Chosen Solution

I found an older version of the APC Smart-UPS 600 UPS (delivers up to 600 VA or 400W, or at least so it claims). I don’t actually know how old it is, but the printing on the PCB says that it’s from revision 5A, designed in 1990. The old batteries that were inside were manufactured in 2005. Any way I look at it, it’s not exactly new. I’ve attached a picture from the front so you can see what buttons it has.

After putting in a brand new pair of batteries, I switched it on, and I could hear the inverter doing its thing. So far, so good. However, I couldn’t make the UPS switch from battery operation to on-line operation. I tried waiting for 5-10 minutes (in case it did a self-test this long), I tried pushing the lower-right ON button, then the lower-left ON/OFF button, then both simultaneously; I tried pressing them, then pressing and holding. I tried the Test button, though I wasn’t expecting it to work (in order to test batteries, you should be in on-line mode in the first place). Nothing did anything. My first idea was that the input rectifier was shot. I identified 4 diodes right next to each other (the PCB calls them D16 to D19), and they all seemed healthy. I also tested any fuses I could see, and they are all good. For good measure, I also tested the output semiconductors; as expected, no problems there. I’m kind of lost on where to look next. There are a couple of relays on the board and both of them seem to be acting janky (see next picture; I added a few extra labels that I’m not sure about, but which are probably right), but without a schematic and a description of the relays I can’t know if I’m barking up the wrong tree. Should I just replace the relays and see if the problem persists, then go from there?

Thanks a lot in advance!

Hi @enakistehen , It’s a bit problematic testing the resistance of the relay coils whilst they’re still connected to the board as you don’t know what else may be connected to them that could affect the resistance reading. The only way to truly know is to unsolder them from the board and then measure them. Here’s the user manual for the UPS that will help. It details how it operates from a user’s point of view. At least you will know what it should do and indicate under various conditions, even if it isn’t at the moment. Here’s the schematic (I think) that will definitely help you to find out what’s going on. Hopefully it is the correct one.