I haven't done a lot this week, busy with work and today was fine so I took the pup for a long walk up the Kelvin to examine the remains of the CR.

However, I did note that when removing a plug in signal there is the potential for the lamp supply contact to touch the chassis, thus creating a momentary short circuit. Simple way round it, make a power supply that can withstand indefinite short circuits by limiting the current available.

Here is the circuit diagram . Conventional enough, but the 82 ohm resistor makes it short proof, even if the 4k7 pot is up for max brightness.

Might be useful for similar applications.