8570w black screen, turning on, but no bios splash screen, no boot

So, I bought an 8570w with a supposedly defective graphics card via ebay a couple of days ago, which I wanted to fix with a an AMD m4000, which I also bought for cheap on ebay previously.
Upon receiving the laptop, I plugged in the battery, plugged in the charger, turned it on, and hey it worked no problem at all! Wasn't defective after all, or so it seemed.
It had a k1000M in it, which worked fine. I installed Windows 10 on it, ran a couple of tests, everything hunky dory. I then replaced the Nvidia with the m4000 as an upgrade, replaced the thermal grease and so on, the usual stuff, temps were looking great, did some tests and some casual gaming for a few days.
Now, today I put the system into standby, which I've never done previously, iirc.
When I went back to wake it up, I pressed the power button, but the screen remained blank, no backlight, no bios splash, fan is spinning, but no boot or nothing. A plugged in USB keyboard gets power (can see it via LED). I figured that maybe the m4000 had died, so I put back the k1000M, but it's the same beaviour.
Tried everything so far. Taking out the battery, powering up on AC alone, unplugging the BIOS battery, doing a BIOS reset, BIOS reset via pressing Win+B-Key as suggested by HP, hooking up to an external monitor via VGA, leaving out various RAM modules, the whole lot, but the thing won't POST anymore.
Sometimes the Caps-lock key stays lit, sometimes it is off, the fuction key LEDs are always on.
Anyone have an idea what's going on here? Could this behaviour be down to an empty BIOS battery?
Or do I have a serious problem here? Looks like it. Any tips welcome.

Edit: Wifi and mute button inciator are red, instead of white as on the browser hotkey and the
calculator. Don't know whether that means anything, though...

 

whats the hdd light doing?

fn+f4 to turn on the display maybe? different power supply? check all the ribbon cables in case you disconnected one.

 

Hdd light is white. Couldn't tell from the SSD in the main bay what was going on, so I put in an HDD.
It is powered and spins up. The same when I put it inside a SATA-adapter and insert it into the DVD-bay.
It spins up and the HDD light is on.
Already tried Fn+F4, it doesn't do anything.
The ribbon cables don't make any difference, whether connected or disconnected.

Also doesn't make a difference whether I power up from the battery (0% wear, btw)
or the AC adapter.

 

Decided to run the laptop off the battery for a few minutes, then plugged in the AC adaper.
Battery indicator went white, which according to the service manual means:
"White: The computer is connected to external power and the battery is charged from 90 to 99
percent." So, there is still life in this machine, but why is it not coming on?
Caps-lock key and num-lock briefly blink once upon powering on.
Dead CPU? Really? How unlikely would that be?

 

Is there any led blinks of the kays shift and num lock?

 

When I power up, one brief blink on the caps-lock and num-lock simultaneously which does not repeat, the two keys stay dark after that. The WiFi hotkey and mute key alre lit amber-orange, the browser and calculator keys are white.
The front WiFi led is also amber-orange.

 

The CPU is dead:

 

Nope, that's not the issue. I was very lucky to get my hands on another 8570w for cheap without a bottom cover, no battery and no power supply, and with a cheap i5 in it (it has 4 ram slots, though).
Swapped all RAM, i7 CPU, m4000, battery and power supply into this machine, and everything works just fine.
The i5 and K1000m in the troubled machine show the problem described above. Also swapped the top cover around to check whether that makes a difference.
So, there seems to be a short on the mainboard that prevents the machine from fully powering on. The system simply halts, so to speak. I suppose I will have to take out the entire board, hook up everything to start it up, and have a look whether there is any component that gets unusually hot when the laptop is powered on, and check the mosfets for (missing) voltage.

 

Oh, now I remembered I had similar problem with one 8760W and the culprit was a faulty USB 3.0 board.
I replaced it and everything was working perfect again. Try to pull the USB 3.0 board out and start the system without it. Do the same with the audio/USB 2.0 board on the other side.

 

Thanks for your input, will definitely try that when I get round to it.

 

Unfortunately thats not the problem either. Took out both USB boards, the VGA port, and the modem module.
The CPU doesn't really get warm when powered on. Maybe the Super I/0 KBC1126-NU has gone bad? Buth then, wouldn't that prevent battery charging?