Dears,

I am facing a critical problem on my RB800 where i am not able to access it via winbox nor via serial.
I think the firmware is deleted and i need to install a new one.
But the problem that i am not able to access the board using serial connection.

Note that the board reboots every 2 minutes.

Any advise ?

It didn't give any output on serial? It should be, even it its damaged. Have you tried another serial cable?

I already used the same cable with another routerboard and it worked fine.

After firmware upgrade, i got the below

"file transfer failed: receive timeout
invalid upgrade file id"

Any advice ?

netinstall help?
if not then email to mikrotik support.

RB800 is a problematic production i have one of early verions.
It did not show its ram speed shows me unkown in routerboard settings, it was freezing time to time and i had to downclock without success it is still runing in 800Mhz ..

what RouterOS version you are using?

Janisk if you ask me i use latest version all the time and upgrading boot loader always.