ASUS Prime Z390-A not starting up - possible issue with PCH

[karlp028]

I'm working on an ASUS Prime Z390-A that isn't reacting to the power button. All of the standby voltages and signals are good. The PCH is receiving O_PWRBTN#, but it is not de-asserting SLP_S3#, SLP_S4#, or SLP_S5#. I started probing around and noticed the voltage on RTCX2 is around 1.5V, which seems crazy high. On RTCX1 it is around 300mV, which is normal. I don't see any waveform either. When the PCH is not powered, the resistance on RTCX1 is about 2.5M ohms and on RTCX2 it's about 8.5M ohms. When the PCH is powered on, RTCX1 shows about 7M ohms and RTCX2 shows OL.

I suspect the PCH is faulty, but I'm hoping it is something easier to fix. I replaced the crystal, just in case, but there is no difference. Has anyone seen something like this before?

[DynaxSC]

Without clock pch will not work. Check with scope on pin 2 of bios spi chip if there is any activity after power on signal. If not, pch does not work. It might be that pch is in reset state due to some external signal beeing in wrong state, eg. some short, but this is not likely. Most probably the pch has decided to go to heaven. 1.5V at RTCX2 is not normal at all. PCH failure is unfortunately a quite common issue. Intel PCH's are not the best quality, durability is much more worse than Intel CPU's.
Should there be an activity on spi chip, then progam the chip with a stock bios, sometime the bios image goes corrupted, and mobo does not POST normally. Do not forget to backup the BIOS so preserve the DMI data and MAC address.
BTW to check the clock directly on the hot pch crystal pins you need a very good probe with low parasitic capacity, and set the probe to 10:1 dividing ratio, so input capacity is minimal then. If parasitic capacity is too high it will stop the clock gen before you can measure anything, as it is very sensitive to such addidional parasitic "load".

[karlp028]

I did not see BIOS the chip select (SPI_CS0#) go low at any time so it looks like there was no activity toward the BIOS. It appears there was a crash log (FLOG) written to the ME region though. I will try to analyze it later.

While the board was on standby power, I was doing some voltage checking and a very strange thing happened. When I probed the capacitor on the RTCX1 side of the oscillator circuit (see below), it caused SLP_S3# to go high and the PSU came on. I was able to repeat it several times. I have no explanation for why this happens.

The PCH appears to have a mind of it's own and probably needs to be replaced.

Note: I have another Z390 board and I am able to see the RTC clock signal on the output side with my probe set to 10x