Good morning;
There are two ways that problems come up with the Selenium cells. That first already mentioned is the contacts. The use of DeOxit or a similar cleaner will work nicely. Just keep it where it is to do its job. The use of a "Q-tip" type cotton swab or even a toothpick to put just what you need on the spot where it is to go is important.
Normally I do not recommend the use of a pencil erasure or something similar in an attempt to remove any suspected corrosion from electrical contacts, especially those on printed circuit boards (PCB). Most often they will have a very thin gold flash coating only a few molecules thick that serves as an anti-oxidant covering on the contacts. The pencil erasure will easily remove that gold flashing and leave the base copper layer underneath now exposed to the atmosphere where it really begins to develop an obvious green corrosion.
In most cases, just the use of a cotton swab and alcohol is all that is needed to restore a clean contact surface.
The second and more serious failure mode with Selenium light cells is a failure of the sealing around the edges of the cell layers allowing moisture in the atmosphere to work in there and degrade the cell over time. There is no real indication that this is accellerated by exposure to light. In this case, the only real cure is to replace the Selenium cell. There are still companies who can make the size Selenium cell to replace one in your camera.
Here in the United States, Quality Light Metric in Hollywood is probably our best for Selenium cell light meter repair. There is another company in London whose name begins with the letter "M," but I cannot think of it just now. I will probably remember after hitting the "Post Quick Reply" button.
