Background information about Spektrum receivers and how they affect gyro initialisation.
Background information: The great majority of RC receivers produce no signals at their servo outputs until the link with the transmitter has be established and up-to-date servo position information is received from the transmitter. Spektrum and JR 2.4GHz receivers however operate in a different way. When the receiver is powered on it will output the default servo positions which it had memorised during binding. When the link with the transmitter is established the receiver with then drive the servos to the positions defined by the transmitter.
How gyros work: All heading hold gyros including the Spartan range need to have very accurate knowledge of the rudder stick neutral. For this purpose the majority of gyros will first wait until a rudder signal is seen at its input, it will then perform the 2-3 second sensor calibration and last it will sample the position of the rudder stick and memorise it as a new rudder stick neutral.
The problem: The non-standard operation of the Spektrum and JR 2.4GHz receivers creates the following issue which affects almost all modern heading hold gyros. Because the receiver drives its servo outputs instantly after power on the gyro starts calibrating at the same time. If the gyro completes calibration before the receiver has linked to the transmitter, the gyro will see and memorise the bind/default rudder position. If the receiver links first the gyro will see the real rudder stick position. Ideally the bind/default rudder position should be the same as the real and as such no harm is done. However, if the rudder trim is moved and the default/bind position is not updated the gyro may occasionally see the incorrect default/bind rudder position rather than the new real position that now includes the trim adjustment. The same may happen for small shifts of the rudder neutral due to normal joystick potentiometer wear.
The solution: To better deal with the above issue Spartan gyros employ a mechanism that allows the gyro to understand when the Spektrum or JR 2.4GHz receiver has linked with the transmitter and only then initiate their own calibration. To achieve this the receiver must be bound while the gyro gain is set below 5% (gyro). After binding the gyro gain can be returned to its normal operating level. When the helicopter is powered on the gyro sees the very low gain value and waits until the receiver has linked. At this point the gyro gain is raised by the transmitter to the normal operating level which triggers the gyro's calibration and sampling of the real rudder stick neutral. Using this method is optional, however it is likely to save a lot of tail drift headaches in the event that the rudder trim is accidentally adjusted. This method also allows more time for the user to enter the gyro setup mode through switch toggling and is therefore recommended for users that may have difficulties in doing so.
Note: Previous versions of the ds760 gyro firmware offer the same functionality by binding the receiver while holding the rudder stick off centre. This method is no longer recommended and will be removed in future firmware versions. Please use gyro gain value less than 5% instead.