Asus X99-S motherboard will not start

[Sully115]

I have an older Asus X99-S mother board which has quit working on me, I don't have money for an upgrade at the moment so thought I would have a go at figuring out what up. Before it stopped working the pc would intermittently not start on the first try, but normally would on the second (I don't know if this is relevant)

I have tried the usual, reseating everything, using a single Stick of ram etc, tried flashing the bios from USB but nothing happened . when the psu is turned on the power and reset buttons light up but nothing happens when pressed, I have checked the psu and all the voltages are present when I jump it. Cmos battery is fine.

I have checked the board with a multimeter and 5vsb is fine, goes into the mosfet for 3vsb which is present at the super io onthe right pins (only at 2.6ish volts though). The 3vsb mosfet appears to be getting a little warm and the capacitor connected is getting quite hot (40 degrees), strangely there is a capacitor close by on the 12v rail that is also getting hot but there isn't any voltage across it. When the button is pressed the pin drops to 0v but the psu does not start up an the super io output pin does not change.

I am at a bit of a loss of what could be the cause without just changing the caps and the mosfet to see if that changes anything. The 12v cap heating up seems to indicate a short somewhere as the is no 12v rail live from the psu.

I have a boardvoew file from a very similar mother board, the data sheet for the super io chip.

Would be greatful if anyone has any suggestions

[DynaxSC]

First check whether there is present 5V Stand-by voltage in the big ATX connector.

If yes, check whether the board asserts the PS_ON signal in the big ATX connector (asserting = low = 0V) after shorting Power On pins in the System Connector.

If yes, check all input resistances for ATX powers, 12V, 5V and 3,3V. If there is an overcurrent of the PSU due to short on any of these power lines, the PSU will not start and block any further start attempts until unpowered from mains for 2-3 minutes.

If no 5V Stand-by present look for shorts on the board, i.e. on any Stand-by rails generated from 5V Stand-by: 3,3V and chipset power rails 1,05V, and probably 1,8V (not sure here). Chipset should be always powered even if board is off.

Chipsets tend to get degraded with long time working, and either get partly broken (working unreliable, no correct POST, or no start at all), getting hot (anything above 55-60 C without radiator on it is suspicious) or get fully shorted power rails finally. Chipset partial or full failure is often the reason for not starting of the board. Such strange behaviour you described is often sign of a degraded chipset. For old boards this means usually RIP, as repair makes economically almost no sense.

After taking off the radiator from chipset, you can measure the presence of the chipset powers on the MLCC caps mounted on the silicon chip carrier - the inner pads are usually the positive power lines, the outer is usually GND.