I'm still trying to get my head wrapped around this. Here's what I think I know. The 3308 has no internal "heading" mechanism, it must be driven from something external. Very common is a Remote Gyro like a Mid Continent Remote Directional Gyro. In many other HSI applications it can be either slaved or not slaved. If it is, it keeps up with precession issues and keeps the Gyro aligned with reference to magnetic north. It does this with a Fluxgate which is nothing more than a magnetic compass that has electrical outputs that can be processed and used to do stuff like keeping the Directional Gyro set to magnetic heading. If its not slaved then you set it yourself. You can push a button and select either slaved or not (presuming of course that it has the slaving feature to begin with.) But the Sandel 3308 has no way to do that. If the Fluxgate fails, you lose heading information. Even though the Gyro may still be working just fine, the 3308 starts having an electrical nervous breakdown and goes into "fast slaving," trying to slave to something that isn't there or is erratic and unreliable.
Reading more, it seems like there is a way to take the Gyro out of the loop and run the heading indicator(compass card) in the 3308 directly from the Fluxgate. But even though the Gyro is still out of the loop the 3308 still "dampens" the input from the Fluxgate limiting it to 3 degrees of change per minute. Doing that when the Fluxgate is keeping the Gyro magnetic makes sense, we only did it once about every 20 minutes or so the old fashioned way. Why not just let it straight through to the heading indicator if the gyro is out of the loop?
Hold the phone. Actually answer the phone.
Charlie from Sandel just returned my call. Letting the Fluxgate through in real time isn't good. They are notoriously noisey(electronically) and must be dampened and averaged out. The nature of the closed loop system that is necessary (I don't really get the details on this) make it unable to have heading manually put in by the pilot. There is a solution though, guys way up north like in Alaska do it. It's the King KCS305 and KA51B.