Denford Triac Spindle Issue

[m_c]

That's handy, they've even marked out what area of the board does :)

First thing I'd be checking, is if the NME0512D (5 to 12V DCDC converter) is outputting 12V.
If it's not, then I'd be checking to see if it's receiving 5VDC.

Beyond that, you really need an oscilloscope to check things.
It's a case of seeing if the opto-isolator is receiving a signal (from that previous thread, the Baldor outputs a PWM signal, which is amplified to drive the opto-isolator), and if the opto-isolator is switching it's output. It's a case of working through the circuitry to see where you are, and are not getting the required signals.

Off course, the problem is doing all that while the card is installed in the rack.
I'd probably solder on jumper wires to test the key points (8 wires - both sides of DCDC converter, and opto) to establish at least what side of the isolation is at fault.