Partway through Chapter 5 you will reach an area where you have to portal from an office (Make a new save file here in case you miss it first time round) onto a moving conveyor belt carrying scrap pieces of metal including parts of scrapped turrets. Continue along this first belt until you get to some air ducts you can jump onto proceed onto the next moving conveyor belt. When on the second belt turn right and walk against the direction the belts going. You will come across a defective turret still in one piece and talking. Pick this little guy up for your Trophy.
There is a time limit on this. Easy to do in the time given, but don't leave the game running un-paused. If you do miss it load your last save game.
Lastly this guy has some funny stuff to say and is worth keeping hold of for a wee while.