Dell E5470 LA-C631p - Short removed now turns on but no post/display

[retrorepair2]

Got a really frustrating issue here, got a Dell laptop as above which initially had a short on the +POWER_SRC rail, discovered a short cap in a bank of 3 near PU801. Removed and verified the short was gone, it was and the laptop powered on, tried to load windows but just kept rebooting. Sometimes didn't post. Battery was charging.

Stripped it back down and replaced the cap (assumed looking at the LA-632p schematic it's a 10U cap, no boardview so who really knows). Now the board doesn't charge the battery and while it powers on with a brief spin of the fans, there's no display on the internal lcd or external via hdmi and no post. Stays on for 60 secs then turns off.

I'm losing my mind with this one, all seems fine and can't find another short, the display test works fine, I get an error with no RAM (blinking orange/white led) and power button turns it off with a brief spin of the fan.

CPU gets warm, not hot.

I can't find any missing voltages. Help please

[retrorepair2]

Any ideas would be very much appreciated, at a complete loss here. Have removed the cap I replaced not that I really suspect that was an issue. No change

Maybe corrupt bios? Just don't get why it worked before I replaced that cap and now refuses to post at all. No idea where to go from here but really could do with this working.

[mon2]

Test the voltage of the RTC (real time clock) battery. Often it will be a CR2032 coin cell type. Be sure this is ~3v else must be replaced.

[retrorepair2]

Thank you for the reply. Would that cause this issue? It's 2.9v so a bit low but assumed these are rechargable? I've seen videos of people booting the motherboard without it (or even the fan).

[lapfix]

I get an error with no RAM (blinking orange/white led) : how much !
LA-C631p is similir to LA-C621P :
Brd : https://www.badcaps.net/forum/troubl...65#post1669965

i guess that you have try a trusted ram at different slot!

[retrorepair2]

They are similar but not the same. At least in layout. Yes tried in a different slot and RAM tests good

[retrorepair2]

Still no idea what's wrong with this thing. So frustrating that I've probably killedbit replacing that cap, being the main power rail it could be anything but then maybe it's just a corrupt bios? Not got a programmer unfortunately though

[reformatt]

If you had a short on the main power rail, you should always check the first two DC input FET's as usually one goes short. If your coin cell battery is missing, then the machine should go through a CMOS reset each time it's powered on. You should be able to assess what is happening by looking at the input current from your DC power supply. Providing you have all supplies up to CPU Vcore, It should cycle on/off a few times, and some Dell's can take a long time to get through that process.

Check current with and without RAM. You should see a discernible difference between the two. Try both slots and with a known good stick of RAM. Ignore the display at this point, you should be looking for activity on a USB port with something like a USB Chipmunk, or any other USB device that has a LED for data access. If you get a blinking, this indicates the CPU has finished it's POST and is looking for something to boot off.

[retrorepair2]

Thanks for the advice, turns out the bios had somehow corrupt, wrote a clean bios and it's working now, though battery suddenly not charging, so something different to investigate at least.

[mon2]

Test with a known good battery. Test the linked mosfets. Confirm the current sense loop for the battery charge feature is matching the schematic / ref designs for the charger IC.

[retrorepair2]

Turns out it's charging fine, the battery was fully charged! Been sat out of the machine for a month so shocked but hey, all seems well Will give it a few cycles to balance it but yeah, happy days