Is your light meter showing only + / – or some doubtful values? Or is it dead completely?
The most common problem here is that one or more wires connecting the aperture/shutter speed rings with the light metering unit are broken.
How this could happen? Well, every movement of your lens especially when “opening/closing” the camera slightly hurts the wires and after 40 years of service, they just break.
The solution is simple, let’s replace them. The only one complication is the fact I need to fully disassemble the camera to do so. Perfect moment for a CLA.