I just realized that the AR2 has a display. So, I must have the AR1. I added a sawtooth voltage in place of the tuning pot, and fed the audio to the X on a scope and the sawtooth to the Y. It made a nice, crude spectrum analyzer.
Thank you, Ken. The AR2 display is highly convenient to have and be able to tune accurately instead of a guessy dial. I will look at the AR1 you mention to see if could be used by adding a frequency display without much complications.
Another reason for the AR2 unit is that would be good to find/build its AR2L option am after.