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.