[Eric T]

it would make sense to be able to replace antiquated hardware with modern replacement... a photodiode seem to make sense. I find the whole idea intriguing. I hope good will come out of this and to have a beam of light replacement guide posted somewhere. Perhaps even a "make your own beam of light"?

I hope to get this thing working and be able to share information about it here.

Here's a little update:
I checked the original photocell a couple years ago and it tested good.
Last time I tested it, last Winter I think, it was bad.
I just tested it again and it works.  Whiskey Tango Hotel, over.

Maybe it's susceptible to humidity or some other environmental factor.

I think I'll just proceed as if it's bad, and go with a photodiode of some type. They are supposed to have better high frequency response than the original, anyway.