Asus N750JK won't start

My 7 years old Asus N750JK started having problems with booting up. Usually, when I press the power button, first the keyboard lights up, there's a click sound and then I get the Asus logo on screen and at last it gets to the GRUB (since I have a dual boot Win 10 + Ubuntu 18.10). But now it mostly won't turn on. When I press the power button, the power LED lights up, the HDD LED starts flashing, fans start spinning but display and keyboard doesn't light up. After a few seconds, it starts clicking and HDD LED starts flashing. The click and the LED flash happens each second, at the same time. Sometimes it starts up after 20-30 seconds of clicking, sometimes it doesn't start at all even after 5 minutes, sometimes I have to turn it off and again on for a few times till it succeeds to boot and sometimes it won't start until I don't leave it settle for a few hours before trying again.

I opened it few weeks ago and found out that the "clicking" comes out of the DVD drive so I took it out. After that, the laptop has been working fine for some time but I haven't used it much. But the issue is now back, so I just removed the clicking but I didn't fixed the issue. Now just the HDD LED blinks each second.

The issue started happening a few months ago and at first it was only occasionally but now it's practically impossible to turn it on. It succeeds once in 100 tries. I have a Intel 180 GB SSD + HGST 1 TB HDD inside but both disks seem fine and show good health without issues. Once it manages to start, the laptop works fine except it sometimes doesn't wake up from sleep - again black screen and fans running at max and then, when I force-turn it off, I can't start it again.

A few things that might be related:
- the battery is practically dead, it holds for barely 5 minutes but until few months ago, except the short duration, it was working normally - the battery LED is white while the battery is full and orange while it's charging but now it acts weirdly - when it's empty, it does light up orange like it's charging but it never reaches 100% and at some point, the battery LED starts flashing orange and it never gets to the white light, it just flashes orange all the time which might indicate some issues with battery or charging circuit? I have two original charges and it's the same on both so it's not the charger.
- I heard that it can be the CMOS battery. Related to that, I'm having some weird issues with time on Windows 10 - when I turn it on, it's always 2 hours back and I have to turn off and on automatic time to get it right. But there are no such issues on Ubuntu.
- A few months ago, I got that big Windows 10 update so I'm thinking if it could be related to Windows? I noticed that everytime I'm unable to start it, after I finally succeed, it starts booting Windows directly, skipping the GRUB screen. Also, when I attempt to shut down Windows, I always end up with black screen and fans running at max speed.

I'm planning to try to take out the battery, the HDDs and to take out and back in the RAM to test but it's hard to test anything since the d*mn thing start acting normally when I'm testing it was the same with the DVD drive. After I took it out, I tried to reboot the machine many times and it worked every time. But when I closed everything and started using it daily, the issue returned. That's why I need your help. Or at least, maybe somebody knows to decipher the HDD LED or the battery LED blinking so that I know where to start.

I know that I should be looking for a new laptop after 7 years but I don't know what to buy yet so it would be good to get this one running until I don't decide.

Thank you in advance!

 

So it seems that the motherboard is dead. I tried everything today but I can't get it to boot. I've disassembled it till the last screw, I cleaned everything, I replaced the CMOS battery with a new one, I left it one hour without any battery, I replaced the thermal paste, I cleaned everything, all the connections and flat cables, I've carefully assembled everything back but it won't work. Then I started removing one by one device: battery, DVD, SSD, HDD, mSSD, WiFi, webcam, keyboard and touchpad and RAM at last but nothing, it just won't work. I managed to boot it once, when I removed all three HDDs and it showed that there's no drive but after that, I couldn't get it boot anymore.

I tried to experiment with both RAM modules in all combinations but nothing. Then I started experiencing something even more weird: when I put a 4 GB RAM module into both slots, the power LED turns on and off after 2 seconds. When I put a 8 GB RAM module into lower slot, it does the same. When I put 8 GB RAM into higher slot, it doesn't get off after 2 seconds but it doesn't want to boot either.

Then I found this: https://forums.tomsguide.com/thread...-screen-unable-to-access-bios-no-boot.370269/

But it didn't help in my case. I tried to short the jumpers while it was on; the first jumper will turn it off but it will turn again on immediately. The second jumper didn't do anything.

PS: I forgot to say that caps lock light is on which, I've read somewhere, means that it's stuck in POST.