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4.7UH Surface mount inductor question with picture.

Low_Boy

Low_Boy

Sr. Member
Jan 21, 2010
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Rochester N.Y.
Here is a picture of a surface mount inductor which i believe is blown open. Please let me know if this is what I am seeing. The part number for this is LEM2520 and is supposedly a 4.7 UH. I can not seem to find one. I am thinking about just removing it and soldering a through hole to the pads. What do you think?
 

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Does it really need to be a SMD?
No; it just needs to be 4.7uh choke. It might be more difficult to use a thru hole part in a small SMD space; but it will still work just fine. Keep the leads as short as possible - tho.
Another thing: why would an inductor blow open - unless there is a short in that circuit? It blew because it was the weakest link and took the whole load. Might have to solve for the causal short - first, otherwise you will just blow the inductor again.
 
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Robb said:
Does it really need to be a SMD?
No; it just needs to be 4.7uh choke. It might be more difficult to use a thru hole part in a small SMD space; but it will still work just fine. Keep the leads as short as possible - tho.
Another thing: why would an inductor blow open - unless there is a short in that circuit? It blew because it was the weakest link and took the whole load. Might have to solve for the causal short - first, otherwise you will just blow the inductor again.
Click to expand...

This is a radio with a electret microphone and there is voltage at the plug. I was wondering what was plugged in to it or was someone sticking a screwdriver in there to try to key the radio and possibly have shorted something. I do know a known good stock mic. has no modulation but a power mic. works perfect.
 
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electret condenser microphones require a small external voltage source and this is usually provided by the radio through the mic line. the only "stock" mic. that will work on that radio is the one supplied with the radio or another electret condenser mic, wired for an htx-10.

there are two 4.7 uH. rf chokes at the mic. chassis connector in series with pin #1 and pin # 6.
 
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Yes I neglected to mention that all the output voltages at the microphone plug are correct. Up and down buttons work, Receive sound with mic plugged in, None when unplugged and factory Mic keys radio.
 
These are such a bear, this is EPOXY pop - where the part did heat up and pop - but not truly blown.

Actually it should still work.
upload_2021-8-10_19-15-28.png​

The BEAR here, is that plated thru hole to the other side - that looks like it popped open like a fuse - the part survived, but the traces to it did not. That's smoke residue from the rapid rise and heating - blowing of the phenolic.

Sorry man, this might be a bane...

Possible solution is to place a STRAND - 1 strand of copper from a 14AWG wire - one single strand - solder from the top thru to the bottom - this requires disassembly and careful attention to details - because what you have to do now is re-establish that plated thru conductor to the other side.

No guarantees.
 
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My bad, in my haste to post the info, forgot the pic.

Kinda had to leave the post without it, because XYL called needing some help - we're ok - just had to switch gears to "family moment"

upload_2021-8-10_19-17-22.png

It's a closer look - there's smoke stain from around that area.

The hole is not smooth round - it's looks a little rough.

This is plated thru "epoxy" residue, did you reheat this area?
​
 
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No i did not reheat anything. I did notice what you are talking about and was not sure what to think about it. I also checked the choke next to that with a ohm meter and it showed continuity the one in question showed nothing. I will have to pull the face plate off the radio and see what is on the other side of that. I imagine that would be a big help.
 
It may be salvageable, but the next time it powers up - unless it was a external short, whatever that caused this - it will take out a trace somewhere else.

The concept is to "bridge" the bottom conductor to the top - that plated thru hole should have done it. But when something like this happens you have to make a "fusible" link to complete, what the surge burnt up in the hole did.

This was a current - like a short - shows "white residue" from what the smoke left behind.

I've seen this before - and thankfully many are recoverable - but not all - because whatever caused the failure can then take out the part in question.

Right now, that parts' epoxy encapsulating - just popped from the heat, and I hope that is all that residue is from - but the hole - it's not smooth - so cannot guarantee this will work - seen them lift right off, trace and all, and it's all from the warping and heat - releasing the adhesive below it.

Be gentle.
 
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I will never know what happened. I am hoping someone plugged in the wrong mic. Or wired a mic plug wrong. I will take the front off tomorrow. Not looking forward to it it is a HTX 10.
 
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I was looking at that yesterday. The picture of the control PCB is pretty useless.
A big bell just went off. I have a factory service manual of a TRC-485 which is basically the same radio.
 

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