Dell Inspiron 3147 motherboard unusual behaviour

[BinaryOperatingSystem]

Hi guys, I have a few Dell Inspiron 3147 motherboards that are exhibiting the same issue. I am new to motherboard repair so if this is a very simple rookie obstacle bear this in mind I have the same issue on multiple 3147 boards (Winstron 13270-1) in which I have my DCBATOUT rail, my 3d3v_aux, and my 5v_aux and they are stable. However, when I turn on the bench power supply to power the motherboard my 3d3v_s5 and rail powers up to 3.3v for 1-2 seconds and turns off. The 5v_s5 rail gets to 5v before powering off. All rails turn off at the same time. The 3 and 5v regulator "PU4503" generates the 3v_5v_POK signal before all of the rails excluding the first 3 rails shut down. The 1d8v rail powers on before the board shuts down etc. etc.. I have measured the resistances of the 3d3v_s5 and 5v_s5 rails to ground and have them both at ~390 ohms. I have followed the schematic checking each rail, and prior to turning itself off, the motherboard looks good until RSMRST# which stays low the entire time.

Like I have said I have run into this type of issue on multiple boards (not only on Inspiron 3147's) as well and I am curious to know what would cause something to seemingly power on for only a few seconds only to turn off and stay off. Thank you for any help you can render!

[mcplslg123]

Re: Dell Inspiron 3147 motherboard unusual behaviour

Check Q2403/Q2404- part of PSL logic.

[BinaryOperatingSystem]

Re: Dell Inspiron 3147 motherboard unusual behaviour

Thanks for the quick reply mcplslg123, I checked Q2403 and Q2404 mosfets. Keep in mind that I may have a slow multimeter and I may be not capturing quick on and offs accurately. Here are the measurements:

Q 2403 N-Channel Mosfet

Source
GND
Drain
570 Ohms
~1v before quickly turning off
Gate
OL Ohms
0V when board is on, stays zero as long as board is on. Shoots up to 3.3v when board is de-energized then quickly fades to 0v.

Q 2404 P-Channel Mosfet

Source
468 Ohms
3.27 on and stays on
Drain
400 ohms
3.27 on and stays on
Gate
OL Ohms
0V when board is on, stays zero as long as board is on. Shoots up to 3.3v when board is de-energized then quickly fades to 0v.

[mcplslg123]

Re: Dell Inspiron 3147 motherboard unusual behaviour

Your measurements seems ok as mb turns off quickly. What about S5_Enable signal??

[BinaryOperatingSystem]

Re: Dell Inspiron 3147 motherboard unusual behaviour

S5_Enable gets up to ~2 volts before it turns off quickly. What does it mean when a line goes up to 3.3 only when the board is turned off? Is that unusual?

[mcplslg123]

Re: Dell Inspiron 3147 motherboard unusual behaviour

Voltage peaking to peak level and then cuts off-surely it is getting some thing wrong on power sequence after that signal. There is likely to be some cap shorted or leaky somewhere. Check mb under microscope carefully for rusted/corroded elements.