The commonly overlooked weak points of cheap ATX PSUs

[Th3_uN1Qu3]

I've noticed more folks rebuilding crap ATX PSUs lately, and it doesn't seem like much work because they're rebuilding something that was working in the first place. Tackling a blown one can be a bit trickier...

So here we go. I won't mention the rectifiers, caps in 5vsb primary and things like that as those are already common knowledge.

1. Driver transistors. These are TO-92 types, located on the secondary side, somewhere around the base driver transformer. Follow the traces coming from the transformer to find them. I've blown more of them than i can remember. A primary short can take the driver transistors out as well, and when only one of them is shorted you get pretty interesting results, which may lead you to search for the failure in a totally unrelated area.
   
   
2. 1N4148 diodes in base driving circuit. Found in the primary near the main switchers. When these go short, they will make the primary switchers fail open because of reverse voltage over the base-emitter junctions. If you want to replace them with something else, you must use an ultrafast diode with very low recovery time.
   
   
3. Base resistors. Found in the primary, in the base of the main switchers. Sometimes these are omitted entirely, and replaced with jumper links. Many times they are too low in value, around 1 ohm. That might work fine for the tiny capacitors they put on the output, but when you've got a significant capacitance to charge at startup, too low value base resistors will blow the switching transistors' bases open because of too high current. Particularly if the NTC thermistor in the primary is too small or missing. Had it happen one too many times - a power supply that works just fine under load, and then you turn it off, and when you turn it back on... it doesn't.
   These must be at least 2.2 ohms, 4.7 ohms preferred. This takes a little bite of efficiency, but it's insignificant, and certainly better than having your primary transistors die.

That concludes it for today.