Help! Rock64 won't boot anything!

[trohn_javolta]

Hello, new Rock64 owner here but not completely new to the world of sbc's. I already own a raspi 1, rapi zero, orange pi pc, odroid hc2 and several "amlogic china android boxes".

I recently bought a Rock64, preowned and I cannot get it to boot anything. The initial owner ensured me for him it was working fine.
So far I already seeked help via discord channel, irc and via pine64 ticket system without any success.

I also found and tried several troubleshooting articles and threads..

The main reason I don't wanna quit yet is that I see 3 straight lights when I plug the Rock64 in   I never heard of a sbc that "suddenly" gets defective without any reason. I only heard of sbc's that were dead on arrival or got damaged by the user by applying wrong power supply etc.
Of course the initial owner could be lying..idk. But also the board has no visible damage..

I already wrote explanations of what I tried a couple times so please excuse me just pasting the initial ticket here:

Hi, I recently bought a preowned Rock64 2gb v2.0 board which I cannot get to boot anything.

Since I guess there isn't any warranty on it I'm solely relying on your goodwill here.

The preowner assured me that the board is working and he could boot up different images...

I found the NOOB section in your wiki and this is what I tried so far:

Run h2testw on the sd card I used

Tried different sd cards

Tried different images

also Tried the u-boot erase image from ayufans github

tried via usb (I thought maybe the board is set to boot from usb, there is no emmc module in place) with different usb sticks and images (also erase u-boot image)

Tried different power supplies (one rated 5.1V@2.5A others rated at 5V@2.5A)

All I get upon plugging the barrel jack in is straight light from the 3 leds (green, red and white) and that's it. No lights on ethernet port, nothing shows up in my network and no hdmi output.

Only thing I could notice with sd card (erase u-boot image) inserted is when I plug in the barrel jack I can see the white led flashing just real short at the beginning (not even one sec) then again straight lights.

Can you help me troubleshoot that?

Also what are the 3 buttons (recovery, reset, power) doing in detail? I couldn't find any documentation on that anywhere.

Lastly I now wanted to get output via serial console.Set everything up correctly but all I get (no media plugged in, only sd card, only usb stick plugged in) is this:

...Is this standard output when nothing boots or does it mean sth. else, maybe defective board?

https://imgur.com/a/TkhCaq8

Pine64 support did not react to my questions concerning serial output and what the buttons do, after some basic troubleshooting they concluded that the board is defective but I cannot apply for RMA because I'm not the original buyer..which is totally true and fine.
Still, I cannot believe that the board is defective...it doesn't get hot anywhere upon powering it, there are leds that give a straight light and there is no visual damage.....I must be sth. wrong or sth. is set up that hinders the board from booting.

Can anyone help me here and suggest things to try?
Especially it would be nice to have the questions answered regarding serial console output and the 3 buttons....
It's basically my last hope. Especially if I could get some serial output...I think there should be some even if the board doesn't boot? or am I wrong?