Broken UPS - faulty power supply?

[Khaveer]

Hello. I'm trying to repair an APS UPS (Back-UPS CS500) that seems to have a power supply problem. After connecting it to the mains the "on battery" led starts flashing rapidly. Nothing happens when I try to press or hold the power button. I measured the voltage rail used by most of the logic ICs and it turned out to be at around 2,7 V. Probing it with an oscilloscope revealed that there's some significant ripple. Now here's when it gets weird. I managed to get it working once. I have no idea how. I unplugged it, then after a few minutes plugged it back in and it just turned on and beeped at me because it had no battery connected. I turned it off, connected the battery, plugged it back in and it was dead again. I have no idea how to continue with troubleshooting. I can't find the schematic anywhere on the internet.

[wraper]

Replace all of the electrolytic capacitors (except SMT one as it is of the good brand). APC is well known for using junk capacitors. The fact you once got it to start proves that the fault should be in the caps.

[bookaboo]

+1
I think we need to update the goto technical response:

(1) Have you tried turning it on and off again?
(2) OK replace all the electrolytics.

[wraper]

From my not too big experience, if I repair low end APC which is at least a few years old, it's not uncommon to find that half of the electrolytic capacitors are dead already. Others obviously would fail soon too if left unreplaced.

[Khaveer]

Thanks for suggestions. I didn't consider replacing the caps because they look fine on the outside and I don't have an ESR meter to measure them. They're made by Jamicon, so I'll replace them and see what happens. Unfortunately I have to wait till monday to buy new ones.

[wraper]

If those are Jamicons, it's actually not that bad. Usually they use awful JH capacitors. Then quiet likely only C25 and C26 could be faulty. Though replacing all of them wouldn't hurt anyway.

[Mechatrommer]

APC is well known for using junk capacitors.

and junk design. my low end 625 floats the battery on Mains...

[Khaveer]

I have replaced all electrolytic caps and it's the same.

Update:
I just noticed starting it with the transformer disconnected, turning it off, connecting the transformer and turning it back on makes it turn on properly. After pressing the power button located on the front of the unit the green power led turns on and replace battery led starts to flash. The unit beeps twice every few seconds.

[TheElectricChicken]

argh, I'm too late to save you replacing the caps. Usually it's something simpler. As you get older, you're more likely to completely rebuild equipment because it is plugged into a faulty power point.

That beeping is normal, it's the UPS's way of saying there is a power failure. Most of them do it, and you can download a pdf user manual from the net and find out how to tell it not to do that.

The other thing I would suggest for this would have been to charge the batteries on a different charger then try it, being careful that it can make lethal power on it's own.

[Khaveer]

I forgot to mention that to make this thing turn on I have to reconnect the transformer every time I connect it to the mains. I checked all the solder joints.

[Ampera]

If you still can't find the issue, then just plop a switch on to the transformer and call it a day.

[Khaveer]

According to apcupsd the UPS thinks that it has no battery connected, but it shows battery charge properly. There battery itself isn't dead and it's charging the way it should be.

[TheElectricChicken]

According to apcupsd the UPS thinks that it has no battery connected, but it shows battery charge properly. There battery itself isn't dead and it's charging the way it should be.

Just like me, it's lost it's sensors ! 

[Khaveer]

I think I've found a schematic for this model. I will verify it as soon as I get home.
https://dl.dropboxusercontent.com/u/55507051/schematics/backcs350_500.pdf

Update: Nope, that's another model/revision. There are some major differences

Update 2: It turns out the power on problem was caused by a dodgy solder joint. I resoldered every THT component and now it turns on wihout any problems, but it still doesn't like it's battery.

Update 3: I tried connecting it to an external 12V power supply instead of a battery and turning it on without mains connected. It makes a very loud buzz noise and turns itself off.