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Is it possible to deduce the identity of damaged, burnt out, or otherwise missing components (or suitable substitutes)?

For example, below is the PCB of a 2 speed electric razor I've disassembled. The missing component had three legs, like a transistor. It originally had writing on it, but was covered in what I can only describe as a thin white layer of soot/charcoal. I tried to clean it by gently swabbing it with an isopropyl alcohol solution and soft brush; it just crumbled to pieces.

The low battery LED is constantly lit and the NiMH cells hold a charge, but the electric motor will not engage. It was left on charge overnight, as usual. When I went to use it the next day, it was hot and non-responsive.

enter image description here enter image description here enter image description here This is all the remains of the absent component. enter image description here enter image description here

voices
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    I do Failure Analysis (FA) as part of my job, yes it is possible to determine what went wrong, and even restore boards like this with some rework tricks, but at a minimum you need schematics , if you don't have one then reverse engineer. No it's not worth it unless this razer has extreme sentimental meaning to you. I would start by pulling off everything that is burnt and cleaning the board with isopropyl, that burnt IC dust is quite nasty and toxic – crasic Aug 11 '18 at 17:37
  • @crasic - Hi, You're answering the OP's question and IMHO it is absolutely suitable to be written as one. Can you move it to become an answer instead? (and then I can remove this comment too). Thanks. – SamGibson Aug 11 '18 at 17:46
  • It appears to be a discrete boost converter with the mystery part surrounded by two diodes for anode cathode. I wonder how NiCad overtemp was sensed to shutoff the charger. – Tony Stewart EE75 Aug 11 '18 at 17:48
  • @SamGIbson I flagged this question off topic. I thought it would be disingenuous to post an answer and flag at the same time, but I wanted to give OP a nudge in the right direction. If the question is not closed I will move comment to an answer. – crasic Aug 11 '18 at 17:53
  • @crasic - Agreed about VTC *and* answering; like you, I try to avoid doing both. However, we [shouldn't answer in comments either](https://electronics.stackexchange.com/help/privileges/comment). Anyway, I'll leave it with you. – SamGibson Aug 11 '18 at 18:19
  • @TonyEErocketscientist It seems it wasn't sensed. The charger was quite hot. – voices Aug 15 '18 at 12:54
  • @crasic it's not sentiment, but I have my own reasons for wanting to attempt a repair, sure. – voices Aug 15 '18 at 13:00
  • the most puzzling thing about it is, it doesn't do much more than charge a battery and turn a small electric motor. WTF are all these components! – voices Aug 15 '18 at 13:03
  • There is a regulated battery charger and a DCDC converter for the motor to keep at constant speed – Tony Stewart EE75 Aug 15 '18 at 16:34

1 Answers1

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Is it possible to deduce the identity of damaged, burnt out, or otherwise missing components (or suitable substitutes)?

Without a schematic one can never be sure, but you do have a few clues:

enter image description here

The footprint left on the board is for an (most likely) an SOT-23 package:

enter image description here

This means it was probably a transistor because it has three terminals, but could be any three terminal device. It could be many things.

The easiest way would be to find a working board and find the codes on the top of the device, Here is a guide for identifying components.

The problem with this is even if replaced the problem that led to the failure of this part also could have caused failures in other parts. The traces on the PCB may also be damaged due to the high heat (or high current) of the component failure .

Voltage Spike
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    +1 Pretty good analysis. It's possibly an SOT-23 power MOSFET (P or N we don't know as the pinout is pretty much standardized a multimeter check could find out). I would also check for a bad (ie. shorted) diode or ceramic capacitor directly across the motor or near the inductor. – Spehro Pefhany Aug 11 '18 at 19:22
  • I thought it was one size smaller than SOT-23. But you're probably correct. – voices Aug 15 '18 at 12:55
  • I also believe it's a transistor. I wish I had something more to go on, so I could try some substitutes. – voices Aug 15 '18 at 12:58
  • @SpehroPefhany How does the multimeter check work? I'll give it a shot. – voices Aug 15 '18 at 13:07
  • Use the diode function, compare with standard pinouts for 2N7002 (N-ch) BSS84 (P-ch) MMBT4401 (PNP) MMBT4403 (NPN) for example and see where the junctions are. Don't forget the body diode junction in the MOSFETs even if it isn't shown on the datasheet. – Spehro Pefhany Aug 15 '18 at 13:12