On this page of our guide to BioShock Infinite you will find the first part of a detailed walkthrough of Chapter 19 - Return to Plaza of Zeal. Destroy more enemies on your way to gunsmith's workshop. You also have a chance to solve a riddle associated with Vox code.
Expect serious resistance from the police units after arriving at the plaza. All the machines you've destroyed will appear again, along with heavily armed opponents equipped with Volley Guns.
You will now have a chance to finish an optional mission involving the Vox code in the watchmaker's store (due to the cipher obtained in chapter 18). Return to that shop and follow the instruction by using the clock found on the wall. You'll be rewarded with Infusion #14, Voxophone #42 and several less important items. Once you're done exploring the plaza return to Chen Lin's workshop.