Samsung LN-S3251D Black Screen

[Sam I Am]

This set is a later model (Nov. '06 vs May '06) than the last one y'all helped me diagnose.
When I first got it, powered it up, the chime sounded, the backlights flashed one time, then went black screen.
Opened it up and found about half a dozen broken backlight bulbs. Replaced them with known good bulbs, which was a very tedious process, and fired it back up thinking I had it licked.
So much for that line of reasoning as it still does the same thing, except that all of the backlights now flash once.
Checked all of my voltages and they appear to be within acceptable limits.
I am leaning toward an inverter problem.
Am I on the right track and how do I confirm?

[PlainBill]

Re: Samsung LN-S3251D Black Screen

This set is a later model (Nov. '06 vs May '06) than the last one y'all helped me diagnose.
When I first got it, powered it up, the chime sounded, the backlights flashed one time, then went black screen.
Opened it up and found about half a dozen broken backlight bulbs. Replaced them with known good bulbs, which was a very tedious process, and fired it back up thinking I had it licked.
So much for that line of reasoning as it still does the same thing, except that all of the backlights now flash once.
Checked all of my voltages and they appear to be within acceptable limits.
I am leaning toward an inverter problem.
Am I on the right track and how do I confirm?

The easy way is to examine the connector to the inverter. With luck the pins are identified. Check the state of the BL_ON pin when the backlights go off. If the voltage doesn't change, it's the inverter.

PlainBill

[Sam I Am]

Re: Samsung LN-S3251D Black Screen

There is a pin designated B/L ON/OFF on the PS board that connects the ribbon cable to the inverter board.
When the power button is pushed, the voltage reads 4.7V then the backlights flash once. The voltage reading remains unchanged.

[PlainBill]

Re: Samsung LN-S3251D Black Screen

There is a pin designated B/L ON/OFF on the PS board that connects the ribbon cable to the inverter board.
When the power button is pushed, the voltage reads 4.7V then the backlights flash once. The voltage reading remains unchanged.

Sorry, I should have mentioned one more test. Some of the pins on that connector are probably labeled with a voltage - 24 volts probably. If the voltage doesn't drop when the backlights go off it's either the CCFLs or the inverter. You should know the drill by now. Pictures of the inverter, top and bottom, sharp, as close to 2000 x 2000 pixels as possible.

PlainBill

[reidy-]

Re: Samsung LN-S3251D Black Screen

most probably a faulty inverter, test the impedances across the pins of the inverters and look for one that doesnt have the same impedance as the rest of the pack (not to say you couldnt have 2 faulty inverters)

[PlainBill]

Re: Samsung LN-S3251D Black Screen

most probably a faulty inverter, test the impedances across the pins of the inverters and look for one that doesnt have the same impedance as the rest of the pack (not to say you couldnt have 2 faulty inverters)

A minor detail, but using correct terminology helps avoid confusion. 'Inverter' usually refers to the PC board that provides the high voltage to the CCFLs. You are suggesting he measure the impedance of the inverter transformers. I agree, there is a very good chance that is the problem. Or it could be the inverter controller.

Now to be a real PITA, I'm going to point out that most of us don't have an impedance meter, we have to make do with the ohmmeter function of a DMM. (But an impedance meter would certainly be the best way to test the transformers).

PlainBill

[Sam I Am]

Re: Samsung LN-S3251D Black Screen

Okay.... here are the pics.
If you need more detail, I will have to redo them as left/right since the board is fairly long.
The first five pins (ribbon conn.) from the left are showing 24V from the PS.
Starting with the first pair of pins on the far left, the secondary side of the inverter transformers are showing 1.5 ohms on all 8 pair without variance.

[PlainBill]

Re: Samsung LN-S3251D Black Screen

Okay.... here are the pics.
If you need more detail, I will have to redo them as left/right since the board is fairly long.
The first five pins (ribbon conn.) from the left are showing 24V from the PS.
Starting with the first pair of pins on the far left, the secondary side of the inverter transformers are showing 1.5 ohms on all 8 pair without variance.

No they aren't. They might be reading 1.5K ohms, but there is no way the secondaries could be reading 1.5 ohms. Recheck them to the full accuracy of your DMM.

What is the part number of the 20 pin IC in the center of the board?

PlainBill

[Sam I Am]

Re: Samsung LN-S3251D Black Screen

Oops.....
Make that 1.5k ohms on the secondary outputs.
The 20-pin IC is a BD9882.

[PlainBill]

Re: Samsung LN-S3251D Black Screen

Oops.....
Make that 1.5k ohms on the secondary outputs.
The 20-pin IC is a BD9882.

One more thing I forgot - check the fuses.

Does your DMM only read 2 digits?

Look, I'm being serious here. There are a number of causes for the 'two seconds to black' problem. One is a bad CCFL. Another is a transformer with shorted turns. A third is a bad component in the sense circuitry.

Shorted turns often cause a slight change in the resistance of the secondary - anything over 3% is significant. That would be the difference between 1.51K and 1.55K.

Replacing a transformer is a PITA, but it is much easier than identifying which diode, resistor, capacitor or transistor is bad. If you would rather just buy a new inverter, or toss the whole TV, just say so.

PlainBill

[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.

[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.

[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

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

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

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

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

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

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

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.