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Need help identifying logic board component

J

johnnyzero

macrumors newbie
Original poster
Sep 14, 2010
7
0
I'm trying to troubleshoot an A1150 logic board that won't boot. After looking very closely at the board, I noticed there's a surface-mount inductor (L2507) that's broken off.

mbp2.jpg


Trouble is, I have no clue about the value of the inductor. I managed to get ahold of a schematic, but as luck would have it the schematic doesn't indicate the actual value of L2507 in mH or µH.

L2507_schem.jpg


It does list some sort of resistance rating (0.28 Ohms) and what I believe to be the package type (1206), but not the inductance value.

Does this mean that the inductance value doesn't matter? Maybe someone who knows more about electronics than I do can enlighten me on this.

Any help will be much appreciated. It would be a shame to have to trash this logic board because of a missing 50-cent inductor!

JohnB
 
Don't suppose you have access to a spare logic board to test the inductance of the component? That'd be the surest way to find out.

The inductance might or might not matter, depending on the context. Might help to see some more of the schematic if you can post it please?
 
Just a thought - could it be a wire-wound resistor (hence the impedance is specified)? That wouldn't have an inductance rating as such, although would have some inductance since it's a coil. Looking at it again, seems like this is a choke to cut out high-frequency noise, so the exact value of the inductance might not matter too much.
 
Thanks for your replies, tpg.

Yes, I do have a spare logic board that I can use to test the component. I'm gonna try to borrow an LC meter. Would I be able to accurately measure the inductance with the component in-circuit, though?

Please excuse my ignorance - I'm sure you know much more about electronics than I do.

JohnB
 
johnnyzero said:
Thanks for your replies, tpg.

Yes, I do have a spare logic board that I can use to test the component. I'm gonna try to borrow an LC meter. Would I be able to accurately measure the inductance with the component in-circuit, though?

Please excuse my ignorance - I'm sure you know much more about electronics than I do.

JohnB
Click to expand...

In general no you can't, since there might be another path with inductance in parallel to the component you're interested in. This is especially likely on a complex piece of kit like a logic board.

However, you are lucky in that you have a logic board with the piece missing! You can use this to work out any stray inductance.

See the section here for the rules of adding inductances in parallel (same as resistors):

http://en.wikipedia.org/wiki/Series_and_parallel_circuits

Take a measurement across the component on the good board, then across the gap on the faulty board. Then a bit of basic algebra using the rules above should give you the missing inductance :)
 
Thanks again. I suspected that other inductive components in the circuit could make it difficult to get an accurate measurement.

Let me try to get ahold of a meter and I'll see how it goes. I may need some assistance with the calculation, though.

I wonder why Apple doesn't just specify an inductor value in their schematic? Then again, that would make it too easy! :)

JohnB
 
Also, on further consideration, even that might not work. It depends how the LC meter you find works. If it works by finding an L-C resonance peak, then capacitance on the board may mess up the measurements.

Try it using the above, but if practical then removing the component is the only way to guarantee a reliable result.

Having said that, if this is just a choke, then the exact value shouldn't matter. I reckon that the component is a standard wire-wound choke with an impedance of 2.8 ohms, as indicated. I'd look for a surface mount component of the right size to meet those specs. The entries at the top of this page are the kind of thing, although don't have quite the right spec:

http://www.vishay.com/inductors/surface-mount/
 
So you're saying that matching the resistance (impedence) value of 0.28Ω is probably more critical than matching the inductance value of the original component? I'll defer to your expertise here.

As I said in my original post, maybe that's why the schematic only specifies the resistance/impedence value.

My next problem would be to actually find a 1206 SMD inductor with the correct rating of 0.28Ω. I tried searching on Mouser & Digi-Key, but it just gave me a headache. :)

JohnB
 
johnnyzero said:
So you're saying that matching the resistance (impedence) value of 0.28Ω is probably more critical than matching the inductance value of the original component?
Click to expand...

I'm not an expert - but that would be my best guess. I believe it is common practice to label these kinds of chokes by resistance/impedance. I mean, you could even hand wind a coil and it would probably work! (Not recommended though :p )

Farnell are excellent for parts. Can't find a case style 1206, but here's some 0.28 ohm surface mount chokes:

http://uk.farnell.com/jsp/search/br...e=en_UK&catalogId=&prevNValues=500006+1004213
 
Thanks for the link. I was actually able to find a 0.28ohm/1206 inductor on the Farnell site, but shipping from the UK would probably be ridiculous.

Anyway, I did a little cross-referencing on Mouser and was able to find what I believe to be a suitable inductor here.

Hopefully it will work, unless you see any reason why it wouldn't.

I'll keep you posted.

best,
JohnB
 
Can you verify if this actually worked? I've got an iMac with the same broken inductor. I went ahead and bought a few from mouser.
 
Unfortunately, the board still didn't work after I replaced the inductor with the one I ordered from Mouser. However, that particular board was a DOA that someone gave me, so it may have had other problems besides the broken inductor. Sorry I can't give you a more definitive answer.

Good luck,
JohnB
 
Thanks for the quick reply. Im the one that broke mine. I took it off to put on a Mac mini board, then ended up ripping the traces off the mini. Tried removing it from the mini and ended up ripping the pad off the bottom. That's what happens when you don't have proper equipment. All because my 7300gt died in the iMac. Was upgrading the mini with the iMac CPU in removing the heatsink from the mini I sheared off that inductor with my leather man. I've since fixed my iMac gpu and want to rebuild it. We'll see on Tuesday.
 
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