I'm just thinking is the hydraulic unit on its own heat sink plate about six inches by six and can you see the thyristors, I'm trying to visualize this unit from years ago, I think it might be the sevcon unit that has gone down, The one I removed from a truck back in the nineties I took to Sevcon in team valley Gateshead, it turned out to be faulty but they where amazed at its performance it had done over twenty five thousand hours before failing. You could phone them and give them the serial no of the unit' and I'm sure there technical department will tell you how to check it out, they were always very helpfully when I took them units, the hydraulic contractor if it is an allbright single pole contractor check to see if it is closed properly sometimes they look closed but their stuck open slightly. The thyristors are quite large and will have a 2volt gate signal to fire them. I hope some of my comments are usfull as I'm guessing at this.