On Thu, 4 Feb 2016 18:41:31 -0000 (UTC), Cursitor Doom
wrote:

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...

Your Towers would have suggested 2n3439/2N3440 or a BUY60.
TO5 body sizes are no longer common, but those types are still listed
by digikey.

BSW68 is 150V, 1A, 40MHz, 800mW, 30min hfe.

A ZTX857, in a silicone E-Line body, could probably do it.

RL