ASUS h81m-ct motherboard CPU fan turns to stop and cannot be turned on

Main board model: ASUS H81M-CT 1.00

Malfunction: CPU fan turns to stop and cannot be turned on.

Maintenance process:

An ASUS H81M-CT motherboard, the fault is that the CPU fan turns around and stops, and after more than ten minutes of repeated turning and stopping, the motherboard can enter the boot screen after the temperature is high, and then the auto power off restarts irregularly.

The motherboard VCC, memory 1.5V, bridge VCC 1.5V, bus VCC 1.05V VCC are all normal, PCIE VCC A12 A13 is 0.4V The first pin of BIOS measured by oscilloscope also has chip selection and pull-down action.

VCC is available, but BIOS is useless.

Oscilloscope measures the waveform, measuring PCH_PWROK and ProcPWRGD, and PCH _ PWROK drops first.

ASUS h81m-ct motherboard CPU fan turns to stop and cannot be turned on Figure 1.

Measure SLP_S3 and PCH_PWROK, or PCH_PWROK first auto power off.

ASUS h81m-ct motherboard CPU fan turns to stop and cannot be turned on Figure 2.

Open the timing diagram and find the signal that affected auto power off before PCH_PWRGD.

Pch_pwrok APwrok SYS_pwrok is the same VCC! Check the components connected with pch_pwrok first, and remove the components that are not connected after cutting off.TriodeThe problem.

ASUS h81m-ct motherboard CPU fan turns to stop and cannot be turned on Figure 3.

Find schematic, timing chart, and the signal in front of timing chart PCH_pwrok is VR_Reandy, this signal is 1.2V, look Boardview went through several triodes to the South Bridge!

ASUS h81m-ct motherboard CPU fan turns to stop and cannot be turned on Figure 4.

The measured PQ804b pole VCC, pole B has a high level of 1.2V, pole C is 5.12V, which is extremely abnormal, and the normal pole C should be 0 V.

ASUS h81m-ct motherboard CPU fan turns to stop and cannot be turned on Figure 5.

The problem is that PQ804 is damaged, which causes PCH_PWROk to be pulled down. After replacement, power-on test,Normally booting up, test function is all right, and maintenance is over.