I forgot that SMPS have their earth connected. I forgot to turn the unit off while soldering, and I think I fried a pin on the main IC that happens to be crucial for the stepper motor to function...
Now the unit will not function, period. Only the timer display works. I'm beyond angry with myself right now.
I guess I could program a 3D printer board to stand in for the (relatively) obscure ST Micro MCU, but... :(
The new board is in the mail, so I will try to make the best of this situation.