Samsung LN-S3251D Black Screen

[Sam I Am]

Re: Samsung LN-S3251D Black Screen

A more clarified pic.
Still as to the disappearance of almost 5V.

[Sam I Am]

Re: Samsung LN-S3251D Black Screen

I've modded the last pic to reflect the latest measurements, etc.
I have no idea what type of component is K72 T9 so I don't know what to make of the voltage reading.
And where did the 4.7V go?

[PlainBill]

Re: Samsung LN-S3251D Black Screen

Per the attached pic, backtracking the trace at Pin 11 leads me to a resistor that is holding the voltage at 4.4V.
Does this mean that this resistor is the culprit or
Additionally, grounding either bottom pin of any DN component as well as momentarily grounding Pin 11 results in the CCFL's coming on for about three seconds then promptly shutting down.

No, I don't think the resistor is the problem. I think they used a voltage divider for multiple pirposes - note that there is another trace running from the 4.4 volt end of the resistor. Odds are that runs to another resistor, the other end of which is tied to ground. And the odds are at least one other point is tied to it.


Right now I'm longing for the days when all you needed to repair a TV was a tube tester, a full tube caddy, and a diddle stick.

PlainBill

[Sam I Am]

Re: Samsung LN-S3251D Black Screen

Hey ....what happened to the pic????
Let's try this again.
There we go...

[Sam I Am]

Re: Samsung LN-S3251D Black Screen

Per the attached pic, backtracking the trace at Pin 11 leads me to a resistor that is holding the voltage at 4.4V.
Does this mean that this resistor is the culprit or
Additionally, grounding either bottom pin of any DN component as well as momentarily grounding Pin 11 results in the CCFL's coming on for about three seconds then promptly shutting down.

[PlainBill]

Re: Samsung LN-S3251D Black Screen

KJJ T9 is the marking code on all of the "DN" diode sets on this board.

OK!!! That's the BAV70 configuration. The first test is easy. I usually think of these diode packs as oriented with the two pins on the 'bottom' and the single pin at the 'top'. (Sort of like one of the objects from Space Invaders'). Are the 'top' pins on the DN1-DN8 diode nets all connected together? If they are not, stop here.

The next step might be a little more complicated. Can you trace pin 11 from the inverter controller to the source holding it at 4.4 volts? This is probably a pair of resistors. Is anything else tide to the junction of these resistors? IF this seems to be an isolated point, you should be able to cycle the inverter by momentarily grounding pin 11 of the controller. (It will be easier if you momentarily ground the junction of the resistors). If you can't be sure, stop here.

Is the top of the inverter exposed when it is installed in the TV? If so, the next test should be relatively simple. Rig two short (12" max) wires with probes at one end. Either leave the other end bare of attach a mini-gator clip. Ground the bare ends of both wires to one of the mounting screws for the inverter. With the TV on, use one probe to ground one of the 'bottom' pins of DN1. Now use the other probe to momentarily ground pin 11 of the controller. Do the CCFLs stay on? If not, move the first probe to the second pin of DN1. Repeat cycling the inverter by momentarily grounding pin 11. If the CCFLs still won't stay on, repeat with DN2, DN3, etc.

PlainBill

[Sam I Am]

Re: Samsung LN-S3251D Black Screen

KJJ T9 is the marking code on all of the "DN" diode sets on this board.

[PlainBill]

Re: Samsung LN-S3251D Black Screen

Many inverter controllers are turned on / off by turning the power to them on and off. The sample schematic does that. This chip also has an on / off input -STB. The datasheet says that if STB is below .8 volts the controller shuts down the outputs, and turns them on when the STB input goes over 1.8 volts. The maximum allowable voltage on STB is Vcc. So that voltage is not a problem.

You are correct, Comp is too high; that's what's shutting the inverter down. I've spent the last 15 minutes staring at schematics and datasheets. IF the sample application note is correct, that would appear to be triggered by the output voltage being too high, but more studying is required.

PlainBill

One more point. Earlier I had you take pictures of the components between the transformers. I notice one picture shows DN2 and DN3; I suspect there are also DN1 and DN4 - DN8. What are the making codes on those diode nets?

PlainBill

[PlainBill]

Re: Samsung LN-S3251D Black Screen

Okay.... this is what I have found concerning voltages with respect to the BD9882 chip.
Pin 20 (Vcc) = 9.1V acceptable per spec
Pin 12 (Comp) = 3.1V unacceptable max is 2.8V
Pin 11 (STB) = 4.4V unacceptable should be Vcc?
Pin 13 (Reg) = 3.1V acceptable per spec
So the question I have ....... is the chip shutting down the CCFL's due to over-voltage detect and why is Pin 11 voltage more or less half of what it should be?
I don't know if I am on the right track or somewhere out in left field.

Many inverter controllers are turned on / off by turning the power to them on and off. The sample schematic does that. This chip also has an on / off input -STB. The datasheet says that if STB is below .8 volts the controller shuts down the outputs, and turns them on when the STB input goes over 1.8 volts. The maximum allowable voltage on STB is Vcc. So that voltage is not a problem.

You are correct, Comp is too high; that's what's shutting the inverter down. I've spent the last 15 minutes staring at schematics and datasheets. IF the sample application note is correct, that would appear to be triggered by the output voltage being too high, but more studying is required.

PlainBill

[Sam I Am]

Re: Samsung LN-S3251D Black Screen

Okay.... this is what I have found concerning voltages with respect to the BD9882 chip.
Pin 20 (Vcc) = 9.1V acceptable per spec
Pin 12 (Comp) = 3.1V unacceptable max is 2.8V
Pin 11 (STB) = 4.4V unacceptable should be Vcc?
Pin 13 (Reg) = 3.1V acceptable per spec
So the question I have ....... is the chip shutting down the CCFL's due to over-voltage detect and why is Pin 11 voltage more or less half of what it should be?
I don't know if I am on the right track or somewhere out in left field.

[Sam I Am]

Re: Samsung LN-S3251D Black Screen

The resistor values vary from 9.75k to 10k ohms.
Agreed, this board is busy.
It never ceases to amaze me how one can take something that should be simple and complicate it beyond reason.
In the meantime, I will further study the schematic as well as take a few voltage readings to see if I can isolate the problem.
Enjoy the games.

[PlainBill]

Re: Samsung LN-S3251D Black Screen

How about these....
Good grief, how much AC voltage do those puppies output?
My Fluke will handle up to 750.

Those pictures will do. What is the value of the resistors?

I looked over the schematic, and there is a LOT going on in that circuit. It's going to take a little research. No, that's not true - it's going to take a LOT of research!

In the meantime, I've got a full day of watching football games scheduled for tomorrow. Enjoy my day off.

PlainBill

[Sam I Am]

Re: Samsung LN-S3251D Black Screen

How about these....
Good grief, how much AC voltage do those puppies output?
My Fluke will handle up to 750.

[PlainBill]

Re: Samsung LN-S3251D Black Screen

So.... that means I should be able to check the output voltages of the transformers to determine if they are within spec. Right?
Also, is there a fairly straight-forward way of testing a CCFL?

There are several ways to troubleshoot this. As I said, I found a schematic and PC layout for a similar inverter. This design is interesting because the high voltage divider caps are actually 'plates' etched in the board. Note the picture of the back of the board.

Do NOT attempt to measure the output voltage of the inverter with a standard DMM. It might or might not survive.


If you look at the top of the inverter you will notice small clusters of SMD components between each pair of transformers. I'd like a picture of them, please.

PlainBill

[Sam I Am]

Re: Samsung LN-S3251D Black Screen

So.... that means I should be able to check the output voltages of the transformers to determine if they are within spec. Right?
Also, is there a fairly straight-forward way of testing a CCFL?

[PlainBill]

Re: Samsung LN-S3251D Black Screen

I'm fairly confident all the CCFL's are firing. I am unable to get a direct line of sight on each and every one. I have power cycled the set several times to determine whether they are all firing as I can see a flash at each inverter board tab. Unfortunately, that is the best I can determine given it's design.
The earlier model 3251D has a much "friendlier" design as you could see each lamp fire as well as unplug each lamp individually. So much for progress.
Will one bad lamp keep the rest from staying on?
I check the ESR on the two caps when I checked the fuses and they were .03.
Once this set is up and going, I will change out all of the substandard caps that reside on the power as well as the inverter board.
I have located a couple of used ones that are reasonably priced (less than $60).
How confident are you that the inverter board is the culprit?

Yes, one CCFL not lighting will result in the controller shutting them down. For example, it measures the voltage out of each transformer. If a CCFL is not connected properly the voltage out of that transformer will be higher, and the controller will shut them all down. Also, if the current through any CCFL is too low or too high, the controller shuts them all down.

That ESR reading indicates that the caps are good (unless you skipped a decimal point or two).

I am very confident that the problem is the backlight system. No tests have been done to prove it isn't a CCFL.

PlainBill

[Sam I Am]

Re: Samsung LN-S3251D Black Screen

I'm fairly confident all the CCFL's are firing. I am unable to get a direct line of sight on each and every one. I have power cycled the set several times to determine whether they are all firing as I can see a flash at each inverter board tab. Unfortunately, that is the best I can determine given it's design.
The earlier model 3251D has a much "friendlier" design as you could see each lamp fire as well as unplug each lamp individually. So much for progress.
Will one bad lamp keep the rest from staying on?
I check the ESR on the two caps when I checked the fuses and they were .03.
Once this set is up and going, I will change out all of the substandard caps that reside on the power as well as the inverter board.
I have located a couple of used ones that are reasonably priced (less than $60).
How confident are you that the inverter board is the culprit?

[PlainBill]

Re: Samsung LN-S3251D Black Screen

Starting from the far left, the first pair reads 1.5284k, second = 1.5226k, third = 1.5262k, 1.5229k, 1.5275k, 1.5251k, 1.5172k, and 1.5158k ohms.

Dang!! Those values are REALLY close. They must have used very good wire to wind them.

I didn't think it was a fuse, but hope springs eternal.

How confident are you that the CCFLs all light? Can you see them directly? (In case it isn't obvious, I'm reluctant to begin detailed troubleshooting on this.)

The good news is I have found a schematic for an inverter built using the same concept.

The caps look good. Are they Sam Young NXH series? If so, replace them.

PlainBill

[Sam I Am]

Re: Samsung LN-S3251D Black Screen

Starting from the far left, the first pair reads 1.5284k, second = 1.5226k, third = 1.5262k, 1.5229k, 1.5275k, 1.5251k, 1.5172k, and 1.5158k ohms.

[Sam I Am]

Re: Samsung LN-S3251D Black Screen

I'm using a Fluke 8050A DMM which reads five digits.
Both fuses were checked initially and found to be good.
I will double check the secondaries later this afternoon and post my findings.
Thanks PB.