I have a perfectly good light bulb installed on my Apple /// keyboard but it doesn't come on when the Apple /// is powered up and the voltage at the bulb socket is extremely low. This being said, the Apple /// works perfectly well even if the light bulb is removed. My understanding of the Apple /// operations is that it wouldn't progress beyond the RAM diagnostics without a working bulb but in this case it doesn't seem to affect operations at all. I would still like to fix this issue that that I see the light when the computer is turned on.