On 26/03/2011 13:16, Paul Bird wrote:
Jules Richardson wrote:
On Fri, 25 Mar 2011 20:09:26 +0000, tony sayer wrote:

We have a rather odd specialist desktop PC that got a knackered power
unit.

Is fixing it not an option? The same few things cause most of the
faults...


Given the apparent complexity of a SMPS, and the fact that you can't
learn much when it's not running what is the usual diagnostic approach?
Blown caps, burned resistors?

PB

OK, start he

Visulal inspection.
Bulging caps?
Blown fuses?
Burnt components?

Fuse blown? Blown to buggery, totally blackened?
- fault is probably on the input side. Suspect main diode bridge
rectifier. Check with meter.

Next:
Check for over 300vDC on the main reservoir caps.
If 300ish V on caps, then we move on to the switchers.

Blown fuse may suggest shorted switching transistor. Check with meter.

Simple meter check of semiconductor devices on the output dide, looking
for shorted devices.

THat's the basics.
If you need to go deeper, it's time to start tracing circuits etc.

--
Ron