On Sat, 6 Feb 2016 12:34:17 -0000 (UTC), Cursitor Doom
wrote:

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.

ft must be equal or greater, similarly beta, which puts a modest
amount of doubt for the 3439/40 parts. They are slower general purpose
parts, with higher voltage ratings that are accompanied by lower beta
(possibly 25% lower at 1A).

The ZTX part is offered as an example of more recent physical
equivalents of higher speed switches in the same voltage range. It's
silicone package has the same temperature limts as hermetic and it can
benefit just as much by heatsinking methods common to TO5, if present
in the current application. It should gunction with whatever base
drive and SOA load line tailoring that was applied to the original,
with reduced switching and conduction losses.

I wouldn't advise a switch technology swap until you become more
familiar with the circuit functioning as originally intended. There is
seldom any noticeable benefit in low/medium power applications.

RL