Systematically working through all suggestions as to the possible source of the hum, next in line was to check that there wasn't any pick-up from within the volume control potentiometer having previously by-passed the mains connections to the on/off switch.
Volume pot duly disassembled:-
The interior was first cleaned with IPA and this removed any crud on the carbon track and contacts. The carbon track was then cleaned with a light application of DeoxIT Fader F5
NOT D5! The inner metal ring was cleaned up with a fibre glass pencil as were all contacts. An overall clean up of the external tabs and everything reassembled and back in the set.
And the result - no change to the hum, which was disappointing!
A few more things to try, especially around the heater chain, but we are running out of options ...
I fully appreciate that tracking down the source of hum in a vintage valve radio is a bit like chasing unicorns but as always this has proved to be an educational, albeit frustrating, journey.
