Switch-Mode PSU Failure Modes, Repairs, & Parts Substitutions

[Tony Duell]

> 
> Hi,
> I have 3 Patriot book pc psu's which I want to get repaired.

You don't give us much to go on :-)

What are they doing?  In general you'll either get mo outputs at all, or 
a supply that's tripping. It will 'tweet' about once a second. If you get 
that, connect a voltermeter (or better a 'scope) to each output in turn 
and see if one of them is not coming up at all (if so, then suspect 
shorted secondary-side components on that output).

If it's totally dead, look at the mains fuse. If it's 
shattered/blackened, you have a major short on the primary side. Check 
the rectifier diodes and chopper transistor, etc.

If it's gently blown, replace it and try again.

If it's OK and the supply seems dead, suspect a startup failure. Often 
there's a high-value resistor from the +ve side of the mains smoothing 
capacitor to some point in the control circuit to get it going. That 
resistor (may be 2 in series) often fails open-circuit.

And don't overlook the obvious. I've just spend some considerable time 
debugging a very complex HP SMPSU (on 6 PCBs, with 5 chopper transistors, 
etc -- the supply in a 7245A printer/plotter), only to find the fault was 
that an intneral cable wasn't plugged in properly.

-tony