Samsung 940b - classic nod backlight problem

[bocca]

Hi peole My first post here, although I've been following unregistered for months now


So, i have two Samsungs 940b with identical problem from the topic title. opened one up and found of course 3 bulged capacitors. Those are the infamous 820uf 25v. I replaced them with 2x1000uf + 1x470uf. Plugged everything back, but tough luck.
So consulting posts from another topic about 740n ( https://www.badcaps.net/forum/showth...?t=5136&page=6 )
i looked for the F301 red fuse. Seems like it's ok.

So now i'm lost basicly

My motherboard model is: IP-35135B
Capacitors were capxcon, replaced them with Jamicon.

Oh and one last thing, before anyone answers. I'm from Serbia, and we don't have a lot of options on buying here. Ebay or any other online store is not an option.

Current here is 230v, 50/60hz

So I'm posting pics now

http://imgur.com/a/sRb0f

if you need anything else just tell me. it would mean a bunch if i fixed these two bad boys

[retiredcaps]

Re: Samsung 940b - classic nod backlight problem

So I'm posting pics now

http://imgur.com/a/sRb0f

For many reasons, I will NOT look at pictures hosted offsite. Follow my sig file below EXACTLY to upload your pictures here. Do not post pictures inline as well.

[Jasgriff]

Re: Samsung 940b - classic nod backlight problem

Hi,

When you say there is no back light do you mean it comes on then goes off or there is no back light at all?

Does the power light come on at all?

If there is power have your tried the flash light method?

The pics are good quality you might get a better response if you add them here using the manage attachment button. Could really do with a nice clear one of the bottom of the board not on an angle so we can see what's going on.

[bocca]

Re: Samsung 940b - classic nod backlight problem

@retiredcaps. I posted photo offsite for two very good reasons:

1. Not to garbae your database. I administrate 3 forums and it is not desirable to post anything through forum

2. Max size is 2000x2000. My photos are bigger so there's much more details to see for those with poor eyesight, or just those who want a detailed look.


If you still want me to post pictures here I will do it, but I'll leave the link for those who don;t mind external links if you don;t mind.

@Jasgriff:

1. When they ended their life about two years a go, behavior was, very very brief flash, 1/10 sec and then off. If you look carefully you see the picture but no backlight so it's very dim of course.
Now, nothing flashes, no backlight at all, and there is a repeating sound. I don't know how to describe it in english, but it's like a clock mechanism being stuck. Repeating "tak tak tak tak" sound, pretty faint. However I can't seem to find it's source

2. Power button is lit (blue).

[Jasgriff]

Re: Samsung 940b - classic nod backlight problem

If they are hosted on here then they will remain here forever to help other people out in the future. If they are hosted externally then they might vanish.

Here is an example of some pics hosted on here and they are fine.
https://www.badcaps.net/forum/showpo...91&postcount=1

Do you own a Digital multimeter and have you read retiredcaps post below?

https://www.badcaps.net/forum/showthread.php?t=10419

Could the clicking noise be like and electric arc noise?

[bocca]

Re: Samsung 940b - classic nod backlight problem

Jasgriff, did you want some angle photographed different? I did not quite understand that last thingy in your post

[Jasgriff]

Re: Samsung 940b - classic nod backlight problem

A good picture of the bottom of the board. Similar to your first picture.

[bocca]

Re: Samsung 940b - classic nod backlight problem

Jasgriff, yes I do own a multimeter. It's Mastech MAS830.

Also i'm pretty good at soldering. yes I read his post a while a go, will read it again tonight

Well, it could be electric arc noise now that you mention it


EDIT: Here's the bottom of the board and a detail on the soldered capacitors and resoldered 301 fuse

[Jasgriff]

Re: Samsung 940b - classic nod backlight problem

It could be a number of things.

We could do with knowing if it is just the backlight that isn't being powered up. can you see your desktop very faint when it is connected to your computer? if you shine a flash light on the screen this will help to identify if you can see your desktop.

[bocca]

Re: Samsung 940b - classic nod backlight problem

i can't see my desktop. all it says on the screen is " check the signal cable". Tried it on other monitor and computer is showing picture of course.

[Jasgriff]

Re: Samsung 940b - classic nod backlight problem

Your other monitor might be set to a higher resolution which would give you that message the key thing is you can see it in the background very faintly.

See those 3 chips on pic 4. Can you read the labels on them?

[bocca]

Re: Samsung 940b - classic nod backlight problem

Yup. two smaller ones are "4502c AA5VD"

and the bigger one is very hard to read but i'll give it a shot

B1131 G
FF984 7528

might not be right thou


oh and other monitor is the same resolution as this one, 1280x1024, so it should show the picture

[Jasgriff]

Re: Samsung 940b - classic nod backlight problem

Before we check those chips can you use your multimeter to confirm the labels and voltages at CN2? the connector with the on/off label.

[bocca]

Re: Samsung 940b - classic nod backlight problem

i'll have to do that tomorrow because my multimeter is at my home, and all this stuff i'm doing at work, so I be replying tomorrow (approx 18 hours from now) when I get back to work.

Thanks for the help so far!

[Jasgriff]

Re: Samsung 940b - classic nod backlight problem

That's fine I am sure that with the help on here you will find out what's going on with them.

[retiredcaps]

Re: Samsung 940b - classic nod backlight problem

Check U202 and U203 (mosfets) for shorts. Instructions on how to check for shorts in my troubleshooting guide.

[bocca]

Re: Samsung 940b - classic nod backlight problem

@Jasgriff:

I just checked microfuse F301 (right by the bulged capacitors that i changed) and it's resistance is infinite, which i guess means it's blown. Only thing that confuses me is that it looks ok, but i've never seen blown fuse thats not ceramic or glass so someone more experienced will judge.

OK, how do I test the on/off switch? I'm not particulary good with multimetre so i'll need some help there, if you don't mind.


@retiredcaps

OK, mosfet testing results are:

left mosfet

S1 G1 - multimeter briefly flashes ~153.23 and returns to default 1
S1 D1 - nothing happens
S2 G2 - 01.3
S2 D2 - multimeter briefly flashes something between 190 and 64 and returns to 1


right mosfet

S1 G1 - multimeter briefly flashes ~162.33 and returns to default 1
S1 D1 - nothing happens
S2 G2 - 01.0
S2 D2 - multimeter briefly flashes something between 190 and 64 and returns to 1

i found the data sheet for this mosfet 4502c

https://cdn.badcaps-static.com/pdfs/...8330ca5905.pdf

and I worked in accordance to it and your troubleshooting guide

[Jasgriff]

Re: Samsung 940b - classic nod backlight problem

If that fuse is showing as infinite resistance then its definitely blown. You will need need to read the small writing on the fuse so that you can order a replacement.

You can check to see if the Logic board is telling the Power supply to turn on the back light by setting your DMM to measure DC voltage then negative probe on the GND pin and the positive probe on the ON/OFF pin. Make sure that you don't touch anything else with the probes.

[bocca]

Re: Samsung 940b - classic nod backlight problem

well multimeter stays at default 1. Absolutley no movement at all, so i guess that's infinity and it's blown.

i'm going to buy few of these fuses cause the store will close in half an hour, and i'll have to wait till monday to get'em

[retiredcaps]

Re: Samsung 940b - classic nod backlight problem

OK, mosfet testing results are:

left mosfet
S2 G2 - 01.3

right mosfet
S2 G2 - 01.0

i found the data sheet for this mosfet 4502c

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.