3rd day of 3D printer assembly

Unfortunately, I was only able to spend about 1 hour working on the 3D printer today, so I don't have much more done.

I got the extruder barrel assembled.  I should be ready to attach the barrel to the extruder, the extruder to the printer, then try printing something!

I spent some time trying to diagnose the non-functional optical end stop.  I had already tried a different cable and a different stepper controller yesterday.  Today I visually inspected the board, re-soldered all the joints, and verified that the LED is working and in the right way.  I then powered up the both the bad end stop and a good one to compare voltages.  I measured every joint on the board, and everything appears identical between the good and bad end stops.  The only difference is that if I block the sensor on the good end stop, the LED turns on and the signal line goes low.  On the bad end stop, nothing at all happens when I block the sensor.  I'm thinking that the sensor must be bad.

I sent a message to MakerBot.  I hope they'll send me a replacement.

In the mean time, this will not stop me from getting the printer up and running.  I understand that it's possible to run the bot with no end stops at all, or to run with just 3 (for homing.)