SC50 heading error?

[dmitry_gdv]

I have a Gyro feeding data to Radar/ECDIS and SC50 for backup on newly got boat. Gyro heading equals to SC-50, but noted angle discrepancy between radar overlay and ECDIS screen causing misalign of radar overlay and AIS targets as well as charted shore line. Upon detecting of Solar azimuth and subsequent error input in gyro radar overlay exactly coincided with chart and AIS targets, but now I have a difference between gyro which looks correct (as error eliminated) and SC-50.
Can't understand how to input error correction in SC-50, the matter does not explained in user manual.