It doesn't need to point its dish at Earth, it can simply aim for the Sun. It is so far away they are as good as in the same position relative to itself. And the Sun conveniently lights up making aiming a lot easier.
Looking up the math on that, the high gain antenna is within 3dB of max at up to .3 degrees off-target. It's 132 AU out, so Earth is within .41 degrees of the sun. Good enough!
Did they have digital light detectors in the 70s? I mean, it’s trivial now to aim something at the sun now, but they didn’t have image recognition then. A primitive heat sensor perhaps?
