I don;t think I've recently described the typical failure mode of the Boschert 2-satge SMPSU (on-topic, since it's used in classic computers, including the PERQ 1). I'll start by describing the basic circuit : Mains in is rectified and smoothed, giving 350V DC This feeds a (non-isolating) step-down switching regulator (using one chopper transsitor and an inductor). The output of this is around 150V (but see below) This feed as pair of chopper transistors which are coupled to a small transformer making a free-running oscillator, the freqeuncy of which is determined by the transfomer properties The output of that feeds the main chooper transformer, the secondaries of which are rectified and smoothed in the usual way. A voltage sense signal is taken from one of the outputs (almost always the +5V line), compared to a reference, and fed back (opto-isolator) across the isolation barrier. This controls the first step-down regulator, thus controlling the voltage to the second choppers, and thus the output. Oh yes, there's a current sense resistor in the first regulator circuit which will shut that chopper down (and thus remove the outputs) if there's an overload. Adn a crowbar on at least one of the outputs. Now for the failure mode : The chopper transistor in the first regulator circuit goes short-circuit. The otuput of this stage (should be 150V remember) leaps to 350V. The second choppers carry on working, so all the outputs try to go up to over twice their normal value. You'd better hope the crowbar fires... Let's assume it does. That short-circuits one of the outputs, thus causing a much higher current in the chopper circuit. This is detected by the overcurrent sense circuit, which removes the drive from the first regulator chopper. Alas that's what has shorted, so removing the drive doesn't do a darn thing!. The 2 second-stage choppers now go short-circuit. This means that across the 350V rectiifed mains you have essentially the 3 transistors (all dead short), some inductor and transformer windings (almost no DC resistance either) and the current sense resistor. The last burns out. This now applies 350V to the current sense inputs of the controller IC (a good old 723) which blows apart. Oh yes, a couple of small signal transistors blow up too, and a PCB track is blwon off the board. Finally the fuse fails And yes, I have seen the results of this happening, and indeed rebuilt the PSU. -tony