I honestly have no idea why your zener diode wasn't working properly. It could have been a simple soldering issue, though those things are so small that it's pretty tough to get a cold solder joint on them. These parts are only really on that board because they also go in the Eros 1.1, where we really need that 6.3V power supply biased up about 50V. In the Mainline, it works just as well to have it grounded, but we kept the instructions consistent between the two kits.