Although I'm not sure, in MAN mode, I can only get ALT HOLD to work. I think it's supposed to be like that in MAN mode. I sometimes have to turn the AP on and off a few times while trying to get TACAN or HDG HOLD to work.

This seems to do the trick for me. Set the Mode Selector knob to TACAN, then set heading bug to your desired heading, then turn the Heading Hold switch on. AFTER doing all that, turn the AP servo switch ON.

And don't forget to turn the Flight Director switch ON (it's behind the Pilots yoke). Failing to turn it on will definitely prevent the AP from working. I have made that mistake a few times. Usually after being away from the B-52 for a while. It has me scratching my head trying to think of what I might have done wrong to prevent the AP from working. ::)

Mark