On Thu, 18 Feb 2016 00:47:43 +0100, Dimitrij Klingbeil wrote:

[...]

Dimitrij, you've gone to an awful lot of trouble here for which I'm
extremely grateful. The main impediment at this time is that my fancy,
high-end Marconi sig gen is temporarily out of order. Perhaps I could rig
up something to do the same job from a 555 timer, but that's another
story.
I followed everything you said - except for one important thing: you say
this board has to be under load at *all* times when mains powered or it
will be catastrophically destroyed (or words to that effect).
I'm a complete switcher novice, so there have been *many* times when I've
probed this board under power with no load connected; I didn't know any
better. Before I go any further I need to know if I've therefore toasted
the board. Which components exactly catastrophically fail under no-load
conditions?
Thanks again for all your help.