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Joosh

Feb 25, 2017
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Feb 25, 2017
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Hi

First time posting so hope this is in the right section.

Trying to repair a friends Makita DAB radio board which has decided to melt itself. I'm looking for a bit of help identifying two surface mount components. I've uploaded pictures of a good board that I've managed to borrow and use as a reference and the bad board just so you can enjoy the burnt up destruction.

The first is what I believe to be a zener diode labelled 'L2' (D1 on PCB). Found a little info but two datasheets giving two different voltages, so a little confusing.

The second is the 3 node component that looks to be a transistor labelled as 'BR'. Found no info regarding what the values of this might be.

Any help is much appreciated!

Josh

tmp_29505-20170225_073659-881432278.jpg tmp_29505-20170224_145252704210254.jpg
 

davenn

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Sep 5, 2009
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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Here are the options I have listed for BR (in small 3 pin devices):

br.png

Narrowing it down to SOT-23 doesn't really help :-(

I would be tracing the circuit to determine what the likely options are.

It looks like these components drive Q3, so I'd check Q3 to make sure it's intact too.

Given that there are two SOT-23 devices marked BR in close proximity, they're unlikely to be something even slightly exotic. The chance drops further if there are more of these devices on the board elsewhere. That leads me to thinking they're transistors or diodes. The face that all three pins are used means they're not going to be zener diodes (but they could be a double diode -- however none of these are listed).

After elimination, they are likely to be jelly-bean NPN transistors. Dave pointed out they could be PNP, but I don't have them listed in SOT-23 (and the datasheet concurs with my list -- they're SOT-323, a smaller package)

Dave's other option is also in a smaller package. They look to me like SOT-23, but if they are smaller, Dave's options are better.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Oh, and don't remove those capacitors from the board. Or if you do, make sure you either label them or have the equipment on hand to measure them. They are not marked (this is normal) and there is no way to determine what value they were if you lose them.
 

Joosh

Feb 25, 2017
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Feb 25, 2017
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Thanks so much for your replies.

After a little testing and measuring I am pretty confident it is a PNP transistor in a SOT-323 package which would point to the MSB1218-RT1G.

The only thing, is that it has 9V on the base but the datasheet says a max of 7V? And of course, cannot find anywhere that actually stocks the transistor for a straight swap!

The base of BR transistor is connected to the base of the big transistor (http://uk.rs-online.com/web/p/products/7740685/ - see pic) and the big cap via zener diode L2 (http://uk.rs-online.com/web/p/products/4761759/).

Having looked around, I've found http://uk.rs-online.com/web/p/products/7527959/ as a replacement for BR, but obviously the voltage present is too high if you go off the datasheet. Is this negligible with the voltage readings I've taken, could my 'good' board be bad or have I gone wrong identifying my components here?

Sorry for the wall of text, I'm still learning, I usually fix locomotives and not electronics! :)

Josh

tmp_16513-20170223_1615291671139918.jpg
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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The only thing, is that it has 9V on the base but the datasheet says a max of 7V?

That's with respect to the base. You need to measure the voltage on the base, or the voltage between the base and emitter.
 
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