Today In the Bravo my 297b  altitude preselect decided to go to a black screen. I had several episodes  of black screen then working screen etc.  it was working then "poof" black screen .I dont recall any event that would have caused this .. I was not sure it the 297b was  still providing an input to the autopilot , but it was black then it came back to life then it died again. This is my second replacement unit .  there seems to be no warranty on the unit.. Ive changed most of my old radios and