Electronics > Repair

APC Smart-UPS 1500 (SMT1500)

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Hi there. Occasional lurker, first time poster.

To start, I do have experience with electronics and repairs, but I wouldn't consider myself advanced. I know stuff, but some of what I come across here is totally lost on me. My day job is IT (mostly help desk) and my side hustle is repairing pinball machines.

Ok, so the reason I'm here... I recently purchased an APC Smart-UPS 1500, model SMT1500, on Facebook Marketplace. The seller said it worked fine but would probably need new batteries "soon," so I couldn't fully test it. Well, "soon" really might right away because they're dead, so I bought a set of aftermarket replacements. I installed them and, aside from appearing to charge curiously fast, everything seemed fine.

Alas, everything was not fine. Once the battery reached 100%, the unit threw an "output over volt" error and shut down. This happened no matter what I did or what settings I changed. Strangely, during one test I managed to measure approximately 123 or 125 volts (I can't recall which) in the few second window before it shut down... certainly didn't seem over voltage to me. I also found that it works perfectly fine with the batteries disconnected, but together the batteries measure around 27 volts.

Obviously the guy still says it worked fine for him and won't give me a refund... so I figure I have little to lose and take it apart. I found a section of very charred components on the mainboard. One piece was so fried it just fell out and I have zero clue what it was.


* Does this seem repairable or does the board look too far gone?
* If feasible, what components should I replace?
* Alternatively, does anyone have a clue whether I might be able to swap the entire board (640-3081A_REV06) for one from the rackmount version of this UPS (640-3081_REV03)? They look very similar aside from the battery connector.
I think that's all I have at the moment. Hopefully this thing isn't a lost cause.

I can try to answer some of your questions.  First, I would start by cleaning that board up.  It probably looks a lot worse than it actually is.  So clean it up the best you can.  Most of that soot will just wash off and then you will get a better idea of what might be damaged.

It looks like a MOV is likely the damaged/charred component.  If it is a MOV, and it took the brunt of the damage, it is very likely that the rest of the unit could be okay.  That is if this blew due to an external overvoltage event.  If it was caused by something internally, then it is only a secondary symptom of a primary fault.  You'll have to do some investigating to find that out.  But at first glance, this appears to be repairable.  Do not let all the soot/charring scare you. 

The battery voltage you measured seems fairly normal for these large 12V batteries in series.

Do some cleaning first, and then get back to the forum with some pictures once it is all cleaned up.

You're right, I definitely should've cleaned it up before posting. I agree that it likely appears worse than it is since I was able to wipe soot off that big yellow capacitor.

I thought that very charred/melty bit might be a varistor. I'm pretty sure there's also a missing component that fell out right below that and to the right of the yellow cap. I think it might be another varistor based on pictures of a board that sold on eBay a few months ago: eBay auction: #204631885269.

I didn't mean to suggest the batteries measured high, if it sounded that way. Reading it again I can see how it might be a little unclear.

I'll clean it up this week, maybe tonight, and post a new pic.

I had a chance to clean it tonight and the board looks pretty decent. That big yellow cap was ruined so I just cut it out to make it easier to get in there. Looks like a few wires should probably get replaced as well.

Your first photos look catastrophic, but most of that is just soot.
The clean-up photos reveal it's not as bad as first thought.
From what I can see, one of the MOV (MV4 - the charred one inside the heatshrink) has taken a hit.
This could be due to a lightning strike or there's been one hell of a surge on the mains.
I would also check the other MOV to see if any of them have gone short as well.
As to whether you can swap out the board from a rack mount unit to this one, well the part numbers appear the same (apart from the revision) so it's possible.
You would be better off leaving the rack mount unit intact and just trying to repair this one.
Given that the unit is throwing up an over-volt indication, it's likely whatever took out that MOV is responsible for this indication and there
may be more failed components than what is initially visible. It may have damaged the line voltage sensing circuitry.
You mentioned that the batteries measured 27 volts together, that's about right - assuming your UPS runs the batteries in series for 24 volt operation. Many do.


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