A while back I think I may have plugged the wrong power adapter into my MIDI keyboard and it started to smoke and then never turned on again.

So... I popped it open and it looks like it's a burnt 1N5187 diode.

I come from the completely different world of stats and maths stack exchange so I'm not really qualified to speak on these things and I have little training in electrical engineering and hardware.

But I have taken a class in soldering and so I came here to learn from anyone willing to advise whether you would recommend for/against me getting a soldering iron to replace it if I bought the right part and isopropyl-ed the affected area with a q-tip (assuming I'm a beginner).

Also, I'd like to understand what role this diode plays in the circuit. Just by looking at it, does it seem like the burnt areas would inhibit the flow even if the diode was replaced.

Any commentary, links, or help from experts is greatly appreciated. Main goal here is to learn more about how these things work.

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EDIT: Adding image of the the other side of the circuit board.

enter image description here


closed as off-topic by DoxyLover, PeterJ, Voltage Spike, Nick Alexeev Sep 20 '17 at 18:37

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the repair of consumer electronics, appliances, or other devices must involve specific troubleshooting steps and demonstrate a good understanding of the underlying design of the device being repaired. See also: Is asking on how to fix a faulty circuit on topic?" – DoxyLover, PeterJ, Voltage Spike, Nick Alexeev
If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ A photograph of the underside of the board (where the traces are) would be useful. You'll have to unscrew it, but you'd need to do that anyway and I don't see any other obstacles to removal. \$\endgroup\$ – Kevin Reid Sep 19 '17 at 21:44
  • \$\begingroup\$ Its a standard 1A Schottky power diode that looks like it had either AC or the wrong polarity applied to it. very common \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 20 '17 at 0:07
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    \$\begingroup\$ Carbonization usually conducts, not insulates. Clean the board very well before replacing the diode. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 20 '17 at 1:34
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    \$\begingroup\$ Does it work if powered through USB? \$\endgroup\$ – CL. Sep 20 '17 at 7:12
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    \$\begingroup\$ @MustafaSEisa What might the wrong power supply have been? E.g. a 24 VAC one or... \$\endgroup\$ – Andrew Morton Sep 20 '17 at 10:03

After further consideration, I've added my earlier comment as an answer. This is not an answer to the question that you asked, but it is an answer to the overall situation.

You commented on an answer:

That IC is basically about to fall off. It's totally fried... Suggestions?"

That IC (U1?) seems to line-up with the melted area on the inside of the case, which confirms that it has been destroyed. So your problem is not actually about a burnt diode - instead you have at least one destroyed SMD IC (replacement needed) and possibly other destroyed components too. Replacing the diode alone will be a waste of time. Based on your comments, this repair is not within your skillset, and is off-topic here as a non-trivial repair of a consumer item, sorry :-(

You think its worth finding someone to repair it or is it not worth the cost?

I don't know the availability or cost of replacement parts for that specific device, in your specific part of the world (wherever that is :-) ). This is one reason why repair questions are off-topic - there is too much "question-specific" detail that won't help future readers :-(

Assuming all of the electronics is on that one PCB, then if you can get a replacement PCB, at a price you are prepared to pay, then (depending on the mechanics of actually replacing it) you might be able to exchange that on your own.

Perhaps you were thinking about replacing that SMD IC? Unfortunately, even if you could find a replacement for that damaged IC (and it may not be sold as a spare part anyway, if it is a custom or pre-programmed device) you then need to find someone with SMD soldering skills. And even then, that's assuming that only that IC (and the diode) have been damaged. Without a full evaluation of the whole keyboard, it's possible that even more components have been damaged, so that replacing even the diode and that SMD IC might not be enough! Sorry for the bad news, but I think it is important to be realistic. :-(

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    \$\begingroup\$ By all means, being realistic is what I'm here for, no need to be sorry. Thanks for taking the time to answer, very helpful. I was mainly referring to how complicated it would be to replace the chip assuming I got another pre-programmed one from M-Audio. But I think there's a slim, slim chance of that happening given that they don't even produce that item anymore. Thanks again Sam! \$\endgroup\$ – Mustafa S Eisa Sep 20 '17 at 18:41

Without the back of the PCB I can mostly guess, but from the components and damage it looks like something like this:


simulate this circuit – Schematic created using CircuitLab

DCHARRED is there to protect against reverse polarity conditions. It is really only effective if there is also a fuse that will blow when something is connected in the wrong polarity, because the diode will then conduct as much as the power supply can deliver, and will quickly burn out if nothing stops the current.

In this case, your power supply was stronger than the diode.

Removing DCHARRED, cleaning the area, replacing with another diode of similar performance (I guess anything 3A or higher should do), should make this work well again.

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    \$\begingroup\$ This keyboard expects Vin = 9 V. Might it be possible that the 7805 (or anything behind it) got fried? \$\endgroup\$ – CL. Sep 20 '17 at 7:12
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    \$\begingroup\$ @CL. Well it's M-Audio so I expect it to break by just looking at the circuit ;). Jokes aside, anything could happen of course, and since the diode looks like it has burnt completely through, C5 and U6 could have had the wrong polarity for a while. Still, these are reasonably sturdy components. I don't think anything after U6 would have been damaged. \$\endgroup\$ – pipe Sep 20 '17 at 7:18
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    \$\begingroup\$ Thanks a lot @pipe! I like your "D_charred" naming convention, made me laugh. Just added a photo of the backside, looks like there's more charring going on there as well. You're totally right, M-Audio breaking is like rain in Seattle. \$\endgroup\$ – Mustafa S Eisa Sep 20 '17 at 8:19
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    \$\begingroup\$ @MustafaSEisa If it doesn't work with USB power, there may be more to it than my answer suggests. That IC on the bottom looks horrible too, but it may just be because of a bad solder job. \$\endgroup\$ – pipe Sep 20 '17 at 8:59
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    \$\begingroup\$ That IC is basically about to fall off. It's totally fried... Suggestions? \$\endgroup\$ – Mustafa S Eisa Sep 20 '17 at 9:11

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