Skeleton Man wrote:

> Well I now have a supspect component.. part number LT334D (3 pin transistor
> casing), which I'm guessing is a rectifier.. it's attached to a heatsink and
> all 3 pins measure as a dead short.. I must need a bigger iron to remove
> it though, as my 25W pencil iron won't do it.. (the heatsink absorbs all the
> heat when I try to unsolder it)..

The parts in my 350W Ultra/Wintech with numbers starting with "LT" are
dual Schottky diode packs and are mounted on the low voltage
heatsink. I can't find information for your particular part, but look
for something that at least meets the amp ratings for its particular
voltage rail (some parts are rated for amps per diode, so double this
rating). Also some PSUs use these diodes in parallel pairs for some
voltage rails, on opposite sides of the heatsink, and if one is
defective, replace both, using components with part numbers identical
to one another so that they share the current equally. My Wintech
has two empty spaces for such extra diodes. If you can't find the
right diodes locally, either try Mouser.com or rip them out of junked
PSUs, even ancient AT models (it's amazing how <200W AT PSUs contain
parts with the same ratings as newer 400-500W PSUs do).

> I'm guessing the heatsink is live too, because there's no washer between the
> part and the heatsink..

Some component packages are designed with built-in insulation, but
check any replacement's because it may insulate only the mounting
screw (still need insulator sheet) or insulate nothing at all (need
insulator sheet plus nylon shoulder washer -- don't substitute regular
flat nylon washer). If your insulator sheet is silicone rubber, it's
not absolutely necessary to smear thermal paste on it, but transparent
sheets need it on both sides. Also be sure the paste is
nonconductive, i.e., contains no silver.

I use a 35-40W soldering iron on single-layer circuit boards, and it's
a necessity for the tabs attached to the heatsink.