RE: Spektrum DSP75 servos compatible with GY401?
I have a DSP75 on the tail of my Blade 400 as well. It is recommended in the B400 manual if you want to use the stock gyro in digital mode.
When I first powered up with the new DSP75 servo the heli started fast CW piros. Flipped the Norm/Rev switch on the gyro and all was good again. Did not use the servo programmer to set servo to rudder mode.