It sounds like your fluxgate compass is broken or disconnected. That age autohelm uses a fluxgate compass that looks like a baseball, only it's black. It has some very fine wires connecting the internal compass to the cable to the course computer. Over time those little wires break. The wires are too fine to fix but the old compass is compatible with the newest raymarine compass. The difference being that the new compass cannot be rotated in it's mount.

Before pitching the old unit check the wiring to the compass - if the wiring is good than you could try replacing just the compass.

BTW, I disagree that those old autopilots are junk. The only issue is that no one will repair them anymore. I have quite a pile of parts for them and use an old AH 6000 as a spare. If you do decide to scrap it I would like a chance to buy the old pieces.