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How I Fixed the Beeping Problem of My 'APC Back-UPS ES 700'

Lately, my APC Back-UPS ES 700 was crippled by the problem that seems to kill most of its models: after a surge event, instead of switching to battery, it started to continuously beep. For a while, I could restart it several times and the problem would go away, but in the past few days, nothing would make it turn on again without constant beeping.

Warning
Don't take anything in this article as advice, I'm not an electrical engineer. I did this at my own risk. Opening your UPS exposes you to the danger of electrocution and can cause fires if handled incorrectly.

Before opening

First of all, I wanted to make sure that the beeping is not caused by a normal condition. I checked the user manual's Status Indicators section for Constant tone audible indication.

It was not a normal condition, so it was time to take it apart.

Before opening it up, I unplugged it from the 220V outlet, disconnected the battery and left it sitting for about an hour for any caps to have a chance to discharge.

Inspecting the insides

Once opened, I took a picture of its connections, as there are plenty of them and I wanted to remember how to put it back.

Picture showing the connections on the main circuit board of the UPS

After a quick visual inspection of the main board, it was clear that no parts were blown. Next, I checked on a hint from a YouTube comment, that someone was successful in fixing it by replacing some bad capacitors, but he/she didn't go into any more detail than that.

Time to take out the ESR Meter. This will allow me to determine if a capacitor might be bad without taking it out of the circuit.

Measured all capacitors and sure enough, found 4 caps that were dead as a dodo. I've circled them in red in the following picture.

Note
The capacitors in the picture below are already changed with the good ones, as I forgot to take a picture beforehand.

Picture showing circles around the capacitors that I've changed

The fix

The fix was pretty obvious by now: I needed to replace the bad caps with good ones.

All 4 bad caps were the same: capacitance 22uF and voltage rating of 25V made by Jamicon.

A close-up picture of one of the bad capacitors

I replaced them with better-quality Nichicon capacitors as I didn't want to replace them again anytime soon.

I also made sure to put them back in the same orientation, as these electrolytic capacitors are polarized. The polarity of the caps is indicated by a solid line on the side with the negative lead.

After putting it back together I realized that this also fixed another problem: for as far as I can remember, like 5 seconds after starting the UPS, it started buzzing loudly for a while. Now I can barely hear the buzzing.

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