I think I got you now. The actual speed control is done through the SCRs. The controller card does the checks and is the front side of the 'dimmer,' the rectifiers do the pulse-width depending on the speed control pot setting, and the contactors just let the pulse-width through.

Okay, now the fun begins.

Would an oscilloscope be a handy tool here? I don't know what end is what in regards to the SCR but I'm sure viewing the pulse-width waveform would be handy.