Dell 2700w j728n 3kVa UPS not starting
Good day folks. Another tough one on our hands here I'm afraid, so please bare with my long story: my colleague asked me to have a look at this large DELL UPS which he had set aside for some years after only partly fixing it (from what he told me) before giving up. I have no idea what happened to it or what the original defect was. From what I can see, it must've had some sort of liquid or moisture buildup inside because the boards seem corroded and the insulation has peeled off many of the traces as a result of him cleaning the boards. Some traces even broke off because I can see solder and wires in many places - it's a mess and it's like mission impossible.
Where I currently stand: the battery pack has 97v on it (8x12v cells in series) and when a startup is attempted on battery alone, the DELL logo does appear on the display, I can hear some relay clicks, the fans spin for a while, but it doesn't stay on - it shuts down with no warning or anything after a few seconds and never makes it to the status screen....that's about it. Haven't tested it on mains yet, which is indeed an important test because it should run without the batteries in it and should reveal some hints as to where to troubleshoot, but I don't have one of those fat IEC20 (?) square leads for it yet.
What I found on my own so far: no documentation on this guy, so the closest I got was a similar 3kVA UPS which uses a similar topology and shares some guidelines. There's a PFC circuit, a booster circuit, an inverter circuit (a separate board in this case), a charger board riser module and a "SPS" riser. I can assume for now that the "SPS" riser is fine (NOT pictured yet): this is a small riser board which is a DC-DC converter to step down the 100v straight from the batteries and since (from what I can tell) this is the only path the current from the batteries can take to allow the UPS to come on, even briefly, it's functional.
My next candidate is the booster: takes the battery voltage and with the help of 6 FETs and the transformer, boosts it to 400v - this is not happening: I'm not getting anything at the "BUS" terminals which go to the inverter for further switching. Digging deeper here: I found some wires soldered to the riser board which holds a SG35225a IC (pictured+datasheet) which is responsible for running the FETs. These were soldered to certain pins by my buddy to allow him to measure with the UPS assembled because the inverter board flips up over the main one, making the job impossible otherwise. Pin 15 gets 11v on it as soon as you hit the power button, so it's OK. Pin 10 "SHUTDOWN" however never drops below 4-5v, so this IC is never going to start switching those FETs. In one of my posts about an oscilloscope, I removed this riser and messed around with it on the bench and indeed pulling pin 10 to GND causes it to show some waveforms on pins 11 and 14 (white wires in the pictures) so the IC itself is assumed functional at this point, especially since it's also been replaced at one point, along with the caps as you can see (not my work BTW). Unfortunately this means that the problem could very well be in the uC which may not be sending the "start" command and not assessing that pin LOW either due to an internal defect or some other fault it doesn't like...I shall dig deeper into the main board to at least see where the trace goes exactly - perhaps it's broken or has a series resistor that's gone open (the pull-up one is on the riser itself).
The "computer" riser is the biggest issue: multi-layer, all SMDs, dozens of interconnect pins to the main board...there's no way you're going to fix that at component level....
What do you guys think ? Anyone messed around with large UPSs before ? What would the most common faults be ? Let's at least discuss until I can provide more feedback. I shall return with more photos once I take it apart again.
Where I currently stand: the battery pack has 97v on it (8x12v cells in series) and when a startup is attempted on battery alone, the DELL logo does appear on the display, I can hear some relay clicks, the fans spin for a while, but it doesn't stay on - it shuts down with no warning or anything after a few seconds and never makes it to the status screen....that's about it. Haven't tested it on mains yet, which is indeed an important test because it should run without the batteries in it and should reveal some hints as to where to troubleshoot, but I don't have one of those fat IEC20 (?) square leads for it yet.
What I found on my own so far: no documentation on this guy, so the closest I got was a similar 3kVA UPS which uses a similar topology and shares some guidelines. There's a PFC circuit, a booster circuit, an inverter circuit (a separate board in this case), a charger board riser module and a "SPS" riser. I can assume for now that the "SPS" riser is fine (NOT pictured yet): this is a small riser board which is a DC-DC converter to step down the 100v straight from the batteries and since (from what I can tell) this is the only path the current from the batteries can take to allow the UPS to come on, even briefly, it's functional.
My next candidate is the booster: takes the battery voltage and with the help of 6 FETs and the transformer, boosts it to 400v - this is not happening: I'm not getting anything at the "BUS" terminals which go to the inverter for further switching. Digging deeper here: I found some wires soldered to the riser board which holds a SG35225a IC (pictured+datasheet) which is responsible for running the FETs. These were soldered to certain pins by my buddy to allow him to measure with the UPS assembled because the inverter board flips up over the main one, making the job impossible otherwise. Pin 15 gets 11v on it as soon as you hit the power button, so it's OK. Pin 10 "SHUTDOWN" however never drops below 4-5v, so this IC is never going to start switching those FETs. In one of my posts about an oscilloscope, I removed this riser and messed around with it on the bench and indeed pulling pin 10 to GND causes it to show some waveforms on pins 11 and 14 (white wires in the pictures) so the IC itself is assumed functional at this point, especially since it's also been replaced at one point, along with the caps as you can see (not my work BTW). Unfortunately this means that the problem could very well be in the uC which may not be sending the "start" command and not assessing that pin LOW either due to an internal defect or some other fault it doesn't like...I shall dig deeper into the main board to at least see where the trace goes exactly - perhaps it's broken or has a series resistor that's gone open (the pull-up one is on the riser itself).
The "computer" riser is the biggest issue: multi-layer, all SMDs, dozens of interconnect pins to the main board...there's no way you're going to fix that at component level....
What do you guys think ? Anyone messed around with large UPSs before ? What would the most common faults be ? Let's at least discuss until I can provide more feedback. I shall return with more photos once I take it apart again.
Attached Files
Amazingly the display was still running just fine and displaying some error message, despite everything being up in cloud of smoke so if there's one thing that survived is the charge module (the large power-brick riser that I pictured). To the left of that you can also see the small SPS module. The bridge rectifier, IGBT and two diodes are also OK (on the opposite side of the heatsink).
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