Two blown backplanes...

Skud

Active Member
Jan 3, 2012
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So I picked up a Supermicro four node fat twin.

I brought up one of the nodes just fine with no issues. However, I went to insert a second node and as soon as it made contact it powered up and I smelled the unmistakable smell of burnt electronics.

I pulled out the node and investigated. Eventually, I found that one of the fuses on the front six-drive backplane had popped. It’s labeled F3 and appears to be on the 5V leg of the input. Interestingly, there is a second identical power plug below it and that fuse did NOT blow. I can’t tell if both plugs provide power to the entire backplane or if they each serve three drives. No drives have been inserted yet.

So, I move on to the next node and BAM. Exact same thing. Same fuse. Same spot. Again, all the bays are empty on these nodes.

What’s going on here? As far as I can tell the fuse popping is the only issue. The nodes power up fine and they seem to be okay. I can get into the bios and the IPMI works OK. All the voltages report okay as well.

Something must be making these fuses pop, but I can’t think of what.
 

pricklypunter

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Nov 10, 2015
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Power it all down, plug back in one of the duff nodes and use a multimeter, set on ohms, to test from the bad side of the blown fuse to each power rail and ground. Either it's a short to ground, or a short to another rail. The reason for the short could be just about anything, so once you confirm which it is, begin breaking it down until the short disappears, either by removing bits of the chassis, or the boards entirely in the case of a ground short, or by removing plugs etc in the case of a rail short. Sooner or later you should trip over what is causing it :)

Of course if something else is now damaged, you might see further shorts before you get near the original cause.
 

vanfawx

Active Member
Jan 4, 2015
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Does your fat twin model support node hotswap? I don't know about the version you have, but the 2u4n model I had, required a specific model to support node hot-swapping. What model do you have?
 

Skud

Active Member
Jan 3, 2012
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Does your fat twin model support node hotswap? I don't know about the version you have, but the 2u4n model I had, required a specific model to support node hot-swapping. What model do you have?
Yup!! Good idea though.

From the manual said:
Nodes Each of the four serverboards act as a separate node in the system. As independant nodes, each may be powered off and on without affecting the others. In addition, each node is a hot-swappable unit that may be removed from the front of the chassis.
 

Skud

Active Member
Jan 3, 2012
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So I've done a bit of troubleshooting and it looks like each power input on the backplane feeds half of the drives. So, that narrows it down.

I've tried to probe as best I can and I can't find any short. The only thing I could maybe think of might be the empty drive bays? The legs of the empty sleds aren't fully-pressed against the sides of the drive cage. They do come close to some SMD components on the inside of the backplane. Perhaps the leg touched something and shorted it out? Maybe that's what happened? Shooting in the dark here.

Riley
 

pricklypunter

Well-Known Member
Nov 10, 2015
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If nothing else looks damaged and it all fires up the way it's supposed to otherwise, then replace the fuse, pull the empty sleds and try again. On the subject of hotswap nodes, I just wouldn't want to be doing that, unless I was faced with actually having to, if you know what I mean :)