On Thu, 04 Feb 2016 18:38:58 +0100, Reinhard Zwirner wrote:

Would you be so kind and tell us what was wrong? Thanks in advance.

The problem was in the EHT converter section which creates 1600V from a
45V input. A BSW68 transistor in this section had fused low-resistance
between C&E and was attempting to dissipate 340 watts all by itself - and
it didn't even have a heatsink! The fact that its case had turned blue
also hinted that it had been very hot. I couldn't see it on inspection
initially as it was hidden by HV shielding.
Now I just have to find an equivalent for it...