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| Need help with SMPS / MA3810 chip |
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| nuno:
Ok, just to let you know I've able to fix the board. There was a open 1.5M resistor on the PSU, R34. In the circuit it measures much lower than this, so it's not possible to test it without isolating a leg (I usually do it by cutting a track). This resistor biases the power switch (MA3810 pin 3) to V+ (rectified mains, so, where I live, some 330V). These A/C boards have been beaten to death by the weather, so I try hard not to remove any components for testing or doing any hard intervention. When everything else fails, I start using the physical test; I slightly bend the board in several directions, I pull and push a bit on some components, trying to find an intermittent fault. That's when R34 made contact and the PSU came up. Replaced it and it's back to work. The resistor is visibly corroded at the ends, although still mechanically stiff. In attach my partial PSU reverse-enged schematics; sorry about the hand-cad :D . Don't trust your live on it. The V+ node then goes to a 7812, etc. See what I believe (from part datasheet and my measurements; still don't know why doesn't seem to be a diode at pins 1-2) to be also the internal diagram of MA3810 in this post above. --- Quote from: nuno on April 09, 2020, 04:07:32 pm ---Same problem here, also by water damage. PSU doesn't come up. Everything looks ok except an apparently damaged (open) diode inside the PSU IC (VD1 in the attach). I bypassed it with an external diode but it still doesn't "convert". Voltage gets there. Anyone with further information? I found the internal schematics of MA2810 and I assume the MA3810 is similar just different power handling (similar datasheets; datasheets are incomplete, no pinout nor internal diagram :palm:), (reverse enginneered) schematics makes sense with such internals, see MA2810 internal diagram attached. Can't find any manual, schematics, whatever for this A/C model; can find for the newer models but not for these older ones |O Unfortunately this PSU has also several outputs with other voltages and opto couplers and other stuff, I haven't looked into it in detail, which doesn't make the task of replacing it with something "off board" easy. --- End quote --- |
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