HTC Desire 820 orange LED constantly blinking - phone not turning on.

Hi everyone,

I'm struggling to get my old HTC Desire 820 (dual-SIM model) up and running again.

I've already replaced the battery (which had died out completely), the USB charging board (the complete board), AND the complete LCD display assembly...!!!

Now literally speaking, only the original mainboard remains...!

I tried to hard-reboot the phone using Vol-up + Power, and Vol-down + Power, but no luck :-( ..... the LED keeps blinking constantly (it doesn't blink and the handset shows no activity when I remove the battery and keep the charger plugged-in).

Any ideas or clues as to what may be wrong here...?

Thanks in advance!

PS: For more details and clarity, I’ve posted a video of this issue on YouTube at https://youtu.be/w_333ZaAikQ