I have a Model B with 2 USB ports. From what I can tell this is the very first version. See picture below from when it was functioning long ago.
It was last used in 2014 with a then-current version of Raspbian, but I lost the memory card.
I recently tried to revive it so I bought a new memory card (SD adapter with 32 Gb micro-SD), downloaded and installed the current version of Raspberry Pi OS, and tried to boot up.
On the first boot, there was a message about generating SSH keys (I chose not to do that when creating the boot image).
Then a black screen with the Raspberry Pi logo and some file names passing by written below it. Looks like a normal boot. This goes on for 10-30 seconds. Then the monitor goes black, saying "no signal" as if the PI stopped sending video, and if I wait a couple of minutes, the PI reboots most of the time, repeating these things except the SSH part. Sometimes it just stays black.
I thought there might be some incompatibility between the newest OS and the oldest PI, but are there any other "obvious" suggestions than trying an older/lightweight OS?
I saw this thread which is somewhat related but not a duplicate:
Also saw this post suggesting to move mouse/press keyboard, which does not help in my case My raspberry pi 2 model B boots but then the screen turns black