Suggestions for a Legrand MEGALINE UPS (snubber resistor very hot) - Page 1
Forum section: electronics / repair

author: Dannyx
author_rank: Regular Contributor
timestamp: February 27, 2025, 07:13:45 pm
content: Good day folks. I've got a very complex task on my hands and it's been a while since I've had to tinker with something this large, plus the issue itself is very hard to track down, so any suggestions are greatly appreciated and so is your patience, as there's a lot to talk about.Pictures are HERE.TL;DR: this Legrand "Megaline" UPS does not fully boot up. More specifically, it comes on, the display works fine, I can navigate the menus, but it immediately throws a "Modules Number Error" and shuts down after 10-15 seconds. There's absolutely no information on this fault at all, much less so any schematics or discussions - I reckon it's quite a new model. No components have failed in a catastrophic way, so it's hard to track down. Among other things, I noticed (what looks like) a "snubber" (?) resistor (R14 in the close-up) gets very hot ! My thermal camera shows it reaching up to 130+ celsius ! Even taking into account the inaccuracies of such a camera, that's still VERY hot for comfort !LONG VERSION: Looking inside, we can see it's a "modular" kind of UPS, meaning there's a "backplane" PCB (pictured) at the back and there are add-in cards which slide on rails either side and plug into the backplane, so you can add more "modules" if more output power is required. This particular unit came equipped with only one such module. The modules are pretty much the heart of the whole UPS: they contain the inverter, charger, booster, etc....the backplane PCB consists mostly of DC-DC stuff.The unit produces the same "Modules number error" with or without the module plugged into the backplane, so initially I thought the module had failed somehow, but without another one on hand to check, it's hard to tell for sure.One thing that WAS obvious after hunting around for anything unusual is a charred 10 ohm resistor on the backplane PCB, near one of the DC-DC transformers (the smaller one at the top). I took as many pictures as possible, in hopes that if anything were to go horribly wrong and pop even further, I'd at least see the values of the components. The resistor had gone open-circuit. I replaced this guy and did my best to "cold-check" as many components as possible BEFORE attempting to plug it in again. Everything checked out fine, so with a good 10 ohm resistor in place, I plugged the UPS into mains without the batteries: nothing went pop and I didn't get the the "modules number error" again, I got a "battery failure", which made perfect sense, so it was looking really promising. However, it didn't last long: I connected the batteries and plugged it in again. This time, it actually DID boot all the way to the main status screen, but I noticed the display still said "mains absent", even though the unit was plugged in, so that was not good. A few moments later, it gave me an "unknown error" message and the UPS turned off again.Realizing there's no point pushing it any further and trying again, I powered everything down and took a look at my 10 ohm resistor. Sure enough it had popped again......except it didn't ! See, there's an obvious burn mark in the middle (the second instance of the picture), but it still reads fine on my multimeter, so whatever caused the current draw wasn't enough to actually cook it and open it. STILL, it's clearly not OK to be like that !This is where I got stuck because, ironically, all the DC-DC converters I was able to identify give off stable voltages, even with the resistor half-baked like that. My thermal camera however does reveal that super-hot resistor near the large chopper transformer and I don't like that one bit. It connects between one leg of the chopper (along with the anode of large diode above it) and goes to the small white ceramic cap near it, which then goes to GND. I guess I should pull off the cap to check, but that still doesn't explain what happened to my 10 ohm resistor.....Yet another thing I noticed: there's another small PCB sitting at the front of the unit which serves as the "smarts" of the unit: it's got a microprocessor and some communication stuffz on there and connects to the backplane via a ribbon cable which plugs into that wide connector on the side of the PCB. The display and button PCB also plug into this "computer" board. I believe the failure of the power section may have caused some issues with this board as well: see, if I go into the "status" menu before the UPS shuts down, I get "zeroes" for the "MOD" "Pout MAX" fields. The first picture shows what I got when I fired it up the first time after replacing that 10 ohm resistor - it STILL doesn't look right - it looks like it just spat out a bunch of BS numbers: Mod number 11250 and "POUT Max 7875"......what is that ? It looks like there may be some communication failure between the PCBs. I DID spy a "hot-spot" on the "computer" PCB with my thermal camera - some "MUX" chip, which might explain the failure, but my shift ended and went home for the dayWhew.....that was a lot....if you had the patience of getting this far, I can only salute you