Need some help, Cant find transistor

Thread Starter

tb271

Joined Jun 12, 2018
6
Hello!

I bought a xiaomi mi mini router secondhand and well, it didnt work.
Opened it up and saw that a transistor was broken.
I found a transistor on the other side of the pcb that looks the same but im not sure if i can go on that.
Is there anyone who could help me find the right part so i can fix it Kapotte transistor.jpg Transistor.jpg

Searched on the web for a replacement part and only found one on aliexpress
https://www.aliexpress.com/item/20pcs-lot-BL1551-BL1551-SOT363-100-NEW-AND-ORIGINAL/32852217146.html
But im not sure if its the right one and could really use some advice :)
 

wayneh

Joined Sep 9, 2010
16,102
Well first of all, that's not a transistor, it's an integrated circuit. Unless you can read its number off the back, or find a schematic, it'll be very difficult to identify and replace. Sometimes an expert can make a judgement based on nearby parts, typical designs and so on.

Not to be a total turd in the punchbowl, but the failure of that part could have ruined, or been caused by, the failure of other parts. Even a successful repair of that part may not get you going.
 

Alec_t

Joined Sep 17, 2013
10,267
From the top of the left-hand pic it look as though a good part of the pcb has been cooked. You'll be lucky if it's only one transistor that has failed :(.
Just a wild shot, but can you post a schematic of that circuit?
 

dendad

Joined Feb 20, 2016
2,937
It may be better to try the manufacturer for a replacement board. There will be a reason for the IC failoure and as above, just replacing that part, assuming you can actually identify it, will most likley not fix it anyway.
I really doubt it is worth repair. What is the same model worth new?
 

Thread Starter

tb271

Joined Jun 12, 2018
6
Well first of all, that's not a transistor, it's an integrated circuit. Unless you can read its number off the back, or find a schematic, it'll be very difficult to identify and replace. Sometimes an expert can make a judgement based on nearby parts, typical designs and so on.

Not to be a total turd in the punchbowl, but the failure of that part could have ruined, or been caused by, the failure of other parts. Even a successful repair of that part may not get you going.
Well its says WB4GH , but didnt come far with google
 

Thread Starter

tb271

Joined Jun 12, 2018
6
From the top of the left-hand pic it look as though a good part of the pcb has been cooked. You'll be lucky if it's only one transistor that has failed :(.
Just a wild shot, but can you post a schematic of that circuit?
Been searching the whole web for any kind of schematic or circuit, came up with none, kinda hard to find it for devices out of Asia

I thought it was some kind of solder flux but you could be right, i hope not :(
 

Thread Starter

tb271

Joined Jun 12, 2018
6
It may be better to try the manufacturer for a replacement board. There will be a reason for the IC failoure and as above, just replacing that part, assuming you can actually identify it, will most likley not fix it anyway.
I really doubt it is worth repair. What is the same model worth new?
Well i tryed Xiaomi US but they said they dont sell it in america, got redirected to their global email and they said they couldnt have me any further. Well new it's 30 bucks but i just love to fix things :)
 

dendad

Joined Feb 20, 2016
2,937
As a good excercise, have a go at tracing the circuit.
Just remember to not poke around in it with it plugged in!
 

Thread Starter

tb271

Joined Jun 12, 2018
6
As a good excercise, have a go at tracing the circuit.
Just remember to not poke around in it with it plugged in!
Well i'll try that, had contact with the previous owner and he gave me my money back so nothing to lose atm.
Only a fun way to spend some free time :)
 

wayneh

Joined Sep 9, 2010
16,102
Well i'll try that, had contact with the previous owner and he gave me my money back so nothing to lose atm.
Only a fun way to spend some free time :)
Well that's cool. Low expectations are more likely to be met! :D

I think circuit tracing may be close to impossible because it's probably a multilayer board. Maybe not.

Did you happen to see this? Might be a lead.
 

Thread Starter

tb271

Joined Jun 12, 2018
6
Well that's cool. Low expectations are more likely to be met! :D

I think circuit tracing may be close to impossible because it's probably a multilayer board. Maybe not.

Did you happen to see this? Might be a lead.
I searched the whole web and you just find it with a snap of your fingers? :') damn.

U sure it's the right one? Not enough knowledge on this side of the line :p
 

-live wire-

Joined Dec 22, 2017
878
Here is a good video that talks about how one component failing leads to many other components failing. Imagine a voltage regulator fails, doubling the voltage across all your components. Instantly, many components get destroyed. Maybe a few things can handle it or get saved by other components failing open. And maybe only a few things are visibly damaged. Often you have to test them with a meter to see anything. If you replace those visibly damaged components, they get destroyed again. Replace just the regulator, and there are still many other components to replace.

It can take a HUGE amount of time to troubleshoot every component and figure out where your issues are. You really should just get a replacement board. There may be a few situations where it is just one component, but usually there is little hope. Chips especially, they rarely fail on their own. Here you should spend your time on more valuable things.
 

Ylli

Joined Nov 13, 2015
752
Here is a good video that talks about how one component failing leads to many other components failing. Imagine a voltage regulator fails, doubling the voltage across all your components. Instantly, many components get destroyed. Maybe a few things can handle it or get saved by other components failing open. And maybe only a few things are visibly damaged. Often you have to test them with a meter to see anything. If you replace those visibly damaged components, they get destroyed again. Replace just the regulator, and there are still many other components to replace.

It can take a HUGE amount of time to troubleshoot every component and figure out where your issues are. You really should just get a replacement board. There may be a few situations where it is just one component, but usually there is little hope. Chips especially, they rarely fail on their own. Here you should spend your time on more valuable things.
But what's the fun and what do you learn from that? Look at the part, figure out the circuit, think about what may have gone wrong, determine what downstream components may be damage or be the initial source of failure. Have fun and lean the electronics.

I was a technician for 35 years. I may not know all the math, but I have a lot of practical experience with everything from old tube amps to MRI systems. Didn't always have schematics available for troubleshooting. Had fun, learned a lot, and was good at my job. The TS has an opportunity to gain some knowledge with minimum cost. Go for it, and don't be disappointed if you fail.
 
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