Hi Phosco,
You were right about the green RT photocell controller, I opened it up and sure enough it is electronic. The switching device is a Relay so it's not completely solid state.
I liked your comments about the RT photocell controllers, I have witnessed all three failure modes which you mentioned on various ones of mine.
The failure mode where the "snap action" of the spring strip fails is quite an interesting one. I was testing an RT P42 last night and just before it switched the light off I could hear arcing inside the unit and the light started flickering, then it switched off. I remember a Philips MA50 near my house flickering on a morning just before switch off, this too was controlled by a P42. You can tell when it's going to be a "lazy" spring action as the click isn't quite as sharp, if that makes sense!
It is a good testament to both RT and Horstmann that after 40+ years of service their controllers are still working (well some of them)
I do find the old thermal controllers a miss off the streets of Cumbria. I liked the fact that as they became old they drifted out of calibration, some coming on with cloud cover and some not coming on until almost completely dark. However some of them remained bang on.
Were RT P42s used a lot down your neck of the woods?
Regards,
Andrew.