Don't be so sure.
I clearly recall the sainted Walter Cronkite blowing smoke up my ass about Tet in 1968 -- though I didn't realize it at the time.
My view is a.) they've always been partisan, b.) which didn't become apparent until the Viet Nam War and now c.) they've thrown all pretense of being "independent" and "dispassionate" to the wind, becoming outright cheerleaders for the liberal cause.
Sadly, it wasn't until talk radio or the internet that we began to have our suspicions confirmed...
What I was thinking about even more than network TV news was the old newspaper, which separated news (dispassionate, interest in conveying objective facts) from opinion, which was placed on segregated op-ed pages. Of course, many of the MSM newspapers in those days were acually owned by entrepreneurial individuals and families, and quite a few major ones were unabashedly conservative and Republican on their editorial pages. With the contraction of the newspaper industry and the shifts in ownership of the newspapers that now survive, there is very little diversity in either news coverage or opinion in the major metro dailies, and it is rare to see a story that is “dispassionate,” “independent,” and objective.