The G/S superflag is 28vdc from the nav rxvr,When you tune in a VOR a discriminant circuit switches this voltage in to lift the flags out. Follow the circuit from the nav rxvr and see if you have 28v into and out of the txfr relay, check for 28v before and after each disconnect on the racks and right up to both instruments, it sounds to me that you have a short circuit to earth which is dragging down the voltage, so out with the old megger. Hope this helps.