitely abnormal. First, I ruled out whether there was something wrong with the resistor PR54, and measured Diode value is normal.

After re-testing, PQ19A 5 pin is also 0V normal. At this time, it is still the same to directly disassemble PC44. Is there something wrong with VCC+15V_ALW? But I feel impossible, because this VCC is not normal.

It’s impossible to start the machine without installing Nand, and the current won’t run up, that is, there is no secondary VCC. It’s mainly suspected that PQ16 lowered VCC, and it was directly removed. It was found that the G-class VCC recovered to 15V, and it can be lit by replacing PQ16 with Nand. It was found that the fan didn’t turn, but the measuring fan 5V VCC didn’t. It was directly repaired by 2 pin and 3 pin of VCC U52.