John, et al:
If you install the AU-22 in a glider with a metal instrument panel and have a problem with radio transmissions interfering with the vario, Dave Ellis had a mod:
Cut the -12V trace in the audio box that goes to the chassis.
Believe "That instrument was before my time at Cambridge" was in his explanation.
Also believe mine was an AU-20, the 22 may be sans built in ground loop.
Jim