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

[rburnett at richieburnett.co.uk]

Something doesn't quite add up here.  The schematic shows Q1 as 2SC3359, 
but this is only an 80V rated signal transistor.  The transistor in the 
place of Q1 in an off-line flyback converter will usually be a 
high-power switching transistor rated for at least 600V.  D2 in the 
snubber circuit will be a fast-recovery rectifier diode also rated for 
600V or more.  UF4006 or UF4007 are common here.  If either the snubber 
diode or Q1 have failed s/c then you would expect R2 to fuse open.  Also 
remember that C12 can store a lethal charge at around 340VDC so avoid 
contact!

Switched-mode power supplies can be dangerous and notoriously difficult 
to repair.  Usually the switching transistor fails for a reason, so it's 
worth checking the semiconductors on the secondary side for any failures 
before replacing the switch and trying to power it up again.  A shorted 
diode on the output side will often just pop the switch on the input 
side if you try to power it up.  Also check the electrolytics for any 
obvious signs of overheating (bulging) and replace with good quality 
(Rubicon) low-ESR of identical voltage, capacitance and temperature 
rating.  (Powering-up from the mains via a 100W bulb in series can often 
minimise the fireworks while there are still faults to be found!)

Good luck!  If it was me I think i'd consider retro-fitting a whole new 
SMPSU if you can find one that will fit.

-Richie,



On 2014-03-11 11:36, Gil W. wrote:
> Hi all! I need some help fixing that TX802, thanks in advance !
> 
> My TX802 wan't powering on, I found that Q1 (2SC3559) and D2 (1SS84)
> were short out (well, I think it was D2 but need to check again...)
> 
> Replaced Q1 with BUT11A (instead of BUT11AF which is supposed to be an
> equivalent) and 1N4148 in place of the 1SS84 (is that ok?).
> 
> Then I found the wirewound resistor between the diode bridge (R2) and
> the big filter cap (C12) was dead. This is a 22ohm/3W. I'd like to
> replace it but the lowest wattage I can find is 5W. Would that be fine
> or is it too much in that place ?
> 
> Here's the PS diagram:
> http://www.loscha.com/ybb/Tx802-power-A.gif
> 
> thanks very much for the help !!
> 
> 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy