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Old 06-04-2021, 09:02 AM   #1
Dannyx
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Join Date: Aug 2016
City & State: Constanta
My Country: Romania
Line Voltage: 230VAC 50Hz
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Posts: 3,131
Question APC Smart UPS 620 not working (overload light on)

Good day folks. It feels like absolute ages since I've posted anything here, mostly because I don't get anything interesting to work on here (a-holes keeping it all to themselves >_>), so I have to come up with my own ways of making time fly by quicker, so here's something: I found this APC UPS which seems decent enough to keep, or at least to keep me entertained and awake as I attempt to fix it.

The board looks clean - no visible damage whatsoever. I also managed to find the schematic for it, so I don't go around blindly replacing all the caps, which I doubt is the cause of failure to begin with.

I didn't dare plug it into the mains, but it should work on its own off the battery, according to the manual, so I used a fresh one to test. When I hit the button, I get a beep, I hear some relays closing and that's it. After a little while, the relays go off again. You can try doing this all day long with no change. Occasionally, the overload LED will come on and it will also beep continuously, but I've yet to figure out if it happens at random or there's a pattern to this LED coming on. I also don't hear any specific noises that would indicate the inverter part is working: no buzzing or humming from the transformer and no squeals or ringing noise from the board.

To try and cut the story somewhat short and skip the routine procedures I always go through when diagnosing, like look for shorts or burnt components, I THINK the problem lies somewhere with U9 on page 6, as it appears to be responsible for driving the 8 transistors (4 in my particular unit) that in turn switch the transformer to generate an output. No U9 means no output. Unfortunately this is where I got stuck. There's no info on this part, so I don't know how it operates. I don't have a scope, so I have no way of checking if U9 is doing anything or not. It gets its supply on pin 1 as it should, no problem there. It seems it's driven directly by U1, the MCU, on pins LSB and MSB, though I have no way of checking if this actually happens or not. Then there's that -8v rail below it. If I measure it with respect to GND (the negative battery terminal), I get around -10v on C50 (that's the negative leg before D28). If I put the red probe AFTER D28, I only get around -4v...not sure what this means and what this voltage is for. I'll probably try pulling out C49, though every cap I've tested so far seems to be within spec, since I tried several of them, hoping they're read out of spec, but ended up putting them back on the board, since they checked fine on my capacitance meter.

This might be a good time to bring up that I'm not sure if the capacitance reading can show up fine on a regular multimeter, but the capacitor may still be out of whack enough to cause malfunctions somehow (like ESR ???). I don't have an ESR meter at work, though I'll bring my own from home if it turns out I've exhausted every possibility....

I took a lot of different measurements all over the board, both with and without the battery, but it would take WAY too much space to list them all, so let's discuss as suggestions come in. Any thoughts would be greatly appreciated.
Attached Images
File Type: jpg 20210604_160323.jpg (690.5 KB, 14 views)
File Type: jpg 20210604_151759.jpg (807.4 KB, 6 views)
Attached Files
File Type: pdf apc640-0232.pdf (143.1 KB, 13 views)
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