Hi,

I'm in need of a pointer to a couple of representative circuits so
that I can get my brain around the particular configuration used in
the SMPS I'm working on. It is a Boschert OEM type (XL131-3604E, HP
0950-1879) used in an HP54502 DSO. Before I accidentally blew the
switching transistor, it would only work in bursts causing the several
output series regulators to deliver less than perfect DC with
negative-going saw tooth waveforms on top. The unit has worked in the
past but, before this happened, the mother board would now and then
reset perhaps due to dirty DC.

The switching circuit consists of a single TIPL755A power transistor,
two transformers, a CNY21N opto-isolator (fed from the +5V line), a
2N2647 unijunction transistor and one each of PN2222A and TIP41C
transistors as well as the usual collection of diodes, resistors and
capacitors. My difficulty is that this topology is unfamiliar to me
and I cannot easily see how a single unijunction oscillator with the
help of two other transistors could form a PWM. I would have expected
at least a flip-flop but this circuit is all discrete.

Currently I'm taking the easy way out and am checking everything, as
far as I can, and replacing all the semiconductors on the input side
of the PSU; the multiple output regulator circuits being a nightmare,
I'm hoping the fault isn't in there. I've tried hard to find the
schematic for this unit but don't now ever expect to do so; the HP
'so-called' service manual does not contain any useful information.

BTW, I've read Sam's repair FAQ and 'googled' away for ages.


TIA - Joe G3LLV