"It is an old television."
Like そら said you don't need the な. The reason is the 古い is an i-adjective and they can directly modify the word they precede (I am not sure but I think only nouns can be modified by i-adjective.).
To use な you would need to use a word which is a na-adjective for example:
古風 [こふう]: (P, adj-na, n) old-fashioned, archaic, antique, antiquated
Would have to be the sentence. I would ask そら to confirm my theory/guess.
- My Japanese Is Poor Sorry if it is wrong.