I am seeking the information about the song that was played at the end
of telecast when television was not 24/7.

Hmmm. In the United States, most stations used to play "The Star
Spangled Banner" at sign off.

For many years, television stations in my area (northeastern Oklahoma)
used to close their broadcast day with a video called "High Flight,"
in which a poem was read aloud, accompanied by images of a jet soaring
high in the sky.

There is a wealth of information about the poem "High Flight" here:

http://www.highflightproductions.com

Traditionally, as the comment above says, "The Star-Spangled Banner"
was played at the end of the brodcast day on many U.S. TV stations
before they went off the air for the night.

While this was by no means a universal practice and was not required
by law, it was enough of a tradition on many stations that it may be
the song you're thinking of if you're referring to U.S. television;
the image of a TV going to "The Star-Spangled Banner" became part of
U.S. pop culture in the mid-to-late 20th century, enough so that there
were images of this song on TV in other media, sometimes accompanied
by someone snoring in front of TV, as a shorthand for someone staying
up very late, falling asleep in front of the TV, etc.