By itself the enlarger at full aperture with no neg in the neg carrier shouldn't be noticeably dim. Check the wattage of the bulb and if this is right for the enlarger then as long as the lens aperture dial is working and it opens up as you go down from say f16 to f 3.5 then it suggests that the lens may have very serious fungus or needs a very good clean on the outside of the lens.
It is difinitely inside the lens and I can't open it. It seems like a toy lens anyhow despite it being branded as a Rodenstock. I'll check the bulb soon. It is a CFL type and is very small.