I just assembled another prototype of the clock, this time it worked nearly perfectly. I think I had some older vintage 4017 counters that worked on my breadboard, and the newer ones I used on the PCB version didn't work. I was able to replicate the problem on my breadboard, and I made it work after I changed a few pull-down resistors to a much higher impedance. I also realized the PCB was missing the 11 o'clock hour, and i solved this by removing the zener diode and capacitor on the hours segment. I changed all the zener diodes from 4.1V to 3.9 volts, I think the newer vintage 4017's were to blame for the difference. The tens digits weren't resetting to zero and switching to 3.9V zeners fixed it. I also changed the USB power plug to a USB B plug because it's through hole, and USB B cables are really easy to find. I changed the LED current limiting resistors to resistor networks just to save a few solder points, because it's seriously a lot of soldering! My next step is to get my hands on a really nice oscilloscope to fine tune the capacitors that make the crystal oscillate, and I'll be making yet another PCB before I call it finished. I'm also working on getting my hands on a flatbed inkjet printer so that I can print some cool art on the circuit board instead of being limited to just one silkscreen color. I'm imagining a matrix themed background of green 1's and 0's.