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APC SMT-750 turns off for no reason

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    APC SMT-750 turns off for no reason

    Well guy's and gal's, after 6 years of flawless service, my APC SMT-750 developed a strange behavior. This UPS hooked up to my PS3, PS4, a media streamer, a network switch and a 40" LCD TV. My kids reported that the TV or the PS3 turns off, wait a few minutes and then one can go at it again and it works for days fine as kind. As it goes Friday night = movie night. And? Riiight, middle in the friggin movie, I heard a "click" and everything turned off that was connected to the UPS. Nothing had any power. My investigation and found out that the UPS itself was still on, but it had all power outlets turned off and there was a warning that there was no batteries inserted. Hmm... so I turned the outlets manually on UPS back on via the front panel. Now the front panel shows that the battery life is over 2hrs, no errors or anything else? So I unplugged it form mains and powered it on through the batteries. Again, no errors, everything is working but the "beep" sound sounded really funky (it does this also when plugged in), but the display is nice and crisp and yet it makes power at the outlets. I yanked out the batteries and had a look. I found two 10v 330uF looking pregnant and puking their guts out. I didn't take anything apart yet, in order too see if there are any other caps that need replacing too. Anyone else with a APC SMT series UPS that has bad caps and behaves strangely and replaced caps? Probably my chances to find a schematic for the early SMT series UPS is like nill.
    Last edited by CapLeaker; 12-08-2017, 08:39 PM.

    #2
    Re: APC SMT-750 turns off for no reason

    6years running 24/7?
    total re-cap time.
    6years x365 days x24hours = 52,560 hours.
    no cap is THAT good!

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      #3
      Re: APC SMT-750 turns off for no reason

      So it turned out there are actually 3 caps that are visually bad and measured bad. All other caps were fine. Those 3 bad caps are all made by IC, 25V 330uF, RZMM series caps. They are around that LM317 regulator. You can see them by taking the battery pack out, top left. Replaced those and the "beep" returned back to normal sound. So far so good. Hope that things stay that way. I guess I have to see what happens after running it for a while.
      Last edited by CapLeaker; 12-09-2017, 08:27 AM.

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        #4
        Re: APC SMT-750 turns off for no reason

        I always replaced all the crappy caps even in these new SMT units right on spot. Never trusted that crap. Guess I was right after all
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          #5
          Re: APC SMT-750 turns off for no reason

          A lil update about this APC SMT750 situation of mine. Those 3 caps cured all the problems I had with that unit. Everything is working well again as it should. There are no more glitches, it won't power the outlets off anymore by itself, no more weird error messages that disappear etc. All is good!

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            #6
            Re: APC SMT-750 turns off for no reason

            install a fan .. come back in 12 years . renew batteries before that of course ..p.s you may have given me the fix for one i sold and came back . same symptoms . i thought it might be getting too hot .

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              #7
              Re: APC SMT-750 turns off for no reason

              I kinda doubt somebody wants fanned UPS in a living room. Besides, the whole fan circuitry is usually missing. But if you have the fan circuit schematic, talk no more and give it here
              Less jewellery, more gold into electrotech industry! Half of the computer problems is caused by bad contacts

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                #8
                Re: APC SMT-750 turns off for no reason

                The small UPS's don't really need a fan, plus it would be annoying to have one running all the time. My SMT-2200 has a fan running all the time, but I have so much other things running in the office, one almost can't hear it anymore.

                To check these caps if they are pregnant is very easy. Take the battery pack out, turn the unit upside down with the battery slot facing you, then look at the bottom right. Can't miss them they are the biggest ones there.

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                  #9
                  Re: APC SMT-750 turns off for no reason

                  Never actually seen any of the big boys bad in Smart-UPS, always the small ones, usually 10 and 22uF.
                  Less jewellery, more gold into electrotech industry! Half of the computer problems is caused by bad contacts

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                    #10
                    Re: APC SMT-750 turns off for no reason

                    funny thing is, we have two of them identical to each other. The other one is fine as kind!

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                      #11
                      Re: APC SMT-750 turns off for no reason

                      If you mean the SMT, those are quite different boards though. I meant the old SU/SUA.
                      Less jewellery, more gold into electrotech industry! Half of the computer problems is caused by bad contacts

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                        #12
                        Re: APC SMT-750 turns off for no reason

                        these two APC UPS's here in this threat are both early SMT series.

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