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.
I pulled that component off the board to see if the part number would still be intact. I removed the heatshrink and the MOV is actually in good condition; the missing MOV must've just burnt/melted the the heatshrink of its neighbor. I'll have to replace it anyway seeing as the leads are pretty much nonexistent, but oh well. (Part number is ERZ-V14D471)
Do you have any idea what part MV2 might be? I can't see any useful details in any of the pictures I've come across.
Good to know about the battery. Unfortunately I don't have a power supply like that. I might still have one my dad built from a kit, but I'm pretty sure it only allows you to control voltage.
Good to know about the battery.
MV2 is a MOV (metal oxide varistor). There are other MOVs on the PCB that you can check to see what kind it is. It is very likely the same as ERZ-V14D471
You may be able to cobble something with an incandescent bulb (say 100W) in series with the battery as a current "limiter". May prevent the UPS from working properly but would reduce risk of a catastrophic short.
MV2 is a MOV (metal oxide varistor). There are other MOVs on the PCB that you can check to see what kind it is. It is very likely the same as ERZ-V14D471Right, I know it's a varistor. I'm fairly certain it isn't the same one because the outline for that space is larger and the component looks larger in this picture from one that sold on eBay: https://i.ebayimg.com/images/g/sgEAAOSw~CZltAdV/s-l1600.jpg
Some advice when working on UPSs: do NOT use the battery to troubleshoot/perform testing. Those batteries can release uncontrolled amounts of current if there is a short somewhere on the board. It can be very dangerous. Use a DC power supply with current limiting capabilities to simulate the battery.
Some advice when working on UPSs: do NOT use the battery to troubleshoot/perform testing. Those batteries can release uncontrolled amounts of current if there is a short somewhere on the board. It can be very dangerous. Use a DC power supply with current limiting capabilities to simulate the battery.
is there any safe (safer) way to debug mains part of such device?
This MOV MV2 not being there isn't going to prevent your UPS from working. As I said earlier, these things are very dangerous to work on. You might want to invest in some additional equipment (DC benchtop supply, scope, etc) as well as gain a bit more experience in troubleshooting before proceeding.
It would be appropriate to check the condition of their contact groups, or better yet, dismantle them and check them individually.
You should also separately check the status of the MOSFET's of the inverter and the operation of the charger.
There are several threads on the forum that describe how to do this.
-
The rating of capacitor C53 for UPS in AC220V version is 2.2mF x 275V~.
The precautions one should take for checkin the mains portion or SMPS of the UPS are the same precautions one should take when working on any device powered by mains voltage.
-Be careful when checking HV side or primary side
-It might be benefical to use a lamp series tester to protect from unknown shorts on the primary side
-If you plan to use a scope, make sure the unit is plugged into an isolation transformer. If you plan to probe mains, you should only use methods that are safe - such as a HV differential active probe.
Those are just a few precautions one should take. But the main thing that separates the UPS from other devices powered by mains is the UPS's battery which is usually two large 12VDC batteries in series that can deliver large amounts of current. Keep in mind most of these devices are rated to deliver upwards of 1000W - that's a lot of power. And a large battery that can deliver a lot of current is required. If there is a short on the board in the charging or inverter circuit, this power can be delivered uncontrollably which could cause serious damage to life/property. I've seen APC UPS units with shorts on the inverter MOSFETs that caused catastrophic damage and the battery protection fuses did not prevent this damage (I've done a few videos/threads on these topics). So that is why I voiced my concern for troubleshooting these units without a DC power supply with current limiting.
You don't have to give up on it! Just be careful and take precautions. Use the advice from more experienced members in this forum. And invest a bit in some additional tools.