Instructions for building one were in a QST Hints and Kinks in either 
1979, or maybe 1980.

I think this is the same circuit that HandiHams built.

To connect the circuit, all you have to do is determine which terminals 
on the back of the control box receive the voltage change caused by the 
changing value of a pot in the rotor which turns along with the 
antenna. For my HD73, I believe that is terminals #2 and #5, but don't 
quote me on that.

The circuit is basically a wheatstone bridge that is connected across 
the two control box terminals that receive the voltage change from the 
pot in the rotor to drive the meter in the control box. The wheatstone 
bridge in turn drives an audio osilator. When the voltage on the meter 
is equal to that of the reference circuit in the bridge, the tone is 
nulled out.

By having a scale, or at least a few tactile marks in key places around 
the knob of the pot in the reference circuit, you can set the pot so 
that you get a nul when the antenna is pointed in the direction you want.

Of course, the scale must be set up with the help of someone who can 
read the meter on the control box. But once this is done, you are in business.

I may have the schematic somewhere. If I can find it, I will put it 
where folks who want it can get it, and post that information here.

-- 
Mike Duke, K5XU