Denford Triac Spindle Issue

[m_c]

Next step is electronics knowledge, and tracing the relevant circuits.

The big problem however, is testing the cards under power, as being rack mount, it won't be easy to probe components when installed.

I think I'd start by removing the rack, and tracing the analogue wiring, to find out where the analogue isolation circuit is. From there it would be a case of starting with the basics, like checking the DC/DC converter is getting power, and is working.
The DCDC converter might actually be pretty obvious, and there is possibly only one of them.

It's probably worth posting a photo of the custom card, and we can point you in the right direction of where various bits are likely to be.