dell e7440 LA-9981P board no boot

[Auld_Tinker]

Trying to learn laptop repairs, looking to be guided/mentored on my journey.
Info - I have 3 boards, 1 which i believe was a working unit and 2 bought faulty to try and repair- Only after i purchesed these did i realise the processors were soldered in place- Duh.
The working unit was used with External monitor via display port output to hdmi monitor with adapter (so i have no laptop display). These were put away a few years back and now i have time to try repairs on them.
Ok So when i power up using dell psu without the memory installed i get the bios led flash code= 1 solid and 2 flashing meaning no ram installed and thus boards are working to a certain level.
When i instal a memory module, this changes and i only get the power light staying on.
Cmos battery= 3v.
I followed the schematic and get the 5v and 3.3v always on voltages. With and without Sata hard drive fitted above.
When i test for 5v run and 3.3v run = they appear for 1/2 second with andcwithout hard drive in place.
So this is where i am at. bare board level only - no keyboard etc connected.
Am currently looking at getting a display or old working unit from ebay etc. to use for visual status.
Should i get the 5v and 3.3v run voltages staying on with above configuration ?
All help welcomed- thanks in advance to all who will help me on my journey.
Not sure whats on the sata drive- have a cradle comming to link to my pc to check this over.
What should my next move be.

[FriedFred]

Hi! Check all coils against GND (Minus) to find out, whether there is a short circuit from a DC-DC-Converter to GND.All coils shall have a resistance between let's say 5 Ohm to 500 Ohm. Something in this range.
The coils for the CPU and GPU are special, usually they show a very little resistance. Only with a very good multimeter, you can get a reasonable measurement.
Measure those CPU-/GPU-Coils against the main power rail. This is usually the current measuring resistor in the near of the power supply input. If you have a very high ohmic value, this is fine.

[Auld_Tinker]

FriedFred- checked around all inductors= no shorts. Checking voltages on 1st stage of PM Table = Good, 2nd stage of the PM Table ie 3.3v_sus and 1.35_mem= ok on power standby BUT when power button pressed both of these voltages disappear. No shorts to ground on these.

[Auld_Tinker]

So got a sata cradle and hooked the hard drive in it and checked files are reading ok on a working machine. So hard drive us good. So my question is why i would be losing both 5v run and 3.3v run voltages at the same time if they are driven independantly.

[Auld_Tinker]

ok 1 of the boards got fried, was chasing the missing 3.3v and 5v run voltages shutting down. Was looking around pq200 and pq201 for the 1.35v supply when one of these blew up in spectacular fashion full on flame. So reckon pu11 or one of these mosfets was responsible- by being shorted- might come back to this one later. onto 2nd board now.

[FriedFred]

Do you have a power supply with current adjustment? Your sight seems to me like a dead short, so a lot of current flows. Even if the battery is connected. Can you disconnect the battery and use instead a power supply with current limit?

[Auld_Tinker]

FriedFred, yes i have one, will rig up a connection point and give it a try.
Found this on youtube- good explanation of power voltage rails sequence.
Also got a working e7450 from ebay- so can remove its screen and try on the faulty unit.

https://youtu.be/56FX6UCsab8?si=PRcd-Tnpp_HDpFLs