I have a Leica Meter M I received with an M3 recently. It does not appear to be working. the needle doesn't budge off zero, even with the door that goes over the cell open. The needle bounces a little if I shake the camera, but does not appear to respond to light at all. Is there an on-off switch or anything else I need to do to turn the meter on? Or is it more likely dead after 50-plus years?