"Roy J. Tellason" bravely wrote to "all" (25 Oct 03  04:06:11)
 --- on the heady topic of "power supply"

 RJT> Got a dead ATX power supply here...

 RJT> Anyhow,  fuse is good.  Thermistor is good (about 8 ohms cold),
 RJT> bridge rectifier is good,  caps are good as far as I can tell,  main
 RJT> switchmode transistors are neither shorted or open,  and I have *no*
 RJT> idea about the rest of it.
 RJT> Repair FAQ says "startup"...

 RJT>   anybody
 RJT> have other suggestions to offer on this?  Besides tossing it?  :-)

I only try to fix them for myself when I don't have another handy.
The main switcher circuit on psu's have pretty much settled down to a
standard form.

Here IMO are the 2 most common failures:

If you can follow the base drive of the switchmode Q's you will find a
large value resistor (between 220K to 470K) to the main rectified
supply. One of these tends to open up for no reason. I think it may be
due to AC line spikes.

If these resistors are good then keep following the switchmode Q's base
drive and you will find a diode, 22ohm resistor, and a diode across a
2.2uF electro (maybe 1uF to 4.7uF). These tend to dry up since they are
close to a heat source and also must conduct a substantial current.

The TL494 is hard to kill. The switchmode transistor can lose gain but
that would show up as a lower power output. Good luck.

 Mike
 ****

... I know flyback, ultor, and 47 other dangerous words.
--- Blue Wave/DOS v2.30