Quote Originally Posted by Ken Nadvornick View Post
Not sure where these references to noise are coming from. My Duplex has always operated dead silent. No noise No hum. Nothing.
According to a thread on LFF, when bulbs begin to go bad they can cause the ballasts to make noise. It can sometimes be remedied with a new bulb.

This is what was posted:

Quote Originally Posted by jbarber
I spoke to someone at Thomas Instruments today. They are still selling parts for the Duplex safelight. My question was: why is one of my units so noisy? Answer: Sometimes an old bulb will strain the transformer so much that it is damaged, but in any case, a dying bulb will cause the transformer to be noisy. The transformer is called a "TRD transformer" and costs $72. I have three units (two in one large group darkroom, one in a smaller darkroom. The smaller darkroom's unit is almost silent at all times, yet nice and bright. One or both of the others are nastily noisy.

The nice Thomas rep. suggested trying the bulb from the quiet unit in both of the others. If peace returns, the culprit was the bulb. She said that a properly-functioning unit takes no more than 5 minutes to reach full intensity, so one that takes longer probably has a dying bulb, and/or a bad transformer.

Hope others find this info as useful as I did.