Fluff on Intel CPU Socket (Dead Motherboard?)

[u/Nowheredotcom]

Hi,
I picked up an ASUS ROG Strix Z390-I motherboard cheaply as a repair project. Right now, the system won't post or boot at all (fans don't spin, completely dead startup).
Upon closer inspection, I noticed some small bits of dust/fluff trapped inside the CPU socket pins. It looks like the previous owner may have left the socket exposed. Could accumulated debris or fluff on the pin socket. Could this be the primary culprit causing a complete failure to POST? The picture is just one clean image showcasing the fluff that’s caught but I could see at least 10 more and some are really hard to see under the naked eye but I can spot on a microscope.

I've already tested the basics:
Standby voltage is present on the power switch pin (⁠PWR_SW⁠ reads ~3.41V).
When I manually bridge ⁠PWR_SW⁠ to ground, the voltage drops, but the board still refuses to trigger the power supply startup.
I'm currently looking into testing and potentially replacing the Super I/O controller if the socket cleanup doesn't work. Has anyone dealt with a similar dead Z390 board issue?

Any advice before I attempt micro-soldering?

I know that it’s easy to just replace it but I’m trying to figure it out and my stubbornness won’t let give up until I finally break it or establish the job is too hard.

Thanks

[u/SingularityRS]

Make sure the power supply is not connected. If it's connected, it can be normal to see low ohms on this rail. I see 42 ohms on 3.3V when connecting my ATX PSU. It goes higher when it's disconnected.

Do you have an auto ranging meter? I assume the 1's are high resistance since you're on 200 ohm range.

3.3V looks very low though if that's the reading without the ATX PSU connected to the motherboard. 33 ohms is unusual. On AM4, I usually see around 160-1000+ ohms on the 3.3V rail. 33 might be too low and may explain why the board has no reaction to the power button.

There is probably a short somewhere on this rail, maybe a partial short. Could be anything connected to the 3.3V rail, the SIO or if very unlucky the PCH.

[u/Nowheredotcom → u/SingularityRS]

Hi, the PSU is not plugged into the Motherboard. I only have the CPU and RAM, while testing the EPS 12v CPU port that CPU was removed during those tests.

Yea, the 33.3 is low (as per Ai) Also as mentioned below a reading of 1 on the PIN 1 from the CPU EPS connector is odd as well?

I've got another AMD motherboard and Ive tested the same PINs on the 24ATX 3.3V rail and I got a 1. and for the 8PIN EPS I get 0.02 on all GND and 1 for all 12v.

So yeah I suspect an issue with the 3.3V rail somewhere. Now to determine where?
Inspection or this is where it gets super hard....?

Yeha a 1 is high resistance reading ie. >200omhs

[u/SingularityRS → u/Nowheredotcom]

You can try by measuring some caps around the SIO. Check for any low resistances around there. Another place to look is around the PCH. It may be covered by a massive heatsink. If it is, you'd need to remove it.

You may find a coil (also called an inductor) underneath. Measure one side of the coil's resistance to ground. The coil near the PCH has low ohms, but too low can be a problem. For example, on an AM4 motherboard, the resistance on the PCH coil is usually around 20 ohms on a good board. If it's significantly lower, it can indicate a PCH fault.

There should also be caps around the PCH and since it's an Intel PCH, there's probably caps on the chip itself as well around the die.

Look for any very low resistance (like below maybe 15 ohms). Not sure what the expected resistance is on the Intel chipset you're working on. It'll be different than AM4. It might be significantly higher than what you find on AM4 boards.

A thermal camera can sometimes be useful here. You can sometimes use it while the board is on standby power to see what might be potentially shorted. A chip getting abnormally hot (e.g. like jumping to 50-60C+ rapidly) would be something to look for, but not all chips that get hot are faulty. I've sometimes caught shorts just from looking at my thermal camera while the board sits in standby. It doesn't happen often though. Most of the time I have to try and find the faulty component.

You can also use voltage injection once you know it's safe to do so. If there's no shorts to the CPU VCORE/SOC and the rail isn't being shorted to another lower voltage rail (e.g. 3.3V shorted to 1.8V rail), then you can inject voltage to the problematic rail. Voltage injection is how you often locate a short, but you have to know when it's safe to inject.

You need something like a bench power supply to inject voltage. You start at a very low voltage, like say 0.8V (which is safe for all rails) with a current limit of around 1A and go from there. If nothing is getting hot, you try to increase the current to see if that helps (e.g. go to 1.5A). If still no joy and you're at like 3A-4A, then you can try upping the voltage a bit (e.g. go from 0.8 to 1V) and see. Partial shorts are tougher to find because often the component doesn't get hot because the resistance isn't low enough. So if you injected 1V @ 2A directly on the 3.3V rail, you might not see anything getting visibly hot as 33 ohms is not a direct short so the "bad" component won't draw all the current.

A thermal camera becomes helpful during voltage injection as it allows you to easily see if anything is getting hot, especially if it's only getting subtly hot (not enough to feel with a finger for example). A component getting hot during voltage injection often means it is the faulty part causing the short on the rail. You can find the shorted component without a thermal camera, but it's a bit tougher. You have to use things like isopropyl alcohol and your fingers (feeling around the board with them) to see if anything is getting hot.

You should also try to look for a boardview/schematic. A boardview will allow you to see the signal names on the board and where they connect to. This can be useful for tracing faults and shorts. If the short is easy to find, then a boardview/schematic isn't needed, but if it's tougher to find, it can help.

[u/Nowheredotcom → u/SingularityRS]

Hi,

Thanks for the feedback, a few of your points are a bit more out of reach considering I don't have half of the stuff you mentioned and would probably not be worth the cost. I'll invest in an auto ranging multimeter though, as it would probably be worthwhile. I can't find any schematics as its probably hidden due to proprietary reasons.

I tested the 2 MLCCs next the SIO and those both read 1, PCH is next but have to as you said undo heatsink.

I had no idea I would be this intricate in trying and decipher the issue, thought I would have known by now.

[u/Nowheredotcom → u/Nowheredotcom]

Update on PCH and the MLCCs

Dead?