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UPS Repair

Discussion in 'Troubleshooting and Repair' started by eKretz, Jun 14, 2013.

  1. eKretz

    eKretz

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    Apr 8, 2013
    OK, I got a UPS from the local school corporation that they were going to toss out. Eaton Powerware 3105. I have found by checking the board that it's only getting ~8VDC where it should be getting 12. This in turn seems to be causing a relay not to work. The thing is, I couldn't figure out where the heck the DC voltage was supposed to be coming from.

    No rectifier in sight anywhere. Upon closer examination I found the DC voltage leads traced back to the big honking transformer!! Do they make transformers with built-in rectifiers? I had no idea there was such a thing. So if I'm only getting 8V out, I probably have a damaged rectifier or shorted transformer winding, eh? Can these be serviced at all? Or is the only fix a replacement?

    There are (2) 16AWG 120V input wires, a black and a white; (4) 10AWG wires, a red and black going to the backup battery (with transformer hooked to 120VAC and backup battery disconnected, I'm getting 3VAC output on these, short?), and a black and white going to 2 sets of paralleled 9972GP FETs, with 120VAC into the transformer and backup battery disconnected, these wires output 16VAC. There are also (3) more 16AWG wires coming out, a red, black and white. As near as I can tell, the white and black are carrying AC @ 8V each, and the red is a common or ground. Both white and black check 8VAC w/respect to the red, and 16VAC with each other. I'm thinking they should be 12VAC and combine for 24? I'm digging for a schematic now. Edit: looks like the red should be carrying 12VDC, it has 8VDC with the backup battery plugged in, 0V without. These wires are at J5 on the schematic. Looks like CT1 shows where the DC is coming out of the transformer.

    OK, found a schematic, here's an image of it:

    [​IMG]
     
    Last edited: Jun 14, 2013
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    What is the open circuit voltage on the battery?
     
  3. eKretz

    eKretz

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    Apr 8, 2013
    12.6VDC last I checked. It just died in a storm last night.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    OK, I was just wondering if the battery had failed and that was the cause.

    Is there another page of the schematic which shows the source of some of those signals?
     
  5. eKretz

    eKretz

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    Apr 8, 2013
    Nope, that was the first thing I checked. I'll try and find some more schematics. That's all I've been able to find so far, it does seem a little incomplete.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    There are a number of places which are labelled "12V" and "12V switched". It seems reasonable that with repect to the -ve terminal of the battery, these points should be at the battery voltage.

    I would check to see if the battery negative terminal is connected to the circuit ground.

    In about the middle of the left hand side of this circuit there is Q2 (MPSA56) which appears to switch the 12V. You might check the voltage on the collector of this. I can't see how it can be much higher than 0.6V less than the battery voltage. If it is substantially lower, there may be a fault.

    Failure after a thunderstorm is never a good thing.

    I would be checking my home insurance to see if I was covered for stuff like this.

    I might also take a close look at the MOV across the incoming power (top left of schematic).
     
  7. eKretz

    eKretz

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    Apr 8, 2013
    I think I did check that the battery neg was connected to ground and it was not. (It shouldn't be, right?) I'll check Q2. I did take a look at the MOV's and they looked fine visually, as does everything else, but I'll make a more in-depth check soon. I thought the same about the 12V labelled points, and checked many of them. They were almost all reading ~8V or thereabouts with the battery plugged in, and 0 without it. One reason I'm wondering about the transformer is that even when I removed it from the circuit completely and ran it from 120VAC, it still didn't put out proper DC voltage from the +12V wire, which it should since it has a built-in bridge, shouldn't it? The battery should only be feeding the 12V into the circuit when there's a power cut I would think.

    The item is not covered by my home insurance, as it wasn't mine. It was given to me by the local school corp as my mother works there in the main office that reigns over all the schools and asked their IT/Tech/Maintenance guy if they would give me any stuff they were sending for recycle or scrap, so I could try to fix it or use some of the spare parts. I have a bad back and can't work full time anymore, so I am trying to start learning electronics repair a bit more in-depth so I can maybe earn a few bucks here and there or use some of the stuff I get since I can't really afford to buy much anymore. I would like to use this for myself if I can get it working again.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    OK, I should have warned you that UPS's are dangerous. Obviously they can have mains voltages even when unplugged. If the battery is connected, always treat it as live.

    If you're talking about the output of CT1, that's a current transformer and it doesn't supply power. It exists to monitor the load on the output AC rail.

    If you're talking about the secondary of T1, then 8VAC is about what I'd expect -- although in this configuration it would probably be 16V centre tapped.

    I assume you've tested all the fuses and that none of the mosfets are shorted drain to source?
     
  9. eKretz

    eKretz

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    Apr 8, 2013
    Yup, fully aware that everything in there can be dangerous. The fuses and FETs were one of the very first things I checked. I just checked the MOVs also and they all check open. Hopefully I'll get to investigate a little more later today.
     
  10. eKretz

    eKretz

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    Apr 8, 2013
    OK problem solved. I found one bad cap, which I replaced the day I got it, and a forked over backup battery. When the battery was measured initially, it read 12.6V. Today I checked it again and it read 10.8V. Put it under load and it dropped down under 6V. Apparently it must have been dropping to 8V under load when I was checking voltages in circuit yesterday. I put in a spare 7.2AH SLA and it fired right up.
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Very good.

    I asked you what the open circuit voltage was on the battery. I should have followed up by asking what it was when the UPS was trying to start. (Doh!)
     
  12. eKretz

    eKretz

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    Apr 8, 2013
    Yup, I should've checked it too. I had already checked the OCV and thought the 12.6V meant it was OK. Learned my lesson on that one.
     
  13. eKretz

    eKretz

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    Apr 8, 2013
    Sure. It's tough to find schematics for UPS's actually, I was lucky to find that one. That UPS is not the greatest, as it's one of the non-sinusoidal waveform output models (modified square wave output), but it works in a pinch.
     
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