Hi,
I also just got one HP 4540s with similar problem. After plugging in the power adapter power led blinks few times and the fan spins for a second and after that full shutdown fan stops and all lights go dark. I don't have the battery to test it.
Pressing the power button turns the fan on and few blinks from power led and after a second full shut down by itself.
After disassembly mainboard behaves the same with no cpu, ram, wlan or anything connected to it except power adapter, tried with more adapters, same story, they all work on other laptops.
Power button works fine,...

Re: Problems with Samsung 740N monitors

Just repaired 2 samsung 740N LCDs.
On the first one I just replaced the 3 capacitors on 13V rail and that was it, worked after that.
Second one was a bit tough. It was completely dead, LED also dead. Fuse, thermistor and varistors were OK. ZD102 was found dead, replaced it but it didn't help. After I powered the LCD still nothing, replaced zener didn't burn. Comparing ohm readings form first LCDs psu I saw lower ohm readings across several components in primary side of the second LCDs psu (Zener diodes were not reading as (600,infinity)...