Hi John, Your Nutone model 2540-B uses a 20V center tapped transformer. You can proably get one at radio shack that will work. If you can't get a 20 volt transformer, a 24 volt center tapped unit will work okay. The dial lamps in the model 2540 are type 161 that are rated at 14 volts, and are connecter in series across the transformer input, so 24 volts will be okay for them. The transformer should be rated at 1/2 amp or more. Regards, John