APC Smart UPS SUA1500 does not detect AC / doesn't turn on

[CapLeaker]

Or the sensing circuit has some kind of issue or the MCU has a issue that could be causing the issue but how to troubleshoot the issue is the real question I not even sure where to begin if I were doing this repair this is the reason I paying close attention to this post to see if he can figure out what wrong with it

The way on how to go about this is already explained. Basically I have to get it into a working powered state, then figure out all the permissive crap being good and going to the MCU. Now if everything is good to the MCU and it still doesn't work I am shit out of luck, unless I replace the MCU. And exactly this is what I am afraid of a bad MCU, as I already went over the permissive circuits and detection circuits all the important diodes, transistors, Fet's, and some resistors too. So far besides that SOT-23 diode I found squat.

[CapLeaker]

Hmm... not sure if I found something or not. The site fault wiring circuit works, but at the same time IC20 the opto gives weird readings if I poke around on it. Not sure if it is due to this specific optocoupler being used, but it doesn't react like a normal one.

SFH6156-4

[CapLeaker]

scratch my last post. It is the nature of that beast. But my original blown diode D13 went again. but this time fully open.

[redwire]

You mention hearing a relay click for a second. It's doing a self-check. Relays off - no mains seen, relays on - mains seen etc.
The transfer relays seem weird - it's using two in parallel RY5, RY4 for Line, RY7, RY6 on Neutral. One must make, the other break or something, but the contacts are in series as well for safety.
I wonder how sophisticated the self-test is. Again, with mains applied and some kind of load, I would see why it's not making it through. Trace the path.

The blown diode might be from insulation failure or arc from contacts to coil on that relay. There are MOV's on either side but not in the middle, so a spike from the VR or UPS output maybe caused this. Just a guess.

[CapLeaker]

You mention hearing a relay click for a second. It's doing a self-check. Relays off - no mains seen, relays on - mains seen etc.
The transfer relays seem weird - it's using two in parallel RY5, RY4 for Line, RY7, RY6 on Neutral. One must make, the other break or something, but the contacts are in series as well for safety.
I wonder how sophisticated the self-test is. Again, with mains applied and some kind of load, I would see why it's not making it through. Trace the path.

The blown diode might be from insulation failure or arc from contacts to coil on that relay. There are MOV's on either side but not in the middle, so a spike from the VR or UPS output maybe caused this. Just a guess.

I did. The problem is the output relay isn't getting the command on the coil for it to be energized. All relays besides the output relay need 24V from the battery, otherwise the UPS doesn't work. Check. ✅
As soon as I plug it in or turn the inverter on, I have 24V on the output relay coil. I also have AC waiting for it to be switched via the relay. The path of AC power through the relays isn't the problem.
I am not sure why that diode D13 went totally open, maybe I damaged it myself when I tried to fire the relay up with 15V instead of 24V.
The main IC isn't dead. After a brain dead, plugging in the battery and AC power, it beeps every time I press the ON button. It is some sort of a permissive problem or the MCU doesn't see any AC mains and thinks it is unplugged all the time.
As a temporary measure I swap the diode out wit something more common over 100V rated and just solder it directly over the relay coil on the back of the board. A 1N4002 or better should be good just fine.

[redwire]

The output relay is normally always on (R5/Q2) and the MCU gives the SHUTDOWN command (Q1) to turn it off.
So why is the MCU not enabling the mains output? What does AC-OK signal look like?

[CapLeaker]

The output relay is normally always on (R5/Q2) and the MCU gives the SHUTDOWN command (Q1) to turn it off.
So why is the MCU not enabling the mains output? What does AC-OK signal look like?

I replaced D13 with two regular 1N4148 signal diodes. I did do a couple of measurements see attachment. I did notice something today while I tested in with AC. One time when I plugged it in I saw for a split second all LED's light up. It still behaves the same way. Short press on button and I get a beep, but nothing happens. Here is something new and I don't know if it was there before and just didn't hear it. I hear little continuous arcing somewhere where the 2 little transformers are on the main board.

[redwire]

AC-OK signal is asserted high, looks OK to me. The LED's coming on might be part of its power up/self-check sequence, an LED test then it sees something confusing with its inputs and shuts down.
If you hear something sizzling, I would go after it, it might be a poor connection on the transformer bobbin (crappy soldering) or just gently tap a plastic pen on things to see if you can flush it out. Even a relay could be fizzling inside.

[CapLeaker]

AC-OK signal is asserted high, looks OK to me. The LED's coming on might be part of its power up/self-check sequence, an LED test then it sees something confusing with its inputs and shuts down.
If you hear something sizzling, I would go after it, it might be a poor connection on the transformer bobbin (crappy soldering) or just gently tap a plastic pen on things to see if you can flush it out. Even a relay could be fizzling inside.

I am not sure how I missed that fizzling noise. Keep in mind this thing is on my bench at work, where it isn't a quiet place. This “fizzling” noise appears now as soon as I plug it in with the batteries connected and the relays kick it. No battery, no relays, no fizzling.
I did think the AC-OK is good. I remember early on checking these little transformers and compare them to each other measuring resistance and they were identical. Anyway… I'll see Friday if I can figure out where this noise comes from.

[CapLeaker]

OK. not sure what went on here (it probably didn't help working on this only on certain days when I had time), but I verified T1 against T2 transformer again. T1 is making this sizzling sound and the I measure 1.7kOhm across the primary winding. It's game over with this, as I don't have a replacement transformer.

[sam_sam_sam]

Did you remove both of the transformer from the board and did you notice anything unusual about either transformer winding connections to the pins on rare occasions I have seen bad soldering joints on the connections pins after removing the solder and reapplying solder to the joints corrected the issue I am not saying it the issue you have

[CapLeaker]

Did you remove both of the transformer from the board and did you notice anything unusual about either transformer winding connections to the pins on rare occasions I have seen bad soldering joints on the connections pins after removing the solder and reapplying solder to the joints corrected the issue I am not saying it the issue you have

No, I didn't remove these transformers from the board, I did look at the wire connection to the pins and them are o.k. Also the sizzling, buzzing noise is coming from the inside of T1. We had a very large event going on last week at work, with all the extra noise created for prepping this place over the last 2 months, I couldn't hear the faint fizzling noise inside the transformer. I suppose I could “hack” another transformer in. I could do that no problem if it would be my own. Since this thing is for my workplace, I can't do that.

I am not sure if they let me have that broken UPS or not, the manager went on vacation right after the event.