[sdiy] TX802 Power Supply failure (replacing 3W with 5W resistor)‏

[rburnett at richieburnett.co.uk]

> It's actually a 2SC3559 (800v), now replaced with BUT11A:
> 
> http://pdf1.alldatasheet.com/datasheet-pdf/view/268894/ISC/2SC3559.html
> http://www.classiccmp.org/rtellason/transdata/but11.pdf

You're proposing to replace an 800V switch with a 450V one.  This may 
not have sufficient Vce rating.

> Original diode is 1SS84:
> http://pdf1.alldatasheet.com/datasheet-pdf/view/230245/HITACHI/1SS84.html
> 
> I think I was wrong and it's not D2 but D5 actually.
> 
> Are these two replacements good at these places ?

It depends what voltages they'll experience in practice.

> Still no idea why all those failed in the first place.

Check every other semiconductor in the SMPS for s/c or o/c.

> I do see some
> greenish acid on some components' leads
> so there might be a leaking cap somewhere.

In which case replace any suspect electrolytics too.  (Electrolytic 
capacitors in flyback converters have a hard life with very-high pulsed 
currents and often run quite hot so don't last very long!)

> As for R1, I guess 3W is critical for it to burn (fuse) out on failure 
> ?

If Q1 goes s/c there will be plenty of energy available to fuse a 5W 
resistor or a 3W resistor.  Just don't jumper it with a piece of wire, 
or you'll probably find the PCB trace underneath will fuse instead!

-Richie,