On color enlargers like the Durst M370, the C/M/Y dials usually operate cams that move color filters in and out of the light path. If turning the dials does nothing, two likely causes were mentioned:

1. Filter disengage / white-light lever is active — many enlargers have a lever or dial that lifts the filters completely out of the light path so you can focus with white light. The M370 reportedly has a small lever/dial on the side of the head; if it’s set to white light, the filter dials won’t affect the output.

2. Filters are stuck — after long storage, the filter carriers may be frozen in their spring-loaded tracks, so the dials turn but the filters don’t move.

To inspect it, look for a service or access panel on the enlarger head, often a flat screwed-on panel. A manual was also reported to be available from Jollinger, which should help identify the filter disengage control and access points.