Foxconn b85mx motherboard has been rebooting no booting

Motherboard model: Foxconn b85mx series motherboard

Fault phenomenon: the motherboard has been restarting no booting

Maintenance process:

A Foxconn b85mx series motherboard, the fault is or so auto power off for 20 minutes, and the booting up has been restarted.
Board number of main board:

Foxconn b85mx motherboard has been restarted no booting Figure 1.
multimeter measured that CLK did not reset when rebooting, and the CPU VCC dropped instantly when it came out.The oscillograph displays the waveform of CPU VCC.

Foxconn b85mx motherboard has been restarted no booting Figure 2.
The Capacitor filter is still bad.

Foxconn b85mx motherboard has been restarted no booting Figure 3.
auto power off uses dual channel to grasp waveform.Taking the output of CPU VCC as the unit of comparison, the waveform of auto power off output by memory VCC and CPU VCC is compared

Foxconn b85mx motherboard has been restarted no booting Figure 4.
The output of CPU VCC is taken as the unit of comparison, and the waveform of output and start is compared. It is found that the waveform is turned on first.

Foxconn b85mx motherboard has been restarted no booting Figure 5.
Enlarged image with reduced time base:

Foxconn b85mx motherboard has been restarted no booting Figure 6.
Open before output, enlarge:

Foxconn b85mx motherboard has been restarted no booting Figure 7.
Open source p_CORE_ SLP_EN, measured R62 and C45 are installed, and the source is from R62 pull-up and controlled by other circuits.

Foxconn b85mx motherboard has been restarted no booting Figure 8.
The front-end waveform of R62 is measured, indicating that the front-end waveform is OK.

Foxconn b85mx motherboard has been restarted no booting Figure 9.
If the front-end is OK, check the back-end control pole. The control pole went to two places and randomly found a circuit.

Foxconn b85mx motherboard has been restarted no booting Figure 10.
It was found that when rn26 2 pin 3V reached 1 pin, there was only 1VtriodeThe maximum clamping value here should be 0.7V. If it is greater than 0.7, it means that the triode has not been pulled down completely, which belongs to poor components. If Q75 is directly removed, it is found that rn261 pin is also 3V.Replace the triode.

Foxconn b85mx motherboard has been restarted no booting Figure 11.
The measured q75b level is 0.7V, which indicates that the triode has been working normally, and the q75c level has been successfully pulled down to 0V.