Hello Jim, well first a few questions. From your description of the problem it seems you must be using Mach 3 to control your machine so I guess it has been converted from the original Denford controller but kept the spindle drive - is this correct? And you can currently control the speed from Mach 3?
Assuming that the answer to these is yes, then probably there is a PWM to DC voltage converter in the breakout board between your computer and the stepper and spindle drives. Could you check that there is a wired connection to pin 3 of the Lynx card?
The Triac seems to use the Lynx 08 drive from the Denfordata forum. I have found the manual for this online and will attach a copy. It confirms that the drive DOES have an isolated input, which is fortunate. The Parker 508 DOES NOT have isolated control inputs and is NOT suitable.
You can buy a refurbished Lynx-SM-08 drive from these people:
https://nicontrols.com/uk/lynxsm08-control-techniques.htmlThe SM bit just means that it uses "surface mounted" components on its PCB, yours (being made in 1988) would have used "leaded" components. This looks like it should be a direct replacement for your board.
I think people have also used Sprint drives, for example the 400i, which also has isolated inputs. However that can only supply 4A, but there are other Sprint products with greater capability. (I believe that the Novamill (which I have) originally used a Sprint drive.)
https://www.transdrive.co.uk/shop/products/dc-drives/sprint-electric/analogue/2-quadrant/sprint-electric-400i-0-55kw-4a-240-110vac-2q-isolatedSo you should be able to buy a replacement Lynx drive - if not then there is probably a suitable Sprint one but the wiring will need changing round a bit.