Samsung 940b - classic nod backlight problem

[Behemot]

Re: Samsung 940b - classic nod backlight problem

Bocca please what is the resistance of R316 resistor? It is next to the 33uF C345 cap on the upper side. I have slightly burned it due to short between S1 and G2

[Behemot]

Re: Samsung 940b - classic nod backlight problem

Both mosfets above are bad. Desolder both and retest S2-G2 out of circuit to verify that read around 1.0 ohms. If still shorted, then replacements are available on ebay. 8 for $4.80 + free shipping.

Exactly my problem, thanks bro

Nah, exactly the right transistors are in one shop here but I don't like the price, worth $4 a piece. Pretty expensive, it will barely make any money when display sold after repair…

EDIT// found a thread here on slovakian forums, a guy replaced them with IRF7389. They are third the price and actually on local stock. Slightly worse current parameters, but who gives a shit, there will be less than 0,3 A of current running through both of them…(rating of the backlight is 650 V/7,5 mA which equals to less than 5 W).

[cpp]

Re: Samsung 940b - classic nod backlight problem

you can desolder this little mosfets (8pin smd) with a little pliers.
take the ic whit the pliers and add solder to four legs on both sides,then pull up
you must add heat on both sides quickly 2 or 3 times.

just repaired one samsung syncmaster 153v , whit 2 of 4 AO4600 mosfet shorted,due to dry capacitors.

My soldering iron is a "JBC" 25W, whit soldering tip slightly curved.

sorry for my english,hahaha, i use google translator sometimes..

[bocca]

Re: Samsung 940b - classic nod backlight problem

Wow, thanks retiredcaps

I will do what you both proposed, and try to look for a place to bridge the connection

[Jasgriff]

Re: Samsung 940b - classic nod backlight problem

Basically you have to figure out where those pads connect to in the circuit. If you look at the datasheet the two solder pads you lifted are either pins 6 and 7 or pins 2 and 3. I can't tell from the photo and regulars will notice that in one thread yesterday, I can't tell north from south in a diagram.

If there is any solder pad left, then you may be able to use the continuity beep on your multimeter to find out where the pads connect to. Then you can use some wire to bridge the connection.

Something like this ...

http://www.circuitrework.com/guides/6-1.shtml

That site offers some fantastic examples of patching. Much better than my response P.s Never Eat Shredded Wheat N-E-S-W.

[retiredcaps]

Re: Samsung 940b - classic nod backlight problem

You will have to guide me on that nearest connection business, as I'm not sure I quite got it.

Basically you have to figure out where those pads connect to in the circuit. If you look at the datasheet the two solder pads you lifted are either pins 6 and 7 or pins 2 and 3. I can't tell from the photo and regulars will notice that in one thread yesterday, I can't tell north from south in a diagram.

If there is any solder pad left, then you may be able to use the continuity beep on your multimeter to find out where the pads connect to. Then you can use some wire to bridge the connection.

Something like this ...

http://www.circuitrework.com/guides/6-1.shtml

[bocca]

Re: Samsung 940b - classic nod backlight problem

good, so there is still some hope

You will have to guide me on that nearest connection business, as I'm not sure I quite got it.


Clean in what sense? General cleanliness or soldering stuff? Thnx, by the way

[Jasgriff]

Re: Samsung 940b - classic nod backlight problem

sure can

here's a photo. i believe you'll destinguish my messup

Its not that bad of a mess! these things happen.

I'm pretty sure you can get away with patching the Pins with the missing pads to the next nearest connection.

The board looks clean what did you use?

[bocca]

Re: Samsung 940b - classic nod backlight problem

sure can

here's a photo. i believe you'll destinguish my messup

[Jasgriff]

Re: Samsung 940b - classic nod backlight problem

Can you show us a picture of what's happened?

[bocca]

Re: Samsung 940b - classic nod backlight problem

Two of the 8 legs came off along with all the solder on the board. I guess that can't be that easily fixed...

[bocca]

Re: Samsung 940b - classic nod backlight problem

I do have powerful enough iron, and that razorblade trick is pretty neat. I'll try it asap

[budm]

Re: Samsung 940b - classic nod backlight problem

I use stainless steel razor blade or eXACTO blade inserting under the melted soldeer IC pin of the IC to separate the pin from the pad if you have no other tools to do it with, just take your time doing it. Main key is that you should have 40watts soldering Iron so the tip can maintain the temperature without heating up the traces and damage the trace, most people using 25~3watts iron which does not have enough heat capacity to maintain the temprature.

[Jasgriff]

Re: Samsung 940b - classic nod backlight problem

Even if you did have solder wick you would probably find that it wouldn't get all of the solder off and risk you lifting a pad as you attempt to pull it off.

[bocca]

Re: Samsung 940b - classic nod backlight problem

That's right. One problem is, the soldering wick can't be found here anywhere. (Most never heard of the stuff). Is there any way to do it without it?

[Jasgriff]

Re: Samsung 940b - classic nod backlight problem

Are you trying to remove the 8 pin chip?

[bocca]

Re: Samsung 940b - classic nod backlight problem

ugh, there's not an easy way to desolder these is there? :-\

@jasgriff: yeah, that's what i'm talking about. And if you order something chances are it's gonna "get lost" in the customs. Most of the things I ordered never arrived...

[Jasgriff]

Re: Samsung 940b - classic nod backlight problem

http://www.digikey.rs/rs/en/digihome.html

$120 fixed rate shipping to Serbia? lol

[retiredcaps]

Re: Samsung 940b - classic nod backlight problem

Ebay won't ship to serbia in any case, and if someone agrees to do it outside of ebay it's pretty expensive. I'll have to find another way

Take a copy of the 4502C datasheet to your local electronics store and maybe you can find an equivalent part?

[bocca]

Re: Samsung 940b - classic nod backlight problem

Thank you. I will desolder tehm now and measure again.

Ebay won't ship to serbia in any case, and if someone agrees to do it outside of ebay it's pretty expensive. I'll have to find another way