SMPSU IC explosion... (UC3844)

Tony Duell wrote: I must have been lucky... all the ones I've had to fix before have had failed in other ways... Yes, so would I to be honest. It's not *that* simple, because this is such a compact unit. There are a couple of vertically-mounted boards soldered on the main PCB. One contains the dead UC3844 and holds the circuitry that sits between the high voltage input side and low voltage output. The other board seems to be purely on the HV DC side and has a few surface-mount resistors on it, a small transistor, and a 16 pin IC labelled UC385x (there's a splodge of dye or something on it so I can't read the last number - doubtless it'll be easy to find via google when I come to look it up anyway). Then there are three large L-shaped heatsinks on the board which wrap over the top of the board components, and then another large slab heatsink is screwed to those. The slab heatsink comes off easily enough, but method of PSU construction is to bolt heatsinked components to the L-shaped heatsinks *before* soldering them in place. There's no access to undo the transistor / diode heatsink clamps without desoldering half the board. Component level repair was obviously not on Sun's list :) I can trace stuff with a continuity meter, common sense, and a visual on the PCB traces on the board underside (where luckily 95% of them are) - but a visual conformation of the PCB's topside is out without some major dismantling... (which of course I'm going to have to do if the chopper transistor's dead anyway in order to change it...) cheers Jules