telly vs television set

telly and television set both are nouns.

Word	Noun	Adjective	Verb	Adverb
telly	Yes	No	No	No
television set	Yes	No	No	No

As nouns, telly and television set are synonyms defined as:

telly and television set: an electronic device that receives television signals and displays them on a screen

Other synonyms of telly include boob tube, goggle box, idiot box, television, television receiver, tv, tv set.

telly (noun)	television set (noun)
an electronic device that receives television signals and displays them on a screen	an electronic device that receives television signals and displays them on a screen