i bought a cheap old plasma tv i want to use as monitor for watching movies/series so i can actually see whats happening on the screen instead of my 21 inch moniter im using now. i came home hyped with a 50 inch tv ready to watch some good movies. only, when i plug in the hdmi cable theres a blackscreen on the tv, how can i idintefy of this tv can be used as a pc moniter?