On early models, at least as late as 1982, the switch could be toggled on and left on indefinitely. Probably to compeansate for our slower students who would leave it on constantly, it was changed to a timed momentary-contact switch. My 1988, 1991, and 1993s have all been this way. Pressing the button down activates a timer that stays on for about twenty minutes and illuminates the switch, which returns to the "off" position when released. The bulb is a 2721 inside the switch. Take the switch out and you'll see the two spade connectors that are switched and a small bulb holder behind the window part of the switch. Pull this out and put a new bulb in.
Sometimes the contact on the outside of the bulb holder gets dirty preventing illumination. The best solution is to go on eBay and for about $4 buy a jewelry tool called a "scratch brush" which is great for polishing up electrical contacts.
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