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Old 10-02-2016, 08:39 AM   #1
Dannyx
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City & State: Constanta
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Question LG 32LX2R only 5V standby voltage (YP2632t PSU)

Good day folks. As the title says, I've got this YP2632T power board from an LG TV that won't power on. Couldn't find the schematic, so that would be greatly appreciated as well. The standby LED comes on, but that's about it....I removed the power supply and tried turning it on 'manually' by applying 5V to the ON/OFF pin (standard stuff): the relay clicks and the main cap C102 charges up to around 400V, but the secondary stays dead: no volts other than STBY. I could tell no "switching action" was going on because I couldn't hear anything AT ALL (SMPSs usually make a slight hissing sound when operating).
Many sources suggest replacing every single cap on the secondary, but I find it hard to believe that it would cause the board to turn on, since the problem is somewhere in the primary I believe. I DID replace the 3 small 47uF caps (C105 thru 107) that are part of the startup circuit and the 5VSB ones (C221-C222), but still nothing. SOMETIMES, I'd get 6v on the 12v bus, but nothing on the others, so it's like "half-working". I checked the 3 ICs in the primary: IC101, IC302 and IC303. The first 2 are FAN7601 MOSFET drivers. The last one (on the right in the picture (IC303) is a MIC4423. The first FAN (IC101) seems to operate normally, as it drives the standby circuit which puts out 5v accordingly. The second FAN (IC302) does get 10.4V on its VCC pin, but I don't know what it's supposed to drive after that, since its OUT pin doesn't go to one of the 2 remaining FETs like I expected, instead, it goes through a diode (D307) to the VCC pin of the MIC (IC303). Well, that pin is dead (no power). It seems to me the OUT of IC302 is rectified by that SMD diode and used as the supply for IC303...what a freak setup >_> This is also why I believe IC302 might be faulty and not putting out anything on its OUT pin, which in turn prevents IC303 from starting the switching action. Another mystery is how the second MOSFET (Q102) is driven, since its gate pin doesn't seem to go to any of the ICs - it loops back to the source through some resistors and diodes :|
To further make this more annoying and complicated than it should be, it's one of those bullsh!t double-sided boards which are extremely hard to follow and you have to keep flipping it around It seems a fairly common problem, but mine's a bit different that others I found. Any thoughts ? Let me know if you need close-ups of any other part of the board.
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