LCguy's suggestion of trying to compare reception with that of known-working TVs is a very good one. You might happen to be testing your mac in an RF dead zone, and it would be nice to know that before tearing into your computer. If you happen to have a VCR or some other device that contains an RF modulator, you can use that as a signal source instead.

If it turns out that the problem is indeed with your Mac's tuner, note that a common failure mode is for the tuner connector to suffer damage of one sort or another. Take a close look at it to see if the center pin (socket) has corroded away or worked loose somehow. Just do a thorough visual inspection in that general area; you might get lucky and spot something obvious (and simple to fix).