Try disconnecting the cable from the Motor/LNB then put LNB power on.
What will it do then?
If that works then check your cable ends for shorts. Maybe it is an autoprotection if it senses a short in the cable such as if the silver strands in the cable end touch the center copper.
If the receiver stays on after removing cable also try removing cable from motor and apply direct to LNB. See if it stays on.
Then, if it does, investigate a short in the motor.
If you read my other posts I received a bad
Stab HH120 recently so maybe there is a bad lot.
Brian