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, HP0950-1879) 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), a2N2647 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