Repairing a small electronic circuit

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,655
Hello.

I want to ask this question prior to go on the thread subject.
I don't have any experience whatsoever "in the real world" regarding electronics nor repairing or anything like that!
My question is: when someone tries to start a repair of a random PCB (in this case is a PCB from a handheld vaccum cleaner), is it common that we find schematics on the internet or is it really unlikely? I believe it's really unlikely. I can provide the model of the vaccum cleaner if you people think it can be found.

That was the question I wanted to ask.

Now, into the real problem.
This vaccum cleaner started to do a very weird noise and we couldn't power it off, so it had to be left working until the battery completely drained out.
I isassembled it and I see this PCB. I started looking for shorts on some of the components like mosfets, power regulators, that small chip, etc. I found that the mosfet with the red arrow was on short. I tried to remove it but I didn't know this mosfet was attached to the board the way it was, meaning that it was soldered to a very large metal pad and I started trying to pull it off while heating it but it was simply not coming off until the point where I end up lifting that very large pad from the PCB itself. However, that large pad didn't came completely off. I is still attached to the PCB by the top side.

I replaced this mosfet and as you can see, it's a tiny bit too much to the left but I think it's fine (I hope).
However, when I plug the charger to tat white connector at the bottom center of the image, the little resitor on it's left heats up quite a lot. But I can't find anything else heating up. That seems to be the only thing heating up. I marked in red that that resistor terminal goes to the back of the PCB through that via and returns to the top side next to that STM8L051F3P6 chip pin 8 (PB0 which is an ADC input/Timer according to the datasheet). Then, this same pin is connected to that other via on the center of the PCB and from there, it goes to another resistor behind that bigger connector!

1656972746529.png

Any idea how can I proceed from here to be able to figure out why this resistor is heating up so much? Or what else can I do? What you guys suggest?

Note: The melted plastic of those 2-pin larger connectors was from my previous attempts using hot air to remove the mosfet. No success though!
 

twohats

Joined Oct 28, 2015
294
Have you looked on YouTube?
It helps if you mention the make and model of your little sucker.
Good luck..........
 

DickCappels

Joined Aug 21, 2008
8,815
That depends on luck! It is worth looking if it is a popular product.

I remember back in the 1960's through the 1970's a series of manuals named "Sams Photofact Manual" provided detailed information including voltages and typical waveforms for a variety of electronic appliances. They aren't cheap and if you are in the repair business the amount of business you can do might depend upon how quickly you can identify a circuit problem.

https://www.samswebsite.com/
 

bassbindevil

Joined Jan 23, 2014
457
You can often find service manuals for products from major manufacturers like Sony. But for small appliances, your best hope may be to find a data sheet for the main chip. Or just follow hunches and look for obvious problems like broken connectors, cold solder joints or failed power components.
 
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