Correction to my last message. . . I should have said that
the faulty component (diac)
is marked Z2 07D431K. It is blue, with 2 leads. It measures
low resistance
in both directions. An exactly similar component on the
board measures high resistance both directions.
The component BTB04 600SAP (a triac, on a heatsink) is in
the 300w lamp circuit which is working ok.
In the 12v circuit, which has a transformer, are two
transistors? Q1 and Q2, which are marked U07772.
I don't have a schematic, but should manage a repair without
it. == Sammy ==