AsRock B560 Pro4 Possible Short on +1.8VSB

[zklgfs]

Hi,

I have an AsRock B560 Pro4 (boardview: https://www.badcaps.net/forum/troubl...pro4-schematic) that's completely dead (it doesn't even turn on the power supply when the power button is pressed or the power button pins are shorted). I do get about 3.4V on the power button pin, but 1.8VSB isn't there. The resistance to ground on the inductor PL53 is only about 0.1-0.4 Ohms. Am I correct in assuming that there must be a short to ground on 1.8VSB, or is there a different reason why the resistance could be so low? I don't have a thermal camera, but I do have a power supply I could use to inject voltage if needed, so I could maybe use the trick with watching alcohol evaporate more quickly on hot components, I just wouldn't know where to even start looking. Does anyone have suggestions what I should try first?

Thanks in advance for any help!

[DynaxSC]

From my experience 98% PCH is shorted, another 2% the 1v8 vrm is shorted. You can inject 1.8v, but set current limiter to 5 Amps or so, alcohol is a good idea. However if the PCH gets the high current it might short totally to very close to 0 ohms. Then injecting even 10 Amps might not get it warm. More than 10 Amps might be risky as tracks might get melted, making board not repairable.
To go sure PCH is shorted before injecting voltage first remove the vrm coil and see on which side the short is. If possible disconnect only one pad of the coil, and pull up the coil to disconnect.

[zklgfs]

Hmm, after looking around a bit more, I have a suspicion that the PCH is shorted, in which case I unfortunately won't be able to fix anything. I guess I need to inject voltage to see if it heats up. According to some videos I saw, if I put isopropyl alcohol on the PCH while injecting voltage, and it evaporates more quickly in one spot, that probably means there's a short. Should I start with a voltage of 0.8V or so to be on the safe side, or can I just go straight to 1.8V?

[DynaxSC]

From my experience 98% PCH is shorted, another 2% the 1v8 vrm is shorted. You can inject 1.8v, but set current limiter to 5 Amps or so, alcohol is a good idea. However if the PCH gets the high current it might short totally to very close to 0 ohms. Then injecting even 10 Amps might not get it warm. More than 10 Amps might be risky as tracks might get melted, making board not repairable.
To go sure PCH is shorted before injecting voltage first remove the vrm coil and see on which side the short is. If possible disconnect only one pad of the coil, and pull up the coil to disconnect.

[zklgfs]

Thanks for the tips! Lifting a leg of the coil showed that the short was on the side of the PCH. When I injected voltage, the PCH got warm, and using some alcohol, I could see that it got warm the fastest at one spot along an edge, so I guess that definitely shows the PCH is shorted. Too bad, but at least now I know that I don't need to search for any other issues.