Xbox One power brick repair
Good day folks. I had a bit of a discussion with a colleague of mine today (G - you may remember him from one of my other rambles about reballing) regarding a power brick for an Xbox One which he doesn't quite manage to fix, but is stubborn enough that he doesn't want to A) give up and B) accept other peoples' suggestions...of course...who would listen to the "greenhorn" noob in a tech department ? 
Unfortunately I don't have any pics of it and it turns out finding resources on these bricks is harder than I originally expected. There appear to be different versions or revisions as the pics I was able to source online don't quite match what we've got. If memory serves, it looks similar to this one here, but it's still not identical, especially since ours has a "LTA...something" SMD IC on the bottom, which appears to be the brains of the whole secondary, like a monitoring IC in an ATX supply. There's also 2 (or maybe even 3) photocouplers instead of one and no solid state caps on the output at the right side there, so yeah, not quite there, but still close enough....
The problem is that there's no 12v output. 5v Stand-by is present and the orange light stays on, but the Xbox doesn't turn on (as far as I could make out from his brief explanation). The first conflicting argument with good ol' G was about actually starting the thing: like an ATX supply, Xbox bricks need to be turned on to tell the 12v rail to actually come on - simply plugging it in does nothing. He was aware of this too, but he insisted the blue "PWR_ON" wire should be grounded like an ATX supply, but I know for a fact that it's the other way around: it's supposed to be pulled HIGH to enable the supply and LOW to turn it off (like in most TVs). He reluctantly agreed to try it my way and soldered a jumper between the 5v output and the blue wire (not sure if he actually soldered it correctly, since I didn't check myself up-close). The orange light came on briefly when the supply was plugged in but then immediately faded away to nothingness, as if the plug had been pulled and the caps were discharging. Still no 12v of course. He also went back to trying it with the PS_ON pin shorted to GND which is never going to work and he might end up scrapping a potentially fixable device, but good luck explaining that to a 60 Y/O "veteran"......so I just left He blamed it on that LTA IC on the back which we couldn't source locally to try a replacement, which I don't entirely dismiss but still doubt is the cause.
Another argument was brought about by those two TO-220 devices located on the heatsink at the far-right (where the caps are). He says they're transistors and even claims he found one shorted which he replaced. I however disagree and know for a fact in a SMPS, those devices which immediately follow after the transformer are going to be diodes and their outermost leads are shorted on purpose since they're the anodes which rectify the same output on the transformer. Granted, as far as I can see in those pics, they're using them slightly in reverse here: GND goes through the center pin and comes out the outer ones and it's actually +12V "HOT" which goes the output wires straight off the transformer - usually it's the complete opposite. I can also see a zener (ZD301 and ZD302) and a resistor going between the center leg and one of the outer legs on those TO-220s.
There's also the matter of the gray "RSENSE" wire which goes to the connector but doesn't actually have an output pin. As far as THIS article explains, I assume one end of the resistor goes to the 12v "HOT" output wire and the other end to the gray wire. Inside the adapter, the gray wire is the "high" side of some sort of resistor divider feeding a comparator. If the droop is too high, the supply pumps more voltage to compensate and vice versa.
Now what would some common failures, beside caps, be in supplies like these ? Any ideas are welcome. Thanks for listening to my rants and a good day to you all !
Unfortunately I don't have any pics of it and it turns out finding resources on these bricks is harder than I originally expected. There appear to be different versions or revisions as the pics I was able to source online don't quite match what we've got. If memory serves, it looks similar to this one here, but it's still not identical, especially since ours has a "LTA...something" SMD IC on the bottom, which appears to be the brains of the whole secondary, like a monitoring IC in an ATX supply. There's also 2 (or maybe even 3) photocouplers instead of one and no solid state caps on the output at the right side there, so yeah, not quite there, but still close enough....
The problem is that there's no 12v output. 5v Stand-by is present and the orange light stays on, but the Xbox doesn't turn on (as far as I could make out from his brief explanation). The first conflicting argument with good ol' G was about actually starting the thing: like an ATX supply, Xbox bricks need to be turned on to tell the 12v rail to actually come on - simply plugging it in does nothing. He was aware of this too, but he insisted the blue "PWR_ON" wire should be grounded like an ATX supply, but I know for a fact that it's the other way around: it's supposed to be pulled HIGH to enable the supply and LOW to turn it off (like in most TVs). He reluctantly agreed to try it my way and soldered a jumper between the 5v output and the blue wire (not sure if he actually soldered it correctly, since I didn't check myself up-close). The orange light came on briefly when the supply was plugged in but then immediately faded away to nothingness, as if the plug had been pulled and the caps were discharging. Still no 12v of course. He also went back to trying it with the PS_ON pin shorted to GND which is never going to work and he might end up scrapping a potentially fixable device, but good luck explaining that to a 60 Y/O "veteran"......so I just left
He blamed it on that LTA IC on the back which we couldn't source locally to try a replacement, which I don't entirely dismiss but still doubt is the cause.Another argument was brought about by those two TO-220 devices located on the heatsink at the far-right (where the caps are). He says they're transistors and even claims he found one shorted which he replaced. I however disagree and know for a fact in a SMPS, those devices which immediately follow after the transformer are going to be diodes and their outermost leads are shorted on purpose since they're the anodes which rectify the same output on the transformer. Granted, as far as I can see in those pics, they're using them slightly in reverse here: GND goes through the center pin and comes out the outer ones and it's actually +12V "HOT" which goes the output wires straight off the transformer - usually it's the complete opposite. I can also see a zener (ZD301 and ZD302) and a resistor going between the center leg and one of the outer legs on those TO-220s.
There's also the matter of the gray "RSENSE" wire which goes to the connector but doesn't actually have an output pin. As far as THIS article explains, I assume one end of the resistor goes to the 12v "HOT" output wire and the other end to the gray wire. Inside the adapter, the gray wire is the "high" side of some sort of resistor divider feeding a comparator. If the droop is too high, the supply pumps more voltage to compensate and vice versa.
Now what would some common failures, beside caps, be in supplies like these ? Any ideas are welcome. Thanks for listening to my rants and a good day to you all !
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