Finding a dead short

I have the backplane board from my HP DL380 G9 server here, and it has two 'sides' each fed by a 6 pin power port.

One of those sides is showing a short between 12V and ground.

I've poked around with my multimeter and I can only see these capacitors showing a short.

Any suggestions on how to find out if one of them is internally shorted without taking each one off?

 

One trick is to feed a current source (from a lab power supply) into the short and find what gets hot with a thermal camera.

Another is to use a 4 wire kelvin resistance meter that can resolve down to 0.0001 ohm and play a game of ‘warmer/colder’ as you move around the board.

 

Freeze spray also works if your not a big shot with a thermal camera, or even the finger test.

Having said that thermal cameras are not so expensive got mine at bunnings recently circa 400 but haven't had the pleasure of diagnosing any shorts yet

 

I'm take the board in to work tomorrow and use the thermal camera

On the plus side, there's bugger all stuff on the 12V rail, it's mostly just feeding 12V to the drives

 

could the be picture but the middle far right cap on the right side looks like maybe a crack... or a piece of hair

 

Combining these two methods works pretty well - if you don't have a lab supply, you can put together a good-enough adjustable current source with a few bucks in parts and some scrap, and you can improvise "freeze spray" by turning a canned air duster upside down so it dumps liquid propellant out the spout rather than spraying as gas.

For the current source: Start with a PNP power transistor and a half-decent heatsink, and control the base current (Ib) with an LM317 configured as an adjustable current source (i.e with the combination of a small-value resistor and a multiturn potentiometer between the output and adjust pins).

Since the collector current (Ic) is equal to the transistor's HFe * Ib (which you set with the LM317), a constant Ib results in a proportional(ish) constant Ic.
HFe varies a lot between parts, so predicting Ic ahead of time is too fussy for this project. Instead, just start with the LM317 source at minimum current (maximum adjustable resistance) and measure Ic with your multimeter while you adjust the pot.

Because you're hunting for a very small resistance (a near short) it will take quite a bit of current to cause significant heating - probably several amps. I once tracked down a dead IGBT this way that needed 10 amps before it got hot enough to be obvious!

Yes, there are some details missing here - if you don't know how to fill them in, you probably shouldn't do it this way

 

Thanks for the advice I have a bench supply here and at work that can deliver >10A, so we should be able to make something glow (on the thermal camera at least!)

 

C114? Hrmm, I'll look closely tomorrow, thanks!

 

yes c114 but its a low res picture so hard to say, thought ill take a punt before it goes under the camera

errm 10a at 12v on those little caps? id prob start at 2-3a then move up

 

It won't be 12V, it'll be the fraction of a volt it takes to put 10A through a dead short... but yes, start lower and wind up if you need to.

 

Ended up putting just on 1.5A into the board, and this capacitor glowed!



Strangely it was on slot 9, but fed from the same supply as 1-6.

Inspecting the cap under the 10x scope, and it was visibly damaged.

Removed it from the board and the short circuit is gone

I'll put it back in the server tonight and see if it fixes the issue. Slot 9 is a 10TB media drive for jellyfin so it's mostly my wife's episodes of the real housewives. Oh no, you can't watch that anymore, what a shame...

I do have a couple of spare slots so I'll reconfigure it to skip slot 9

 

it's probably a decoupling cap (being it was shorting supply to ground), which means it'll probably work just fine without it.

 

damn was going to say the same thing. Therman camera is the best way to go with the least fussing about.

 

Nicely done.

It'll work 100% without that cap there. I think they've gone a bit overboard with the decoupling caps here, I doubt they are doing much at all where they're placed.

The best is what you got on hand.

I got a 4-wire kelvin meter on hand, where the Ali-express plug in thermal camera for my phone I got is complete garbage.

 

oh i dont have a thermal camera, wish i did haha.

 

aliexpress... its probably a normal camera with a thermal sw filter

 

It's a real thermal camera, was a few hundred at the time. The frame rate is 2fps, 64 pixels of resolution, and it runs its close shutter recalibration every 5 seconds, just terrible to use.

When the need comes, I got this in mind -

 

Server is back up and running!

Turns out slot 9 wasn't populated after all