You mention connecting the + and - leads of your power supply to
the PMDX-126. This is confusing because the PMDX-126 does not
use DC power, it is connected directly to mains power of 120 VAC
or 240 VAC.
Can you confirm that your 120 VAC power is coming into the small J8
connector directly beside the voltage selector switch, and not the large
J9 connector which is for the big relay contacts?
Also try sliding the voltage select switch back and forth just in case
it was not fully seated.
Lastly check the voltage at the screw terminals of J8 to see if you measure
120 VAC.
If none of this helps, then we need to do a warranty repair and you should
send the board back to us with your description of the problem, your
contact email and/or phone, and your return shipping address on a printout
inside the box with the PMDX-126.