I apologize that I was not able to get back to you within a few hours yesterday. It was my birthday, and I took a day off. (Generally you will find me posting nearly every day of the year, even Sundays, Christmas, Thanksgiving, Easter, etc)
Can you post a couple photos of how the big board is wired?
I believe you have the hardware for the TIP-50 transistors installed incorrectly, which is allowing the metal tab of the transistor to touch the metal heatsink, which is creating the short you measured. Please review the installation instructions in the manual and be sure you have the hardware mounted appropriately.
-PB