[TonyDuell]

I've had another look at this, and it's rather complicated.

It appears that the MOSFET is the real chopper transistor, in that it gets the drive signal. The base of the BUW12A is held at an almost constant voltage. When the fet turns on, the BUW12A base-emitter junction passes current, thus there is current flow in the collector-emitter circuit, the collector going to the chopper transformer and then to the 350V rail.

I say 'almost constant voltage' because there's a current transformer involved. One winding is in series with the BUW12A emitter. The other supplies a signal to the base via a diode. It appears that idea is that once current starts to flow it increases the base drive to make sue the BUW12A turns on properly.

Now the diode friom that current transformer to the base is a BAX12A. Turns out that's a switching diode with a specified reverse avalanche characteristic, it breaks down between 120V and 170V. I can't see how that could happen in this circut but there must be some reason for choosing an obscure diode like that. It still tests OK but I can only test it as a signal diode.

The 'constant voltage' on the base of the BUW12A comes from a BZW06-13. A strange 13V zener thing (at least it's not the bidirectional one). At first glance a normal 13V zener would work, but again why choose something odd. Anyway, it's dead short so I've got to replace that.

Finding the replacement parts is not going to be easy...