Samsung BN44-00261B power board backlight inverter fail

[Axxel]

Hi everybody

I have a samsung TV and when I turn it on, i can see the OSD and the backlight during 2 seconds then turn off. Sometimes, the backlight get ON all the time during the TV is powered but it is very rare. I looked the solder and everything looks OK but with the soldering iron, I have remaked someones, but the problem still continue.

I think it is a condenser problem. I tested all the chemical caps and they looks to have the correct value. On the picture, we can see a sort of blue caps. Do you think it can be possible it can be this the problem?



Regards!

[budm]

Re: Samsung BN44-00261B power board backlight inverter fail

Disconnect the cable between this board and the main board, the backlights then should come on and stay on when you plug the TV into the outlet, if it does, then the problem will be the main board, if not, then it will be this power supply/inverter board combo problem.

[klkmalik]

Re: Samsung BN44-00261B power board backlight inverter fail

Did you try resolder in inverter area and high volt with ccfl conecter . Remove old solder, apply new solder.

[budm]

Re: Samsung BN44-00261B power board backlight inverter fail

This Inverter board uses fingers on the board and sockets on the panel for lamp connections.

[tom66]

Re: Samsung BN44-00261B power board backlight inverter fail

Just to eliminate the small possibility of it being a main board issue, check the inverter on/off pin.

[Axxel]

Re: Samsung BN44-00261B power board backlight inverter fail

Hi! Thanks everybody for the answers

I looked fater the TV and I have the following :

I looked the inverter ON/OFF PIN (the 23) and I have 4,4V. When I turn the ON the TV and when the inverter pilot signal pass from 0V to 4,4V, I can see the backlight during 2 seconds then turn OFF. And the 4,4V is staying.

I tried to disconnect the white cable between the main board and the power board, and it is the same, the inverter turn on 2 second then turn OFF.

I think the problem is the power board... But now I don't have any idea where I can watch the fault... Solders? Condensers?

Regards

[tom66]

Re: Samsung BN44-00261B power board backlight inverter fail

Could be a bad backlight tube. The schematic has several OVP points; check those.

[Axxel]

Re: Samsung BN44-00261B power board backlight inverter fail

Could be a bad backlight tube. The schematic has several OVP points; check those.

Waht do you mean with "OVP Point"? It is a test point on the board?

[tom66]

Re: Samsung BN44-00261B power board backlight inverter fail

There are diodes DI820, DI821, DI822, etc. which go into the point marked SD. I think you need to solder a wire to the common point if this is not easily accessible then measure it. It is relative to cold ground. Any more than a few volts is bad.

[Axxel]

Re: Samsung BN44-00261B power board backlight inverter fail

Hi!

Thanks for your answers!

So with the help of the scheme and your informations, I solded 1 cable to the SD point and another to the COLD mass.

Miracle, with this 2 cables, the backlight looked to stay ON, with a value of 3.2V, and this value is decreasing little by little (look photo).

I turned it OFF, turned ON, the backlight worked normally.

Anther try and now, the backlight goes down after 3 seconds. Impossible to get it staying ON.



Do you think there are the neon backlight tubes or the inverter board?

Regards

[tom66]

Re: Samsung BN44-00261B power board backlight inverter fail

It is one or the other. What you can do is take a resistor of say 100 ohms and connect it to the cold ground. Power on the TV. The backlight should stay on. Then set test pattern to white in service menu or use a computer to generate a full white image. Try and look for any dark bands in the image.

[budm]

Re: Samsung BN44-00261B power board backlight inverter fail

Per diagram on page 2:
There are 3 comparators and 3 Diode OR Gate for open lamp connections detectors and lamp over current detector.
UI802A is the comparator for the lamp over current, the output at pin 1 is normally LO due to +4.22V at the + input (Pin 3) of the UI802A, if the Negative input (Pin 2, DC signal SD) goes higher than 4.22V, the output at pin 1 will swing HIGH and will be fed through Diode DI819 OR GATE (one of the 3 Diode OR GATE) which will turn on MOSFET QI807 which will ground out PIN 9 (COMP) of UI801, then the output of this IC will shutdown.
The other 2 comparators work the same way, you can easily calculate the Voltage at the Negative pins to find out what the voltage at the + pin has to be to cause the output of the comparator to go HIGH.
Right now it may be one of the three detectors has trigger the shutdown.

[Axxel]

Re: Samsung BN44-00261B power board backlight inverter fail

It is one or the other. What you can do is take a resistor of say 100 ohms and connect it to the cold ground. Power on the TV. The backlight should stay on. Then set test pattern to white in service menu or use a computer to generate a full white image. Try and look for any dark bands in the image.

Thanks for the answer!

But I don't understand where I have to connect the 100 ohm resistor to get the tubes forced ON.

budm, I will try to manage to see the offsets of the 3 comparators to see where is the failure...

[tom66]

Re: Samsung BN44-00261B power board backlight inverter fail

You connect it to the cold ground. This is the chassis of the TV but also places like DGND and SGND. Don't connect it to any part of the live side of the power supply (primary side), unless you enjoy fireworks...

[Axxel]

Re: Samsung BN44-00261B power board backlight inverter fail

Hi!

Thanks for the infos!

I made more tests, everything was unsuccessful... I removed the resistor RI825, and I tried to saturate the mosfet through a 10K resistor with the 13V output and it was unsuccessful... And I tried too to put directly the pin 9 of the BD9893 to the ground (I suppose when the mosfet 2N7002 is saturated, it will act as a closed interuptor so directy connects to the ground...)

But I can note a strange thing.

- When i connect only the PSU board to the 230V line, the backlight turn ON only 2 seconds then shutdown.

- When i connect the PSU board to the motherboard of the TV, sometimes, the backlight gets on, but when I turn OFF, at the third o fourth time I turn it OFF/ON, the problem comes back...

Please there is a way to bypass this security? Where exactly I have to connect this 100 ohm resistor to force the inverter? To the cold ground (If I understood well, the cold ground is the same ground of the RCA A/V input plugs? Or what component I have to remove to force the inverter to be turned on?

Regards and thanks a lot for the attention

[budm]

Re: Samsung BN44-00261B power board backlight inverter fail

If the MOSFET is bias on, that will cause the inverter to go into shut down, you need to ground out the GATE of the MOSFET (using 100 Ohms resistor) to keep it from turning on by the error signal.
That MOSFET is normally OFF if there is no error. You are turning it ON instead of OFF.
If that MOSFET is forced off and the backlights still will not stay on, then you have to locate the cause of the problem some where else.

[Axxel]

Re: Samsung BN44-00261B power board backlight inverter fail

Hi everbody!

Thanks for the help.

I grounded the gate of the transistor QI807 trough a 100 ohms resistor and the problem continues to be here...

Regards.

[tom66]

Re: Samsung BN44-00261B power board backlight inverter fail

In which case it is not an over voltage detect (open lamp) but possibly an over current detect fault. For this you need to look at UI802/A in Bud's highlighted drawing.

Also, if you can, check pin COMP (compensation) #9 of the main inverter IC, UI801; if this is going low it could indicate the protection is still firing but if it's staying high it indicates some other problem like lack of PFC voltage, bad inverter IC, cold solder joint, etc.

[jbfr_54]

Re: Samsung BN44-00261B power board backlight inverter fail

Hi everybody, I read carefully the overall thread as I am having the exact same problem. After a lot of unsuccessfull searches around the security circuits, I found the reason of the problem which is that the IS (pin 4) input of the feedback circuit is not correct, about 650mVolts. According to the IC BD9893 datasheet this pin is comparing with internal 1,25 volt. If higher, it will decrease the duty cycle of the IC output lowering the inverter output. If lower, it will do the reverse. The problem is that I do not see any failing component around and when I apply 1,25v to pin 4 using an externa power supply, the inverter works and the backlight fires up and the duty cycle varies with the applied voltage.
If I connect pin 1 of either DI810 or DI811 to pin 1 of DI809 then the backlight is quite dark but works. I wonder if the problem might be due to a bad transformer.
Can anyone tell me the resistance measured on TI801S out put and of TI801 in parallel with TI802 ?
To make it work, I replaced R1811 parallel with R1813 by a 22 ohms carbon and placed a 1,5nf in parallel with R1840. This provides a stable backlight but very dark.
Any idea about this problem ?
Regards.
Jean from France

[leonard]

Re: Samsung BN44-00261B power board backlight inverter fail

check this link there is a power supply that is similar to yours
https://www.youtube.com/watch?v=wrD1d-a2fnA