made a video of the whole problem so maybe it will explain it to you best

However, I will write the problem here too.
Well, as the title says when I plug the power cable the power button on the display just goes on and off every 2 seconds without showing anything on the display. While it's that way I disconnected the screen cable (picture 1) and then after few seconds the screen goes white, back off again for a second and back white and then stays white (I suppose this means the back light is working fine?). After that I connect the screen cable back again and then the screen shows some lines until I press the screen cable down with my hand on different spots until I find the one which makes the screen show the "check signal cable" window going around, I put some tape to press it down and it stays that way. However, when I connect my laptop to the monitor the screen on monitor goes back to its first state, blank and only the power button flashing blue light every 2 seconds, or sometimes it doesn't go back to the first state but the LG logo shows up on monitor screen then after it the "power saving mode" window and the monitor screen goes off with the power button light orange and stays that way until I press the power button, then it goes off completely and then when I press it again it shows again the lg logo, power saving window and goes orange (as you can see on the video). Now back to the first state with the laptop connected to the monitor. While in that state, blinking power button on monitor every 2 seconds, I again disconnect the screen cable, screen goes white and then again connect it back, then the screen shows some lines again and I tried pressing the cable but all it does is being completely black or the lines show up when released, so no more "check signal cable" what makes sense because it's connected.

So I have no idea what could it be, the vga board of monitor, the screen cable, the long thin board of the screen where the screen cable is connected (excuse me for not knowing the name) or something else?

I forgot to mention that I did take the monitor apart because at first I thought it might be some bad capacitors on the power board causing the leds not go on but they all looked fine and the leds were working fine with the screen cable was disconnected (wish I tried that first heh, took the whole monitor apart for nothing).
Also, maybe the sound might say something which you can hear on the video when I go closer to the boad.

Thanks

 

First thing to do is replace the electrolytic caps in the power supply.
Just looking at them may not be enough. Did you measure the capacitance and ESR?
They may be just borderline.
Then look at the soldering around the connectors for dry joints. Often they crack with movement of cables.

 

Ok, I will try to replace them. Only tool i have is a cheap multimeter (and a soldering iron) so i don't think that i can measure them.
https://www.hardwareinsights.com/re...s-sn/#Troubleshooting-identifying-the-problem

This seems to be the exactly same problem as mine and he also replaced the caps as i understood.

Thanks

 

Get yourself one of these component testers on Ebay or similar...

They are quite cheap and work pretty well.
The caps are always the first to suspect.

 

Thanks will probably take one, what's the name for those?
Also, I have some old PSUs so was thinking to take the caps which are same as the ones in monitor but coz I can't test them right now I will have to rely on my eye for the new ones.
Anything specific I should be looking at?

 

https://www.ebay.com.au/itm/Mega328...h=item41db11ef1b:g:tEQAAOSwg0dcPfTL:rk:3:pf:0

Check the capacitance and ESR
https://www.avnet.com/wps/portal/ab...understanding-esr-in-electrolytic-capacitors/

If the capacitance is low, replace them. Low ESR is important in switch mode supplies.

 

Unfortunately I couldn't find the replacement caps, only 2 with the same stats but the board has 8 all together.
Would taking out one by one and then try to power one help?
What i mean is, i take one out,try to power the monitor one,if same i put it back and go for the next one.
I don't have replacement caps now nor a cap tester
Oh and on the other side of the board one thing i saw differing from others, it seems damaged but I don't know what it is and does it mean anything, you can see it on the pictures.

ZD904 looks damaged, all other seem fine.

 

ZD904 visually looks ok to me. Why do you think it is damaged? Have you measured it?
It is pretty hard to fix things like this without the tools needed. I would recommend you get the tester, or something similar.
And new caps are the best way to go if you find a crook one. And in some places, the caps are in pairs, like 2 in series in a voltage doubler power supply, so replacing both together with the same caps is good.

 

Update,
The VGA on my laptop is faulty heh.
The monitor didn't show any picture after I reconnected the cable coz the laptops vga is faulty, guess with a working vga there would be picture on the monitor.
However, the monitor is still faulty.
This way I would always need to disconnect the screen cable and then connect again.
As you said probably the caps are bad but I will have to wait to get the replacement caps and the cap tester.
Thanks again for the time and support