If you look at the CB for STBY IGN (1) it is also labelled underneath MHRS (magnetic heading reference system) as the 2 are powered through the same cb at a guess when STBY IGN is selected there will be insufficient power for the MHRS.. .I don't think the CB is exclusive to JT9 powered a/c it's on the CF6 ones I deal with and I'm pretty certain it is on the RR, maybe it's just the way the JT9 (old a/c!) are wired.
Hope that helps <img src="wink.gif" border="0">