Display Issues post-cleaning

[Lenes]

Hi all, new to the forums after I ran out of ideas on what to do and need some advice. Apologies if this is posted in the wrong section.

Today I decided to clean-up a 12 years old laptop that has never been opened. The laptop in question is a Asus ROG 750JH.

CPU Package temperatures used to be in the 60s while idle, and even shot up to 94C if opening Chrome on 10+ tabs. Could hear the fan spinning loud so figured it might be time for a cleaning.

I first disassembled the laptop down to the fans which had a blanket of dust inside covering the fins. I cleaned these thoroughly and whiel I was at it I decided to also change the thermal paste on the CPU and GPU. I then stripped down all cables/connectors and took apart the motherboard. I managed to replace the paste on the CPU but the GPU heatsink wasn't budging so I decided to leave it be as I was afraid not to break something. I then assembled everything back together and when I plugged it in it would only turn on for half a second, could see the LEDs turn on, fan spin and then it would go off. So I thought it might be a connector issue. I took the laptop apart again up until the keyboard/motherboard. Disconnected all RAM boards and connectors and connected everything back. I then put the keyboard back, connected the plug and turned it on. The laptop turned on fine without issues, Windows loaded, ran some test in HWiNFO and was happy to see temperaturs drop in the 40s while idle and climb to 64C while browsing in multiple tabs. So I decided to turn it off in order to assemble it. When turning it off it asked if to update windows and I let it run the updates. I walked away and when I returned a couple of minutes later, I could hear the fan spinning lightly and see keyboard backlight on but nothing on the screen. Left it like that for a while and seeing how nothing was happening, I force turn it off from the button. Turned it back on and the screen then wouldn't come on. I turned it off again and took it in my office to connect the external screens and to my surprise the external screens would work fine but the laptop screen wouldn't come on.

I have no idea what happened. It originally worked after the cleanup and then after the shutdown/windows update it stopped working. I haven't moved it between the working-not working phase so no clue what happened. I tried reconnect all the wires running from the screen to the motherboard and nothing. Within windows on the display settings it detects both external screens but not the laptop one.

What can I do to troubleshoot it? At this point I am just happy it works as it's the first time I take a laptop apart like this and I will settle with using just the external screens, but if anyone has any suggestions I would appreciate it before I assemble it back.

Thank you in advance

 

[Lenes]

Would the CPU repasting affect the laptop display though? I have used MX-6 to repaste, and it was not using pads previously.

Since yesterday, after a longer shutdown the laptop screen turned on like normal. Shut it down overnight as well and laptop screen turned on like normal again this morning. While the keyboard was not clipped in I touched around the screen cable and connector that goes on the motherboard and could not see any flickering to indicate a bad connection. So I decided to remove all the external screen cables and USB connection and go assemble it back thinking the issue was solved. I clipped in the keyboard and back cover and decided to test it without all other screens connected and the screen wouldn't go on again. I connected the external screens and they work fine again. Only thing I noticed is that when the laptop screen doesn't come on, the two external screens swap around their order. Laptop screen was always duplicating screen 1 as I was keeping the lid closed, but now when it doesn't work, External Screen 1 becomes Screen 2 and Screen 2 becomes Screen 1. I am thinking the keyboard/back cover is pressing on the cable, but then shouldn't it have responded when I touched up the cable earlier?

 

[Lenes]

As an update, I think I might have solved it. This is the chain of events:
Disconnected all KVM Switch cables form laptop in order to assembly. -> Laptop screen stopped coming on.
Connected back all KVM Switch cables. -> Laptop and External screens would all come on.
So I figured it must have to do with the screen/display topology. I had Laptop Screen and External Screen 3 set to duplicative [1/3] and Screen 2 as extended. Whenever I disconnected the external screens, the Laptop Screen wouldn't come on and Screen 2 would jump to be main screen, which why Windows wouldn't turn Laptop Screen on.
I set all three screens to extend and set Laptop Screen as main and tested again without all the KVM Switch cables/screens and Laptop Screen came on upon reboot.

Very strange for this to happen, I would have thought once all the extra screens are disconnected, Windows would figure out that Laptop Screen needs to boot up.
Anyway, thank you for now. My brain is on high alert with all that happened since yesterday. It was my first disassembly and repaste and I just had a bad feeling that I might have messed something up. Anything to monitor over the next couple of days/weeks to put my mind at ease that the repaste was a success?