Had an Isocom in my Super Viking... was never impressed with it
although it functioned..
The on-off switch is just that... When the intercom is turned off, the
radios will still feed through to your headset... The idea is to adjust
the Isocom so the audio level is the same, on or off... There are two
audio adjustment pots...
Look at the manual, he
http://www.davidclark.com/PDFfiles/ISOCOM.pdf

denny