E5540 shorting adapter - unusual

[spleenharvester]

Hi all, really odd one here.

I have this mainboard from an E5540, unknown history but cosmetically in excellent condition. No signs of liquid damage, but slightly melted plastic at one of the HDD posts. Slightly peeled PCB surface at the battery connector, though it didn't appear to be burned.

It is shorted at +SDC_IN and other connected rails. I thought this would just be a usual case of injecting voltage into the shorted rail and seeing what heats up. Unfortunately, I cannot for the life of me find anything heating up when I do this. The power delivery area of the board does get slightly warm when +19V is injected into this shorted rail at PQ404 legs 5-8, but I cannot identify the source of the heat. PQ401 and PQ402 are not shorted at legs 2 or 3.

I'm going to lift off PL400 to figure out what side the short is on later, but I suspect it is somewhere on +SDC_IN as there are more components on this line. Anyone got any other tips? I'm really perplexed by this. All I can think is that it's a shorted ceramic cap I haven't yet identified (this is one of those nightmare boards where very little is screenprinted so I don't 100% know for sure where some of these caps are).

Attached pics, schematics are posted in this thread: https://www.badcaps.net/forum/showthread.php?t=79018

Thanks in advance

(Edit, forgot to add - my injection PSU shuts off totally if it's just applied to the DC connectors directly. I find this odd because it's fine when I directly target the rail)

[piernov]

Re: E5540 shorting adapter - unusual

Always measure exact resistance to ground when a short to ground is suspected. And monitor voltage/current on the PSU to know the dissipated power.

[spleenharvester]

Re: E5540 shorting adapter - unusual

Always measure exact resistance to ground when a short to ground is suspected. And monitor voltage/current on the PSU to know the dissipated power.

Measures 0 ohms to ground when lead resistance is subtracted. I can't remember what the PSU pulled, but it was a substantial amount of current. Unfortunately said PSU just died a death while trying to troubleshoot this board so I can't double check atm.

[khaahk]

Re: E5540 shorting adapter - unusual

From my service history i have one 5540 with marking of shorted capacitor near PQ404. So just in case the history repeats itself check these two guys.

[spleenharvester]

Re: E5540 shorting adapter - unusual

From my service history i have one 5540 with marking of shorted capacitor near PQ404. So just in case the history repeats itself check these two guys.

Interesting you mention that, I'd actually suspected those caps due to proximity to the melted stuff. But they just don't seem to be heating up when voltage is injected.

[spleenharvester]

Re: E5540 shorting adapter - unusual

Okay update, I've removed the two caps closest to the melted area. No dice. More importantly, I've removed PL400 and the short is definitely between there and PQ404. Still nothing heating up when I inject 9V! I'm not 100% sure I have identified all of the other caps on that area correctly though as none of the ceramic caps are screenprinted at all. Any ideas?

[spleenharvester]

Re: E5540 shorting adapter - unusual

FIXED! It was one of the caps on the other side of the board next to hole H37. I noticed a small crack on the side. Not sure why it wasn't heating up when voltage was injected - perhaps an electrical reason someone more knowledgeable than me can fill in on

Only issue is that I have no idea what cap I need to replace it with as nothing is screenprinted. Any ideas? Pic attached. It's either PC401, PC402, PC414, PC412, PC415, or PC416 on the screenshot posted in the OP.

EDIT: Sorry, I forgot to add an arrow to the pic. It is the space closest to the connector (next to those other two caps by "H37").

[spleenharvester]

Re: E5540 shorting adapter - unusual

Bump, anyone know what I can replace it with? It looks like it's just a decoupling cap.

[caspian]

Re: E5540 shorting adapter - unusual

To find the name of the shorted capacitor, you should discover the name of the two neighbor capacitors. Then, the shorted capacitor will be probably the one beside the neighbor capacitors on the schematics too.

[KvnTM]

Re: E5540 shorting adapter - unusual

I'd bet it isn't necessary to have on the board because it is next to a small inductor going to gnd. Probably just for EMI reasons.
Those grey'ish white components are usually inductors but correct me if I'm wrong though.

[spleenharvester]

Re: E5540 shorting adapter - unusual

Thanks guys - I'm going to replace it with an X7R 25V 0.1uF ceramic - my reasoning is that this cap seems to be an 0603 package and the only 0603s on this part of the schematic are 0.1uF. Cheers

[spleenharvester]

Re: E5540 shorting adapter - unusual

So I replaced the cap. Left the system idle on the BIOS screen for an hour while charging - came back and it was dead again. Exactly the same issue, but this time two other decoupling capacitors on that line were shorted. I *think* they were the 22uF caps, and I've had to replace them with several 0.1uFs as I don't have any 22s kicking about. It's alive again now but I'm guessing the same problem is probably just going to happen again?

[spleenharvester]

Re: E5540 shorting adapter - unusual

Just as a follow up, the system still works fine several months later and no further shorts have occurred.