Where is the orifice tube located on a 1993 Chevrolet pickup?

On a 1993 Chevrolet pickup, the orifice tube is located within the A/C system's high-pressure side, usually near the receiver/dryer. It's not externally visible without disassembling part of the A/C system. You'll need to consult a repair manual specific to your truck's year, make, and model (and possibly engine) for the precise location within the system's lines. The manual will show diagrams.