I'm trying to put together a treadmill motor and controller for a lathe build but ran into some issues. Lights flickering, smoke etc..

I had the board wired using a DPDT toggle to reverse the polarity of the DC between the board and the motor. Everything worked great. I tried to put a 3 position, 12 pole rotary switch in to replace the DPDT toggle and as soon as I rolled the pot forward the board made a cracking noise, started smoking and the reset switch on my power bar shut everything down.

I removed the rotary switch, connected the motor directly to the board and turned everything on and again, as soon as I rolled the pot forward it made the cracking noise, started smoking, and the motor went to high speed. I turned everything off and noticed that two resistors on the board (R12?) were fried.

I'm hoping someone can advise as to A) what did I do wrong? and B) is this board done for?

In case it isn't glaringly obvious, I'm new to this so any light you can shed on this would be tremendously helpful.

 

Those are in the trigger gate for the two SCR's I suspect you may have tried to reverse the motor while the SCR's were conducting, and probably fried the SCR's and possible rectifiers.
That is a no-no, boards that allow that have a inhibit/reset which brings the drive back up in acceleration mode, there is no feature like that on these boards, without removing power.
Max.