Burnt resistors in Floston FL450w ATX PSU
Good day folks. I'm on a bit of ATX repairing spree here and I manged to fix around 5 of them in a single day by replacing various caps and stuff and to keep my record going, next in line was going to be this Floston one but I wasn't so lucky here. So yeah, it's a pretty junky PSU...you just look at it and it sucks. It's not like there are any expectations to meet: it's not Corsair or Thermaltake or something cool - it's a friggin' 10 dollar PSU (IF THAT) so I'm perfectly aware it may not be worth repairing, but the reward I get is not within the value of the object but within the skills I hone by doing it.
So: the fan is stuck on this guy, fuse is intact, lots of bulging caps, no problems there...nothing out of the usual, but the main problem is those burnt resistors. There's 4 of them leading into various pins of that control IC...after a bit of searching, I was able to find its datasheet. It's a SDC2921.
The resistors go between the small transformer in the middle and this IC, more specifically to pins 7 and 8 - C1 and C2 which the datasheet tells us are "PWM open drain outputs". Aside from trying to find what value those are supposed to have, there's also the matter of whether or not it can still be repaired, because my uneducated guess is that those resistors burnt up because something tried drawing too much power and since they connect to that IC, I fear the IC itself may be dead, although it doesn't show any immediate shorts. Either that, or the transformer is dead which is even harder to find and not worth replacing.
So how exactly does this circuit work ? Since those are output pins, I'm expecting they provide the control signal for the FETs in the primary through an isolation winding of that transformer ? I never did understand why there's a single optocoupler on these ATXs....ATXs...is there such a plural ?
Why did those go up in smoke anyway ? Is there a schematic for this ? I'm asking because cheap stuff like this is often the same PCB under many brand names, so maybe I'll get lucky and someone will say it looks similar to another PSU for which they'd be kind to provide a schematic. Cheers.
So: the fan is stuck on this guy, fuse is intact, lots of bulging caps, no problems there...nothing out of the usual, but the main problem is those burnt resistors. There's 4 of them leading into various pins of that control IC...after a bit of searching, I was able to find its datasheet. It's a SDC2921.
The resistors go between the small transformer in the middle and this IC, more specifically to pins 7 and 8 - C1 and C2 which the datasheet tells us are "PWM open drain outputs". Aside from trying to find what value those are supposed to have, there's also the matter of whether or not it can still be repaired, because my uneducated guess is that those resistors burnt up because something tried drawing too much power and since they connect to that IC, I fear the IC itself may be dead, although it doesn't show any immediate shorts. Either that, or the transformer is dead which is even harder to find and not worth replacing.
So how exactly does this circuit work ? Since those are output pins, I'm expecting they provide the control signal for the FETs in the primary through an isolation winding of that transformer ? I never did understand why there's a single optocoupler on these ATXs....ATXs...is there such a plural ?
Why did those go up in smoke anyway ? Is there a schematic for this ? I'm asking because cheap stuff like this is often the same PCB under many brand names, so maybe I'll get lucky and someone will say it looks similar to another PSU for which they'd be kind to provide a schematic. Cheers.
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