The off/on switch on my 1973 olympus khc has broken and hass locked in the off position.

You could try to dismantle the microscope to access the switch and try to service the switch, or replace it. It's probably stuck in place because of grime. Dismantle it, clean it out with isopropyl alcohol, make sure it moves, then wait until it dries out and test it.

It can be easily replaced with a simple switch bought at any hardware store. An electrician could easily do it for you, or if you know electrical hardware, you can do it yourself.