On Sat, 06 Feb 2016 02:09:03 -0500, legg wrote:

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

Thanks, legg. I'll have a rummage round my copious junk box. I doubt the
ft figure has much bearing in this application as whatever the rate this
thing switches at it won't be anywhere close to a fraction of that.
Likewise the hfe is definitely on the low side, so I shouldn't have any
trouble matching or bettering that in a subbed part.
Just to reiterate here, this is NOT the main chopper in the smps section;
it's a self-standing HT generator card with its chopper being driven by a
single transistor oscillator stage. In fact it really wouldn't take much
modding to swap the BJT for a MOSFET but I'd prefer to stick with the
original circuit if poss.