QX7 wont turn on & draining its RTC battery

[slimlim]

Hello all, wondering if anyone can help...

My QX7 all of a sudden wouldn't power up, there's nothing happening on the screen & only a clicking sound from the speaker when pressing & releasing the power button.
Nothing shows up in windows hardware manager when plugging in USB, pressing the power button while connected does nothing.

Main battery voltage is fine & I've tried the commonly suggested fixes, removing the SD card & trying a different one with fresh files, removing the RTC coin cell battery & also leaving it un-powered (overnight). The text files on the original SD card did seem to be corrupted (some random jargon in places) when compared to the fresh files.
I've checked the switch board & it seems good. Removed the main board, checked wiring, solder joints (re flowed some pins) & PCB but cant see any obvious visual damage.

The CR1220 RTC battery was at 0.7v, I picked up a new one today but this didn't help.
The fresh cell started off at 3.3v, but when pulling it after a couple of minutes in the radio it was at 2.5v. Checking the voltage of the coin cell holders contacts while the battery was installed resulted in 0.1v.
The original battery had also bounced back after being left out of the radio overnight & was at 2.8v. I consistently get the same results with a battery in the holder, 0.1v & lower voltage when being removed.

There's too much current demand coming from somewhere, seems like it's pretty much shorted out. what could cause this? Dead CPU?

Just before this all happened I was setting up a quad in Betaflight & had left the radio switched on (5 minutes or so). It's got 2x 18650's installed & think it had been buzzing away at the low voltage warning for a minute or two. When I realised the screen was flashing, buzzer was sounding, saw the voltage somewhere in the 6v range (think 6.5v) so switched it off.
It was after charging the battery the radio wouldn't turn on, I've tried different batteries, fully charged 3S & 2s lipos etc.

Any advice would be much appreciated!

[slimlim]

Not to worry, it seems the answer is the MCU is dead. The fix is either a new main board or swapping out the MCU so I might attempt the latter option. Considering the price of a replacement board it would probably make more sense to jump ship like everyone else & try out a jumper or radiomaster .

[jhsa]

Yep, CPU dead, I have one of those radios from a friend here at home with the same problem. Will attempt to replace the CPU as well, when I have some spare time

João