If there is no switch, unscrew the bulb slightly when you are done. This will save on the bulb life. It's also a good idea to Also, keep a spare bulb on the floor of the ATU in its cardboard case. With your vacuum or rag, dust the components, remove all cobwebs, rodent and bee nests. As you work, you will encounter a number of different components that you'll need to both clean and inspect.
Coils are first.
As you clean and dust each coil, be careful not to jiggle or move any of the copper straps that tap the coils. You might grab each tap to einsure they are tight, but do not change their location. If you have a permanent marker, place a mark on either side of the coil clip, so you'll know where it belongs should it fall off. If rotary coils are used, inspect the wheel that rotates around the coil ribbon or tube. Age and arcing can damage or destroy the ribbon or tube.