The Dimmer Rack in the photo the OP included is a photo of a Strand CD80 Rack which was (is still?) a very common rack in North America. Strand still produces Dimmer Modules and replacement Processors for them. There were a few versions over the years (Reporter, Supervisor, etc.) and were first produced around 1980 (hence the CD80 name). One in a local theatre is still going strong after 25 years or so. They are quite robust but they don't last forever. One source of parts is here: https://www.johnsonsystems.com/strand_cd_2000_3000.htm Depending on the version, the CD80 Rack Processor (installed in the bottom of the rack, below the dimmer modules) can either recognise which Dimmer Modules in the rack are Non-Dims, or has to be told (it is a setting in the Processor). A local CD80 I use, for the Non-Dim to operate correctly, has to be told through the face panel menu if I move a Non-Dim from one slot to another in the rack. Perhaps someone has been messing with the settings in the rack processor? Or it is on its way out...