Before I Drive Myself Nuts!!

[Skywave]

I have checked the lamp limiter again - the light is on and doesn't dim.
Rob

Whoops! That signifies a dead - or near dead - short. I think you are getting very close to the cause with the ON / OFF switch.

Thoughts arising.
This exercise amply demonstrates the value and usefulness of a lamp limiter. A Variac wouldn't have given you such an instantaneous clue, although you could have obtained an equivalent indication using a Variac + a.c. voltmeter. But not only is the lamp limiter much less bother to use for this test, it also completely eliminates the risk of damaging an expensive Variac - and replacement 60-watt light bulbs are a lot cheaper !

Al.