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Surface mount SOT-23 transistor help please

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freya
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My Fluke has been great since replacing the faulty LCD display but the backlight has never worked, trivial I know but curiosity got the better of me today and I took a look.
Tracing back from the led brought me to a couple of transistors one leg of which was broken from its track, brilliant I thought and soldered it back. No such luck.
Opening it again I tested the transistor this time and found it was open circuit on the collector leg when testing all directions.

Would any expert out there be able to tell me what it is and what I can replace it with please

 
Posted : 06/06/2014 5:59 pm
freya
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Thanks Jeffrey, I had thought about checking out some scrap boards to implement that idea although the more modern boards seem to have escaped the transistors in the design stage and skipped straight to a socking great IC :)

 
Posted : 06/06/2014 6:43 pm
davegsm82
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I don't quite understand the design of that circuit, looking at your picture, the top pins are the collector usually, lower left is emitter and lower right is base.

By the looks of it, the collector of the top goes through a Via, top left, then the Emitter of that one goes to the collector of the next. The emitter of which must go through the board underneath the device.

The bases of both are fed from the same signal via the 2x resistors to the right. It's not like a Darlington pair.

It looks like sort of a totem-pole output with 2x of the same devices. Can't figure out why.

Try just shorting out the defective device, maybe it will start working.

https://sites.google.com/site/davegsm82/projects/radioputer - A BC5441 Turned into a Media Centre PC.

 
Posted : 06/06/2014 7:28 pm
freya
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Measuring on the bottom left pin of upper transistor and toggling the backlight gives 1.5 volts which is switched from the transistor below. The resistors trace up to two more transistors, when measuring here whilst toggling turns 5 volts on and off.
I have not thought much on this yet and its back together for the time being to stop childs fingers getting to it.
It did make me think that the controller also has access to the backlight so it could be to do with that for auto backlight power off.

 
Posted : 06/06/2014 7:51 pm
Refugee
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I would put a scope on there.
It could be some kind of chopper circuit to step up or down to drive the back light.

 
Posted : 06/06/2014 8:29 pm
freya
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Ok, will try that when children are offline :bba

 
Posted : 06/06/2014 8:31 pm
DavidW
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Squinting at your picture I think I make the marking as 1R, which, according to Mr Towers equates to an MMBT2059 NPN Transistor. SOT23 case and same as conventional type 2N5089.

My perception is my reality.

 
Posted : 06/06/2014 9:11 pm
freya
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Thanks Dave,
Quite by chance I hit this site a few mins ago that lists the actual device as:

1R MMBT5089 Mot N SOT23 MPSA18 Vce 25V

http://www.marsport.org.uk/smd/mainframe.htm

Along the lines of what Jeffrey said something like a BC848A should be fine

 
Posted : 06/06/2014 9:19 pm
Refugee
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That is a good site.
Cheers :thumb

 
Posted : 06/06/2014 9:55 pm
Anonymous
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https://groups.yahoo.com/neo/groups/Fluke_DMM/info

You have to join this group. They may be able to help.

Al

 
Posted : 06/06/2014 10:42 pm
DavidW
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equates to an MMBT2059 NPN Transistor./quote]

Glad you got it sorted..hope you weren't put off by my typo!!! I must read what I write! 2059 of course should say 5089

My perception is my reality.

 
Posted : 07/06/2014 9:09 am
freya
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All working fine now, the first transistor I pulled off a computer motherboard checked out ok, so in it went. Could do with some IPA on a cotton bud to remove the flux but I am out.

Thanks for the help

 
Posted : 07/06/2014 2:58 pm
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