I'm going to assume that the humming is still there even when you use the voltage stabiliser, if so then read-on. Also I'm going to assume that voltage stabiliser outputting 220VAC (rather than 110VAC).
Originally Posted by foto-r3
The most likely cause of the hum would be the step-down transformer (220V-110V). If an under rated (power-wise) or poorly designed step down transformer is used you will over-load it causing hum. If there is a rating on this txformer then see how it's power rating compares to the power needs of the enlarger.
A humming transformer is operating inefficiently and will draw more power than it should. So if the fuse on the voltage stabiliser is chosen exactly for the enlarger (i.e. the ratings were chosen to match) then the additional (unnaccounted) power drain in the humming transformer core will blow the fuse (albeit intermittently if it is just over the limit since fuses are not binary in nature, they obey a time*current curve.)
So if that is the case, I'd get the step-down transformer replaced with one that can handle the power drain of the enlarger. If the step-down transformer is sufficiently rated then get another one anyway because it is probably a bad design ! Toroidal transformers are quieter conventional shaped (cubical ?) txformers and are used in audio amplifiers for this reason.