ASUS b75m-a mainboard is not powered on for Repair

Model 75m: ASUS b75m-a motherboard

Fault phenomenon: the main board no booting,switch,auto power off are not reset

Maintenance process:

One ASUS b75m-a motherboard.

ASUS b75m-a mainboard is not powered on for Repair Figure 1.

After clicking Switch, the main board can be powered on and the fan can be turned on.

ASUS b75m-a mainboard is not powered on for Repair Figure 2.

When the diagnostic card runs 00, the reset lamp is not on.

ASUS b75m-a mainboard is not powered on for Repair Figure 3.

First test the whole board VCC:1

CPU VCC 1.1V is normal;

Memory VCC 1.5V is normal;

1.05v of bridge VCC was normal;

PLL VCC 1.8V was normal;

SA VCC 1v was normal;
VCC.

Maintenance signal part:

PCH 25m Crystal started to vibrate normally;

ASUS b75m-a mainboard is not powered on for Repair Figure 4.

The BIOS chip pin has no chip selection.

ASUS b75m-a mainboard is not powered on for Repair Figure 5.

It is judged that Nanqiao has not read BIOS. According to the Ivy bridge platform timing, PCH will read BIOS only after obtaining valid apwrok and pwrok signals. Normally, the two signals of the motherboard are connected together.

The pwrok signal from Io chip is 3.3V high, and VCC is normal.

ASUS b75m-a mainboard is not powered on for Repair Figure 6.

The apwrok pin bit of Nanqiao is named meprock in the peripheral circuit, which is controlled by the me MOSFET chip. No VCC is detected.

ASUS b75m-a mainboard is not powered on for Repair Figure 7.

After startup, there is no me VCC. This board supports AMT independently.

ASUS b75m-a mainboard is not powered on for Repair Figure 8.

Test VCC chip EN is seriously low, less than 1V, the point is 5V VCC pull-up.

ASUS b75m-a mainboard is not powered on for Repair Figure 9.
The EN was pulled up by a 103 drain resistor, and its appearance was obviously abnormal, so it was replaced directly.